The Society for Photonics

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August 2007 Vol. 21, No. 4 www.i-LEOS.org IEEE NEWS

THE SOCIETY FOR PHOTONICS

HowHow thethe Double-HeterostructureDouble-Heterostructure LaserLaser IdeaIdea gotgot startedstarted

LEOS and the Growth of Photonics

High-PowerHigh-Power Vertical-CavityVertical-Cavity Surface-EmittingSurface-Emitting LaserLaser PumpPump SourcesSources 21leos04.qxd 8/29/07 4:55 PM Page cov2 21leos04.qxd 8/29/07 4:55 PM Page 1

IEEE NEWS

THE SOCIETY FOR PHOTONICS

Page 29, Figure 3: Cross-section schematic of the processed VCSEL array. August 2007 Volume 21, Number 4

FEATURES Special 30th Anniversary Feature: “How the Double-Heterostructure Laser Idea got started,” by Prof. Herbert Kroemer ...... 4 Special 30th Anniversary Feature: Most Cited Article from JSTQE • “Semiconductor Saturable Absorber Mirrors (SESAMs) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers,” (Reprint JSTQE Vol.2, No.3, Sept 1996) by Prof. Ursula Keller et al . . . . 8 • “Semiconductor Saturable Absorber Mirrors (SESAMs) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers,” - “Discovering” the SESAM - Commentary by Ursula Keller ...... 27 Industry Research Highlights: “High-Power Vertical-Cavity Surface-Emitting Laser Pump Sources,” by J.-F. Seurin et al, Princeton Optronics ...... 28 Industry Research Highlights: “High Power Laser Diodes: From Telecom to Industrial Applications,” by Norbert Lichtenstein ...... 33 Column by LEOS Leaders: “LEOS and the Growth of Photonics”, by LEOS President Dr. Fred Leonberger ...... 39

DEPARTMENTS

News ...... 40 • Awards and Recognition at LEOS 2007 • Call for Nominations: 2008 Young Investigator Award • Nomination forms

Careers ...... 42 • Focus on IEEE/LEOS 2006-07 Distinguished Lecturers: Toshihiko Baba, Bishnu P. Pal, and David V. Plant • 2006 IPRM Best Student Award recipient: Kale J. Franz 28 • 2006 LEOS-Newport/Spectra-Physics Research Student Paper Award recipients: - Tomoyuki Takahata, Ali K. Okyay, Hans C. Hansen, Maxim Abashin, and Haltice Altug

Membership ...... 49 • Benefits of IEEE Senior Membership • New Senior Members 31 • Chapter Highlight: Santa Clara Valley Chapter Conferences ...... 52 • Recognition at CLEO/QELS 2006 • Conference Calendar • LEOS 2007 Exhibitor Contract

Publications ...... 52 • Call for Papers: - IEEE Journal of Selected Topics in Quantum Electronics (JSTQE) . . . 55 36 - IEEE Sensors Journal

COLUMNS Editor’s Column………………2 President’s Column………………….3

August 2007 IEEE LEOS NEWSLETTER 1 21leos04.qxd 8/29/07 4:55 PM Page 2

Editor’s IEEE Lasers and Column Electro-Optics Society KRISHNAN PARAMESWARAN President Vice Presidents Alan Willner Conferences – E. Golovchenko University of Southern California Finance & Administration – S. Newton Welcome to the August Newsletter! As we continue to Dept. of EE-Systems/ Rm EEB 538 Membership & Regional Activities celebrate the 30th anniversary of LEOS, this issue is Los Angeles, CA 90089-2565 Americas – S. Unlu Tel: +1 213 740 4664 Asia & Pacific – C. Jagadish dedicated to the semiconductor laser. From CD players Fax: +1 213 740 8729 Europe, Mid-East, Africa – J. Buus Publications – C. Menoni to communications transmitters, to manufacturing Email: [email protected] Technical Affairs – N. Jokerst instruments, semiconductor lasers are ubiquitous, and President-Elect Newsletter Staff have had an immeasurable impact on both our field of John H. Marsh photonics and society in general. We are honored to Intense Photonics, Ltd. Executive Editor 4 Stanley Boulevard Krishnan R. Parameswaran present an article by Nobel Laureate Prof. Herbert Hamilton International Tech Park Physical Sciences, Inc. Kroemer of UC Santa Barbara, inventor of the double High Blantyre G72 0UX Scotland, UK 20 New England Business Center Tel: +44 1698 827 000 Andover, MA 01810 heterostructure laser concept. His article describes how Fax: +44 1698 827 262 Tel: +1 978 738 8187 he arrived at the idea, and should serve as inspiration to Email: [email protected] Email: [email protected]

readers developing the next generation of photonics Secretary-Treasurer Associate Editor of Asia & Pacific technology. Filbert Bartoli Hon Tsang Lehigh University Dept. of Electronic Engineering We also have two Industry Research Highlights arti- 19 West Memorial Drive The Chinese University of Hong Kong cles on diode laser technology. Norbert Lichtenstein of Packard Lab 302 Shatin, Hong Kong Bethlehem, PA 18015 Tel: +852 260 98254 Bookham Switzerland presents a nice review of High Tel: +1 610 758 4069 Fax: +852 260 35558 Email: [email protected] Power Laser Diodes, and Jean-François Seurin and col- Fax: +1 610 758 6279 Email: [email protected]; Associate Editor of Canada leagues at Princeton Optronics describe their recent [email protected] Amr Helmy work in High-Power VCSEL Pump Sources. The Edward S. Rogers, Sr. Department In our continuing series on reprints of the most cited Past President of Electrical and Computer Engineering H. Scott Hinton University of Toronto articles in LEOS journals, we present a seminal paper by Utah State University 10 King’s College Road Prof. Ursula Keller of ETH Zurich and co-workers Dean of Engineering Toronto, Ontario, Canada M5S 3G4 4100 Old Main Hill Tel: +1 416-946-0199 describing the semiconductor saturable absorber mirror Logan, UT 84322-4100 Fax: +1 416-971-3020 (SESAM). This paper has been cited 261 times since its Tel: +1 435 797 2776 Email: [email protected] Fax: +1 435 797 2769 publication in 1996, and the device concept has found Email: [email protected] Associate Editor of Europe/Mid application in a variety of applications. Prof. Keller has East/Africa Executive Director Kevin A. Williams also provided a commentary about the article. This fea- Richard Linke Eindhoven University of Technology ture was inadvertently left out of the April Newsletter, IEEE/LEOS Inter-University Research Institute 445 Hoes Lane COBRA on Communication Technology Department of Electrical Engineering so we are pleased to present it this month. We are also Piscataway, NJ 08855-1331 PO Box 513 Tel: +1 732 562 3891 happy to present a column by LEOS Past President Dr. 5600 MB Eindhoven, The Netherlands Fax: +1 732 562 8434 Email: [email protected] Fred Leonberger, who was instrumental in developing Email: [email protected] the Photonics industry into the robust field it is today. Staff Editor We hope that his column will be the first in a series of Board of Governors Giselle Blandin M. Amann K. Hotate IEEE/LEOS commentaries by LEOS leaders past and present. F. Bartoli D. Huffaker 445 Hoes Lane Please feel free to send any comments and suggestions to K. Choquette H. Kuwahara Piscataway, NJ 08855-1331 C. Doerr C. Menoni Tel: +1 732 981 3405 [email protected]. I would love to hear from you! S. Donati D. Plant Fax: +1 732 562 8434 C. Gmachl A. Seeds Email: [email protected] Krishnan Parameswaran

LEOS Newsletter is published bimonthly by the Lasers and Electro- Optics Society of the Institute of Electrical and Electronics Engineers, Inc., Corporate Office: 3 Park Avenue, 17th Floor, New York, NY 10017-2394. Printed in the USA. One dollar per member per year is included in the Society fee for each member of the Lasers and Electro-Optics Society. Periodicals postage paid at New York, NY and at additional mailing offices. Postmaster: Send address changes to LEOS Newsletter, IEEE, 445 Hoes Lane, Piscataway, NJ 08854. Copyright © 2007 by IEEE: Permission to copy without fee all or part of any material without a copyright notice is granted provided that the copies are not made or distributed for direct commercial advantage, and the title of the publication and its date appear on each copy. To copy material with a copyright notice requires specific permission. Please direct all inquiries or requests to IEEE Copyrights Office. 21leos04.qxd 8/29/07 4:55 PM Page 3

President’s Column ALAN E. WILLNER

“LEOS is Our have a life 30 years from now. LEOS A Fundamental Value Professional Glue” is our professional address, one that of LEOS “The Force … is an energy field created by has permanency to it. “In risk perception, humans act less as indi- all living things. It surrounds us and pene- Let me raise an example that is bit- viduals and more as social beings who have trates us. It binds the galaxy together.” Obi- tersweet and that might seem a bit con- internalized social pressures and delegated wan Kenobi, Star Wars. troversial. When I was a student, many their decision-making processes to institu- “Through the Force, things you will see. of the seminal papers that I embraced tions. ... Institutions are their problem-sim- Reaches across time and space it does. Other came from the Bell System Technical plifying devices.” Mary Douglas & Aaron places. The future... the past. …. Always in Journal (BSTJ). Today, the BSTJ is no Wildavsky, Univ. of California Press, ‘82. motion is the future.” Yoda, Star Wars. more, and many of my students have Your professional reputation comes (OK, so it is a stretch to compare never even heard of it. Does this dimin- from many ingredients, including: (a) LEOS with the “Force” from Star Wars.) ish the long-term archival value of the quality of your work, and (b) your What will your career look like in 5 those papers? Unfortunately, I think ability to work and communicate effec- years? 20 years? These are common yes. How would you feel if JQE or PTL tively with others. These two items can questions if you take a strategic view of ceased to exist in 5 years? Would you make or break most of your future career your professional development. How regret having published in these jour- options. In many ways, our journals, many of us know what path we will nals? LEOS members can feel secure conferences, and volunteer committee want to take in 5 years? How about next that LEOS will be around for decades, year? Life changes, our choices change, as will our venerable journals. (continued on page 59) we change. One key element to long-term plan- ning is simple – do the best you can in your present position. By doing wonderful things, it enhances your reputation and your skill set. Hopefully, the people inside your own organizations will appreciate your contributions. However, those same internal people might or might not be able to help you towards your next career goal. External contacts are critical for solidifying your reputation and widening your strategic career options. In general, we all try to meet people from other organizations, participate on professional committees, give conference presentations, and/or publish journal/newsletter articles. Not coincidentally, this is the bread- and-butter of LEOS, and we provide a high-quality forum that “surrounds” and “binds” our community. Our activities do seem to transcend both space and time. Space: people can migrate to a different institution but are still anchored with a distributed set of colleagues. Time: we can relate to papers published 30 years ago, and we know that our present work will stand the test of time and

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Special 30th Anniversary How the Double-Heterostructure Laser Idea got started Herbert Kroemer

In August 1952, I was hired as “Resident Theorist” by the device designer which enables him to obtain effects with the small semiconductor research group at the Central quasi-electric forces that are basically impossible to obtain Telecommunications Technology Office [Fernmeldetechnisches with ordinary circuit means involving only “real” electric Zentralamt (FTZ)] of the German Postal Service in Darmstadt, fields.’ [Emphasis added] Germany. I had just received a doxtoral degree in Theoretical As an example, I quoted the improved bipolar transistor. Solid State Physics from the University of Göttingen, with a However, that still did not draw on the full power of the idea dissertation on what we would today call hot-electron transport expressed in the general design principle that the quasi-elec- effects, of the kind that were thought to play a role in the col- tric fields ‘enable the device designer to obtain effects that are lector of the then-new transistor. In the process, I had acquired basically impossible to obtain using only “real” electric fields.’ what by 1952 standards was a good background in semicon- It certainly represents major improvements, but does it repre- ductor physics, including the emerging device physics. It was sent something “basically impossible” otherwise? an unusual background for a 1952 Theoretical Physicist, but it Something that was indeed truly impossible to achieve oth- was perfect for what was to come. erwise emerged abruptly in March 1963. I was working at The FTZ group was at that time working on the first bipo- Varian Associates in Palo Alto at the time, where a colleague lar junction transistors. These early devices were far too slow of mine—Dr. Sol Miller—had taken a strong interest in the for practical applications in telecommunications, and I set first semiconductor junction lasers that had emerged in 1962, myself the task of understanding the frequency limitations a topic then outside my own range of interests. In a colloqui- theoretically—and what to do about them. One approach was um on the topic he gave a beautiful review of what had been to speed up the flow of the minority carriers from the emitter achieved, not failing to point out that successful laser action to the collector by incorporating an electric field into the base required either low temperatures or short low-duty-cycle puls- region, by using a non-uniform doping in the base, which es, usually both. Asked what the chances were to achieve con- decreased exponentially from the emitter end to the collector end. While working out the details, I realized that “... a drift field may also be generated through a variation of the energy gap itself, by making the base region from a non- −qF stoichiometric mixed crystal of different semiconductors with different energy gaps (for example, Ge-Si), with a composition (a) that varies continuously through the base.” ([1]; translated) This was not yet a full general design principle, but a seed had been planted. Because of the absence of any credible technology, I did not follow up on this idea until 1957, after I had joined RCA Laboratories in Princeton, NJ. At that time, the +qF 1954 seed had germinated: I had realized the generality of the design principle hinted at in the above sentence, and pub- Fe = 0 lished a (widely ignored) paper in the RCA Review (never publish in obscure journals!), which included the figure shown below, and the accompanying paragraph of text [2]: ‘In a homogeneous semiconductor the band slope under an (b) electric field is the same for all bands and, as a result, the forces upon electrons and holes are equal in magnitude and qF opposite in direction. This is not the case with a varying band h gap. If the concept of a varying band gap is a legitimate one, −qFe the forces would no longer be equal and opposite. It should, for example, be possible to have force acting only upon one kind of the carriers, or to have a force which acts in the same (c) direction for both (Fig. 1). Electrical forces in uniform crystals can never do this. This is why we call these forces “quasi-elec- + tric” forces. They present a new degree of freedom for the qFh

ECE DEPARTMENT AND MATERIALS DEPARTMENT UNIVERSITY OF CALIFORNIA, SANTA BARBARA, CA 93106 Figure. 1: (a) Effect of a true electric field; (b) and (c): Effects of quasi-electric fields.

4 IEEE LEOS NEWSLETTER August 2007 21leos04.qxd 8/29/07 4:56 PM Page 5

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tinuous operation at room temperature, Miller replied that “this device could not possibly have any practical applica- certain experts had concluded that this was fundamentally tions,” or words to that effect. It was a classical case of judg- impossible. The argument ran roughly as follows. In order to ing a fundamentally new technology, not by what new appli- obtain laser action, a population inversion has to be achieved, cations it might create, but merely by what it might do for which means that, in the active region, the occupation proba- already-existing applications. This is extraordinarily short- bility of the lowest states in the conduction band has to be sighted, but the problem is pervasive, as old as technology higher than that of the highest states in the valence band. A itself. The DH laser was simply another example in a long necessary condition for such a population inversion is a for- chain of similar examples. Nor will it be the last. Any detailed ward bias larger than the energy gap. Even then, a population look at history provides staggering evidence for what I have inversion is hard to achieve in an ordinary p-n junction. The called the Lemma of New Technology: carrier concentrations in the active region will always be lower The principal applications of any sufficiently new and innova- than in the doped contact regions. Inversion, therefore, tive technology have always been — and will continue to be — required degenerate doping on both sides. Even then, both the applications created by that technology. electrons and holes would diffuse out of the active region As a rule, such applications have indeed arisen—the DH immediately into the adjacent oppositely doped region, pre- laser is just a good recent example—although usually not venting a population inversion from building up. immediately. I immediately protested against this argument with words somewhat like “but that is a pile of .... , all one has to do is References: give the injector regions a wider energy gap.” To me, this [1] H. Krömer, Archiv d. Elekt. Übertragung 8, 499 (1954). would cause an electron-repelling quasi-electric field to be [2] H. Kroemer, RCA Review 18, 332 (1957). present on the p side, and a similar hole-repelling barrier on [3] —, Proc. IEEE 51, 1782 (1963). the n side. Carrier confinement would thus be achieved. In [4] —, Revs. Mod. Phys. 73, 783 (2001) fact, electron and hole concentrations could be much larger [4] —, US patent 3,309,553 (filed Aug. 16, 1963, issued 1967). than the doping levels in the contact regions (for details, see [5] —, Varian Central Research Report CRR-36 (1963); unpublished my Nobel Lecture [4]), and it would become readily possible (available from the author as PDF copy). to create the population inversion necessary for laser action. This double-heterostructure (DH) laser finally represented a Biography: Herbert Kroemer device truly impossible with only the real electric fields avail- Herbert Kroemer is Professor of Electrical and Computer able in homostructures. Note that the idea for it arose essen- Engineering and of Materials at UCSB. He was born and edu- tially at the instant I had been made aware that there was a cated in Germany. In 1952 he received a Doctorate in Physics problem. from the University of Göttingen, Germany. Since then, he The rest is history. has worked on the physics and technology of semiconductors I wrote up a paper describing the DH idea, along with a and semiconductor devices, especially heterostructures. He patent application. The paper was submitted to Applied originated several key device concepts, including the het- Physics Letters, where it was rejected. I was urged not to fight erostructure bipolar transistor and the double-heterostructure the rejection, but to submit the paper to the Proceedings of laser. He is a Member of the National Academy of the IEEE instead, where it was published [3], but largely Engineering and the National Academy of Sciences. He holds ignored. The patent was issued in 1967 [5]. It is probably a honorary doctorates from the Technical University of Aachen, better paper than the Proc. IEEE letter. It expired in 1985. Germany, from the University of Lund, Sweden, from the Both the IEEE letter and the patent draw on an extensive University of Colorado, and from the University of Duisburg- unpublished corporate report that might be of interest to read- Essen, Germany. He has received numerous awards, most ers wishing to go deeper into the history of the subject [6]. recently, in 2000, the Nobel Prize in Physics, “for developing When I proposed to develop the technology for the DH semiconductor heterostructures used in high-speed and opto- laser, I was refused the resources to do so, on the grounds that electronics,” and in 2002 the IEEE Medal of Honor.

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Special 30th Anniversary Semiconductor Saturable Absorber Mirrors (SESAM’s) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers Reprint of most cited article from JSTQE Vol. 2, No. 3, Sept 1996 Ursula Keller, Kurt J. Weingarten, Franz X. Kärtner, Daniel Kopf, Bernd Braun, Isabella D. Jung, Regula Fluck, Clemens Hönninger, Nicolai Matuschek, and Juerg Aus der Au

Abstract without any external pulse compression, and passive Q-switching Intracavity semiconductor saturable absorber mirrors (SESAM’s) with pulses as short as 56 ps—the shortest pulses ever produced offer unique and exciting possibilities for passively pulsed solid- directly from a Q-switched solid-state laser. Diode-pumping of state laser systems, extending from Q-switched pulses in the such lasers is leading to practical, real-world ultrafast sources, and nanosecond and picosecond regime to mode-locked pulses from we will review results on diode-pumped Cr:LiSAF, Nd:glass, 10’s of picoseconds to sub-10 fs. This paper reviews the design Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO4. requirements of SESAM’s for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device Historical Background and Introduction structure and material parameters for SESAM’s provide sufficient design freedom to choose key parameters such as recovery time, sat- Semiconductor Saturable Absorbers uration intensity, and saturation fluence, in a compact structure for Solid-State Lasers with low insertion loss. We have been able to demonstrate, for The use of saturable absorbers in solid-state lasers is practically example, passive modelocking (with no Q-switching) using an as old as the solid-state laser itself [1]–[3]. However, it was intracavity saturable absorber in solid-state lasers with long upper believed that pure, continuous-wave (CW) modelocking could state lifetimes (e.g., 1-μm neodymium transitions), Kerr lens mod- not be achieved using saturable absorbers with solid-state lasers elocking assisted with pulsewidths as short as 6.5 fs from a Ti:sap- such as Nd:glass, Nd:YAG, or Nd:YLF with long upper state phire laser—the shortest pulses ever produced directly out of a laser lifetimes (i.e., >100 μs) without Q-switching or Q-switched mode-locked behavior (Fig. 1). This limitation was mostly due to the parameter ranges of available saturable absorbers [4]. CW - Running CW - Q - Switching However, the advent of bandgap engineering and modern semiconductor growth technology “Single” has allowed for saturable absorbers with accu- Mode rate control of the device parameters such as Power Power absorption wavelength, saturation energy, and recovery time, and we have been able to Time Time demonstrate pure passive Q-switching, pure Self-Starting Mode Locking CW modelocking or, if desired, Q-switched modelocking behavior [5]–[9]. In addition, semiconductor absorbers have an intrinsic bitem- Q - Switched Mode Locking CW - Mode Locking poral impulse response (Fig. 2): intraband carri- Multi er–carrier scattering and thermalization processes Mode which are in the order of 10 to 100 fs as well as Power Power interband trapping and recombination processes which can be in the order of picoseconds to Time Time nanoseconds depending on the growth parameters [10], [11]. As we will discuss, the faster sat- Figure 1: Different modes of operation of a laser with a saturable absorber. CW Q- urable absorption plays an important role in sta- switching typically occurs with much longer pulses and lower pulse repetition rates than bilizing femtosecond lasers, while the slower CW mode-locking. response is important for starting the pulse formation process and for pulse forming in lasers with pulsewidths of picoseconds or longer. MANUSCRIPT RECEIVED SEPTEMBER 24, 1996; REVISED JANUARY 9, 1997. THE AUTHORS ARE WITH THE INSTITUTE OF QUANTUM ELECTRONICS, SWISS FEDERAL INSTITUTE OF Many other classes of laser can be passively TECHNOLOGY (ETH), ETH-HÖNGGERBERG HPT, CH-8093 ZÜRICH, SWITZERLAND. mode-locked with saturable absorbers. PUBLISHER ITEM IDENTIFIER S 1077-260X(96)09675-X. Previously, semiconductor saturable absorbers

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provide stable pulse generation for a variety of solid-state lasers. Time E Mode-Locking Mechanism for Solid- Interband Recombination State Lasers: Fast-Saturable-Absorber E ≈ ns Mode-Locking or Soliton Mode-Locking Absorption LT Grown Materials: D Passive mode-locking mechanisms are well- Electron Trapping explained by three fundamental models: slow sat- ≈ ps - ns urable absorber mode-locking with dynamic gain D saturation [27], [28] [Fig. 3(a)], fast saturable Intraband absorber mode-locking [29], [30] [Fig. 3(b)] and Thermalization Density of States D soliton mode-locking [31]–[33] [Fig. 3(c)]. In ≈ 100 fs the first two cases, a short net-gain window forms and stabilizes an ultrashort pulse. This net-gain window also forms the minimal stability require- Density of States D ment, i.e., the net loss immediately before and Figure 2: A measured impulse response typical for a semiconductor saturable absorber. after the pulse defines its extent. However, in The optical nonlinearity is based on absorption bleaching. soliton mode-locking, where the pulse formation is dominated by the balance of group velocity dispersion (GVD) Loss Loss Loss and self-phase modulation (SPM), we have shown that the net-gain window can remain open for more than ten times longer than the Gain Gain ultrashort pulse, depending on the specific laser parameters [32]. In Gain this case, the slower saturable absorber only stabilizes the soliton and starts the pulse formation process. Until the end of the 1980’s, ultrashort pulse generation was dominated by dye lasers, where mode-locking was based on a balanced saturation of both gain and loss, opening a steady-state net gain window as short as the pulse duration [Fig. 3(a)] (the slow- Figure 3: The three fundamental passive mode-locking models: (a) absorber with dynamic gain saturation model [27], [28]). Pulses as passive mode-locking with a slow saturable absorber and dynamic short as 27 fs with an average power of ≈10 mW were generated gain saturation [27], [28], (b) fast absorber mode-locking [29], [34]. Shorter pulse durations to 6 fs were achieved through addi- [30], and (c) soliton mode-locking [31]–[33]. tional amplification and fiber-grating pulse compression, although at much lower repetition rates [35]. have been successfully used to mode-locked semiconductor diode The situation changed with the development and commercial- lasers, where the recovery time was reduced by damage induced ization of the Ti:sapphire laser [36], which has a gain-bandwidth either during the aging process [12], by proton bombardment large enough to support ultrashort pulse generation. However, [13], or by multiple quantum wells [14]. More recently, both bulk existing mode-locking techniques were inadequate because of the and multiple quantum-well semiconductor saturable absorbers much longer upper state lifetime and the smaller gain cross section have been used to mode-lock color center lasers [15]. In both cases, of this laser, which results in negligible pulse-to-pulse dynamic the upper state lifetime of the laser medium is in the nanosecond gain saturation. Initially it was assumed that a fast saturable regime, which strongly reduces the tendency for self-Q-switching absorber would be required to generate ultrashort pulses [Fig. 3(b)]. instabilities (discussed further in Section II). This is not the case for Such a fast saturable absorber was discovered [26] and its physical most other solid-state lasers with an upper state laser lifetime in mechanism described as Kerr lens mode-locking (KLM) [19], [37], the microsecond to millisecond regime. First results with SESAM’s [38], where strong self-focusing of the laser beam combined with in solid-state lasers were reported in 1990, and they were initially either a hard aperture or a “soft” gain aperture is used to produce a used in nonlinear coupled cavities [16]–[21], a technique termed self amplitude modulation, i.e., an equivalent fast saturable RPM (resonant passive mode-locking). This paper was motivated absorber. Since then, significant efforts have been directed toward by previously demonstrated soliton lasers [22] and APM (additive optimizing KLM for shorter pulse generation, with the current pulse mode-locking) lasers [23]–[25], where a nonlinear phase results standing at around 8 fs [39]–[41] directly from the laser. shift in a fiber inside a coupled cavity provided an effective sat- Using a broad-band intracavity SESAM device in addition to KLM urable absorption. Most uses of coupled cavity techniques have and higher order dispersion compensation [42], [43] we recently been supplanted by intracavity saturable absorber techniques generated pulses as short as 6.5 fs [Fig. 12(b)] directly out of a based on Kerr lens mode-locking (KLM) [26] and SESAM’s [5], Ti:sapphire laser with 200 mW average output power at a pulse due to their more inherent simplicity. In 1992, we demonstrated repetition rate of ≈85 MHz [44]. External pulse compression tech- a stable, purely CW-mode-locked Nd:YLF and Nd:YAG laser niques based on fiber-grating pulse compressors have been used to using an intracavity SESAM design, referred to as the antiresonant further reduce the pulse duration from a Ti:sapphire laser to ≈5 fs Fabry–Perot saturable absorber (A-FPSA) [5]. Since then, many at a center wavelength of ≈800 nm [45], [46]. These are currently new SESAM designs have been developed (see Section III) that the shortest optical pulses ever generated.

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Besides the tremendous success of KLM, there are some signifi- soliton is absorbed which delays the soliton with respect to the cant limitations for practical or “real-world” ultrafast lasers. First, continuum. the cavity is typically operated near one end of its stability range, In contrast to KLM, soliton mode-locking is obtained over the where the Kerr-lens-induced change of the beam diameter is large full cavity stability regime, and pulses as short as 13 fs have been enough to sustain mode-locking. This results in a requirement for generated currently with a purely soliton-mode-locked Ti:sapphire critical cavity alignment where mirrors and laser crystal have to be laser using a broad-band SESAM [33], [56]. Soliton mode-locking positioned to an accuracy of several hundred microns typically. decouples SPM and self-amplitude modulation, potentially allow- Additionally, the self-focusing required for KLM imposes limita- ing for independent optimization. We justify the introduction of a tions on the cavity design and leads to strong space-time coupling new name for this mode-locking process because previously soliton of the pulses in the laser crystal that results in complex laser dynam- effects were only considered to lead to a moderate additional ics [47], [48]. Once the cavity is correctly aligned, KLM can be very pulsewidth reduction of up to a factor of 2, but the stabilization was stable and under certain conditions even self-starting [49], [50]. still achieved by a short net gain window as discussed for CPM dye However, self-starting KLM lasers in the sub-50-fs regime have not [57]–[60] and for KLM Ti:sapphire lasers [61], [62]. yet been demonstrated without any additional starting mechanisms as for example a SESAM. This is not surprising, since in a 10-fs Design Criteria for a Saturable Absorber Ti:sapphire laser with a 100 MHz repetition rate, the peak power First we consider the basic design parameters of a general saturable changes by six orders of magnitude when the laser switches from absorber. These consist of the saturation intensity Isat and saturation CW to pulsed operation. Therefore, nonlinear effects that are still fluence Esat, which will be seen to influence the mode-locking effective in the sub-10-fs regime are typically too small to initiate build-up and the pulse stability with respect to self-Q-switching. In mode-locking in the CW-operation regime. In contrast, if self-start- addition, the recovery time of the saturable absorber determines the ing is optimized, KLM tends to saturate in the ultrashort pulse dominant mode-locking mechanism, which is either based on fast regime or the large SPM will drive the laser unstable. saturable absorber mode-locking [Fig. 3(b)] in the positive or neg- However, we have shown that a novel mode-locking tech- ative dispersion regime, or soliton mode-locking [Fig. 3(c)], which nique, which we term soliton mode-locking [31]–[33], [51], operates solely in the negative dispersion regime. For solid-state addresses many of these issues. In soliton mode-locking, the pulse lasers we can neglect slow saturable absorber mode-locking as shaping is done solely by soliton formation, i.e., the balance of shown in Fig. 3(a), because no significant dynamic gain saturation GVD and SPM at steady state, with no additional requirements is taking place due to the long upper state lifetime of the laser. on the cavity stability regime. An additional loss mechanism, When the recovery time of the absorber is on the order of or even such as a saturable absorber [31], [33], or an acousto-optic mode- larger than the laser’s cavity round-trip time, the laser will tend to locker [51], [52], is necessary to start the mode-locking process operate in the pure CW-Q-switching regime (Fig. 1). and to stabilize the soliton. In addition, the nonsaturable losses of a saturable absorber This can be explained as follows. The soliton loses energy due to need to be small, because we typically only couple a few per- gain dispersion and losses in the cavity. Gain dispersion and losses cent out of a CW mode-locked solid-state laser. As the non- can be treated as perturbation to the nonlinear Schrödinger equa- saturable losses increase, the laser becomes less efficient and tion for which a soliton is a stable solution [51]. This lost energy, operates fewer times over threshold, which increases the ten- called continuum in soliton perturbation theory [53], is initially dency for instabilities [see (4) and (6) below] such as contained in a low intensity background pulse, which experiences Q-switched mode-locked behavior. negligible bandwidth broadening from SPM, but spreads in time Fig. 4 shows the typical saturation behavior for an absorber on a due to GVD. This continuum experiences a higher gain compared mirror. Initially, the pulses are formed by noise fluctuations in the to the soliton, because it only sees the gain at line center (while the laser, and the saturation amount at this early stage is dominated by soliton sees an effectively lower average gain due to its larger band- the CW intensity I incident on the absorber [Fig. 4(a)]. In general, width). After a sufficient build-up time, the continuum would we can assume that the saturable absorber is barely bleached (i.e., actually grow until it reaches an effective lasing threshold, destabi- I Isat) at CW intensity, because if the absorber were fully lizing the soliton. However, we can stabilize the soliton by intro- bleached at this intensity, there would be insufficient further mod- ducing a “slow” saturable absorber into the cavity. This slow ulation to drive the pulse forming process. absorber adds sufficient additional loss so that the continuum no The saturation intensity Isat is given by longer reaches threshold, but with negligible increased loss for the I = hv short soliton pulse. sat (1) σ T Depending on the specific laser parameters such as gain dis- A A persion, small signal gain, and negative dispersion, a “slow” sat- where hv is the photon energy, σA the absorption cross section and urable absorber can stabilize a soliton with a response time of more TA the absorber recovery time. It is important to note that the than ten times longer than the steady-state soliton pulsewidth absorption cross section is effectively a material parameter. The [Fig. 3(c)]. High-dynamic range autocorrelation measurements absorption coefficient α of the material is then given by have shown ideal transform-limited soliton pulses over more than six orders of magnitude, even though the net gain window is open α = σA ND (2) much longer than the pulse duration [32], [54], [55]. Due to the slow saturable absorber, the soliton undergoes an efficient pulse where ND is the density of absorber atoms or the density of states cleaning mechanism [33]. In each round-trip, the front part of the in semiconductors, for example.

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repetition rate). Equation (4) also indicates that solid-state lasers R(I) with a large upper state lifetime τ2 will have an increased tendency dR for self-Q-switching instabilities. dI The physical interpretation of the Q-switching threshold (4) is as follows: The left-hand side of (4) determines the reduction in losses per cavity round-trip due to the bleaching in the saturable absorber. This loss reduction will increase the intensity inside the hν laser cavity. The right-hand side of (4) determines how much the Isat = σ τ Absorber Reflectivity eff c gain per round-trip saturates, compensating for the reduced losses and keeping the intensity inside the laser cavity constant. If the Intensity on Absorber I gain cannot respond fast enough, the intensity continues to increase (a) as the absorber is bleached, leading to self-Q-switching instabilities or stable Q-switching.

R(Ep) Equations (3) and (4) give an upper and lower bound for the sat- dR uration intensity which results in stable CW mode-locking with- dEp out self-Q-switching. Of course, we can also optimize a saturable Multiple absorber for Q-switching by selecting a small saturation intensity Pulsing and a short cavity length, i.e., a short TR. This will be discussed in Instabilities more detail in Section V. hν Esat = σ If we use a fast saturable absorber with recovery time much

Absorber Reflectivity eff E >> E p sat shorter than the cavity round-trip time (TA TR), then the conditions given by (3) and (4) are typically fulfilled and much shorter Pulse Energy Density on Absorber Ep pulses can be formed. But now, an additional stability requirement (b) has to be fulfilled to prevent Q-switched mode-locking (Fig. 1). For Figure 4: Nonlinear reflectivity change of a saturable absorber this further discussion, we assume that the steady-state pulse dura- τ mirror due to absorption bleaching with the (a) CW intensity and tion p is shorter than the recovery time TA of the saturable τ < (b) short pulses. Isat is the saturation intensity, Esat is the satura- absorber, i.e., p TA. In this case the saturation [Fig. 4(b)] is tion fluence, I is the CW intensity, and Ip is the pulse energy den- determined by the saturation fluence Esat, given by sity incident on the saturable absorber. = hv ( ) Esat σ 5 Referring again to Fig. 4(a), the slope dR/dIat around I ≈ 0 A determines the mode-locking build-up time under certain approx- and the incident pulse energy density Ep on the saturable absorber. imations [9] can be written as The loss reduction per round-trip is now due to bleaching of the saturable absorber by the short pulses, not the CW intensity. This ∝ 1 ( ) Tbuild–up dR 3 is a much larger effect when TA TR. Therefore, in analogy to (4), | ≈ I dI I 0 we can show that the condition to prevent Q-switched mode-lock- As expected, the build-up time is inversely proportional to this ing is given by [9]: slope. This follows directly from Fig. 4(a), which shows that small intensity fluctuations will introduce a larger reflectivity change of no Q-switched mode-locking: the saturable absorber if the slope is larger. Therefore, the mode- dR TR TR locking build-up time decreases with smaller saturation intensities. Ep < r ≈ .(6) dE τ τ However, there is a tradeoff: if the saturation intensity is too small, p 2 stim the laser will start to Q-switch. The condition for no Q-switching is We can easily fulfill this condition by choosing Ep Esat [Fig. derived in [4], [9]: 4(b)]. This also optimizes the modulation depth, resulting in reduced pulse duration. dR TR TR no Q-switching: I < r ≈ .(4) However, there is also an upper limit to Ep, determined by the dI τ τ 2 stim onset of multiple pulsing [63]. Given an energy fluence many times where r is the pump parameter that determines how many times the saturation energy fluence Esat, we can see that the reflectivity is the laser is pumped above threshold, TR is the cavity round trip strongly saturated and no longer a strong function of the pulse ener- time, and τ2 is the upper state lifetime of the laser. The stimulated gy. In addition, shorter pulses see a reduced average gain, due to the lifetime τstim of the upper laser level is given by limited gain bandwidth of the laser. Beyond a certain pulse energy, τstim = τ2/(r − 1) ≈ τ2/r for r 1. The small signal gain of two pulses with lower power, longer duration, and narrower spec- the laser is given by g0 = rl, where l is the total loss coefficient of trum will be preferred, since they see a larger increase of the aver- the laser cavity. From (4), it then follows that Q-switching can be age gain but a smaller increase in the absorption. The threshold for more easily suppressed for a small slope dR/dI(i.e., a large satura- multiple pulsing is lower for shorter pulses, i.e., with spectrums tion intensity), a large r (i.e., a laser that is pumped far above thresh- broad compared to the gain bandwidth of the laser. Our experi- old with a large small-signal gain g0 or small losses l ), a large cav- mentally determined rule of thumb for the pulse energy density on ity round-trip period (i.e., for example a low mode-locked pulse the saturable absorber is three to five times the saturation fluence.

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0.8

0.7 380°C 30

0.6 20 315°C Electron Trapping Time (ps) Time Electron Trapping 0.5 260°C Absorption Bleaching on Mirror 10

200°C 0.4 0 24682468 10 100 1000 200 250 300 350 400 Energy Density (μJ/cm2) MBE Growth Temperature (°C) (a) (b)

Figure 5: Measured absorption bleaching and electron trapping times (i.e., recovery time of saturable absorber) for low-temperature MBE grown InGaAs–GaAs multiple quantum-well absorbers. The MBE growth temperature is the variable parameter used in the nonlinear reflectivity.

A more detailed description of multiple pulsing will be given elsewhere. In general, the incident pulse energy density on the saturable absorber can be adjusted by the incident mode area, i.e., how strongly the cavity mode is focused onto the saturable absorber. Equations (3), (4), and (6) give general criteria for the saturation intensity Isat (1) and saturation fluence Esat (5) of the saturable absorber. Normally, the saturation fluence of the absorber material is a given, fixed parameter, and we have to adjust the incident mode area to set the incident pulse energy density onto the saturable absorber to fulfill the conditions given by (6) and the multiple pulsing instabilities. Therefore, the only parameter left to adjust for the saturation intensity is the absorber recovery time TA (1). However, if we want to use the absorber as a fast saturable absorber, we have to reduce TA. Semiconductor materials are interesting in this regard, because we can adjust TA from the nanosecond to the subpicosecond regime using different growth parameters (Section III-A). In this case, however, it is often necessary to find another parameter with which to adjust Isat rather than with TA. We will show in the next section that this can be obtained by using semiconductor saturable absorbers inside a device structure which allows us to modify the effective absorber cross section σA (1), which is a fixed material parameter. For cases where the cavity design is more restricted and the incident mode area on the

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saturable absorber is not freely adjustable, modifying the device solid-state lasers into Q-switching instabilities (Section II). In addi- structure offers an interesting solution for adjusting the effective tion, nanosecond recovery times do not provide a fast enough sat- saturation fluence of the SESAM device to the incident pulse ener- urable absorber for CW mode-locking. We use low-temperature gy density. This is particularly useful for the passively Q-switched grown III–V semiconductors [5], [7], [67] which exhibit fast carri- monolithic ring lasers [64] and microchip lasers [65], [66], dis- er trapping into point defects formed by the excess group-V atoms cussed in more detail in Section V. incorporated during the LT growth [11], [68], [69]. Fig. 5 shows typical electron trapping times (i.e., absorber recovery times) and Semiconductor Saturable Absorber the nonlinear absorption bleaching as a function of MBE growth Mirror (SESAM) Design temperature. For growth temperatures as low as 250 ◦C, we still obtain a good nonlinear modulation of the saturable absorber with Material and Device Parameters recovery times as low as a few picoseconds. The tradeoff here is that Normally grown semiconductor materials have a carrier recombi- the nonsaturable absorber losses for Ep Esat increase with nation time in the nanosecond regime, which tends to drive many reduced growth temperatures [8]. This tradeoff will ultimately limit the maximum thickness of the absorber material used inside a solid-state laser cavity. For femtosecond pulse generation, we can benefit 140 200 from the intraband thermalization processes that occur 120 Reflectivity Pulse Width, fs with time constants from tens to hundreds of fem- 100 150 toseconds, depending on the excitation intensity and 80 100 energy [70]. A larger femtosecond modulation depth 60 can be obtained for quantum-well structures because 40 50 of the approximately constant density of states above

Output Power, mW Output Power, Output Power 20 Pulse Width the bandgap. However, we can strongly reduce the 0 0 requirements on this fast recovery time if we do not 1.0 1.0 use the semiconductor saturable absorber as a fast saturable absorber, according to Fig. 3(b), but just to start 0.8 0.8 and stabilize soliton mode-locking. In this case, no quantum-well effects are absolutely necessary and, 0.6 0.6 therefore, bulk absorber layers are in most cases sufficient as well. The reduced requirements on the 0.4 0.4 absorber dynamics also allowed us to demonstrate 50-

Pulse Spectra nm tunability of a diode-pumped, soliton-mode- 0.2 0.2 locked Cr:LiSAF laser with a one-quantum-well low- finesse A-FPSA (Fig. 6) [71], [72]. We would not 0.0 0.0 obtain this broad tunability if the excitonic nonlinear- 800820 840 860 880 900 ities in the SESAM provided the dominant pulse for- Wavelength, nm mation process. In addition, in the soliton mode-lock- Figure 6: Tunability of a diode-pumped Cr:LiSAF laser using an intracavity lowing regime we can also obtain pulses in the 10-fs range finesse A-FPSA. Pulsewidth as short as 45 fs has been achieved. The Tunability of or below, even though the mode-locked spectrum ≈ 50 nm was limited by the lower AlGaAs–AlAs Bragg mirror of the A-FPSA. extends beyond the bandgap of the semiconductor saturable absorber, for example [56]. We can further adjust the key High-Finesse Thin Absorber Low-Finesse D-SAM parameters of the saturable absorber if A-FPSA AR-Coated A-FPSA Sat. Abs. and (*SBR) Negative Disp. we integrate the absorber layer into a R=95% device structure. This allows us to modify the effective absorber cross R≈30% ≈ % section σA (2) beyond its material Sat. Abs. R=0% R 30 Sat. Abs. Sat. Abs. Sat. Abs. value, for example. In addition, we can obtain negative dispersion com- % R>99.5% R>98 R>99.5% R>99.5% pensation by using a Gire–Tournois mirror or chirped mirrors. In the following, we will discuss the different device designs in more detail.

April 92 Feb. 95 *June/July 95 April 96 Overview of the Different (a) (b) (c) (d) SESAM Designs Figure 7: Different SESAM devices in historical order. (a) High-finesse A-FPSA. (b) Thin AR- SESAM’s offer a distinct range of coated SESAM. (c) Low-finesse A-FPSA. (d) D-SAM. operating parameters not available

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with other approaches. We use various designs of SESAM’s mode-locking is typically well-described by the soliton mode-lock- [73] to achieve many of the desired properties. ing model [31]–[33]. Fig. 7 shows the different SESAM designs in historical order. The Fig. 9 shows a typical high-finesse A-FPSA design for a laser first intracavity SESAM device was the antiresonant Fabry–Perot wavelength ≈ 1.05 μm. The bottom mirror is a Bragg mirror saturable absorber (A-FPSA) [5], initially used in a design regime formed by 16 pairs of AlAs–GaAs quarter-wave layers with a com- iϕb with a rather high top reflector, which we call now more specifical- plex reflectivity of Rb e . In this case, the phase shift seen from the ly the high-finesse A-FPSA. The Fabry–Perot is typically formed absorber layer to the bottom mirror is ϕb = π with a reflectivity of by the lower semiconductor Bragg mirror and a dielectric top mir- Rb ≈ 98%, and to the top mirror ϕt = 0 with Rt ≈ 96% [8], ror, with a saturable absorber and possibly transparent spacer layers [83]. The multiple-quantum-well (MQW) absorber layer has a in between. The thickness of the total absorber and spacer layers are thickness d chosen such that the antiresonance condition is fulfilled: adjusted such that the Fabry–Perot is operated at antiresonance [(7), ϕ = ϕ + ϕ + ¯ Figs. 8 and 9]. Operation at antiresonance results in a device that is rt b t 2knd broad-band and has minimal group velocity dispersion (Fig. 8). The = (2m + 1)π (7) bandwidth of the A-FPSA is limited by either the free spectral range of the Fabry–Perot or the bandwidth of the mirrors. where ϕrt is the round-trip phase inside the Fabry–Perot, n¯ is the The top reflector of the A-FPSA is an adjustable parameter that average refractive index of the absorber layer, k = 2π/λ is the determines the intensity entering the semiconductor saturable wavevector, λ is the wavelength in vacuum and m is a integer num- absorber and, therefore, the effective saturation intensity or absorber ber. From (7), it follows that: cross section of the device. We have since demonstrated a more gen- λ eral category of SESAM designs, in one limit, for example, by d = m (8) 2n¯ replacing the top mirror with an AR-coating [Fig. 7(b)] [74]. Using the incident laser mode area as an adjustable parameter, we From the calculated intensity distribution in Fig. 9, we see that can adapt the incident pulse energy density to the saturation fluence m = 4. of the device (Section II). However, to reduce the nonsaturable The π-phase shift from the lower Bragg reflector in Fig. 9 may insertion loss of the device, we typically have to reduce the thickness of the saturable absorber layer.

A special intermediate design, which we call Rtop Rbottom the low-finesse A-FPSA [Fig. 7(c)] [75]–[77], is achieved with no additional top coating result- Ic < I0 ing in a top reflector formed by the Fresnel I0 reflection at the semiconductor/air interface, which is typically ≈30%. d The dispersive saturable absorber mirror (D- ΔR SAM) [78] [Fig. 7(d)] incorporates both dispersion and saturable absorption into a device similar to a low-finesse A-FPSA, but operated close to resonance. The different advantages and tradeoffs of these Semiconductor devices will be discussed below. Saturable Absorber

60 100 High-Finesse A-FPSA The high-finesse antiresonant Fabry–Perot saturable absorber (A-FPSA) device [5], [7] (Fig. 9) was the 80

(fs) 40 Reflectivity (

first intracavity saturable absorber that started and ω

/d Antiresonance 60 sustained stable CW mode-locking of Nd:YLF and Φ Nd:YAG lasers in 1992. Since then, other solid-state 20 lasers such as Yb:YAG [77], Nd:LSB [79], Nd:YLF, 40 % and Nd:YVO4 at 1.06 and 1.3 μm [80] have been ) passively mode-locked in the picosecond regime 0

Group Delay d 20 with this design. In addition, high-finesse A-FPSA devices have been used to passively Q-switch microchip lasers, generating pulses as short as 56 ps −20 0 0.95 1.00 1.05 1.10 1.15 [66]. Femtosecond pulse durations τp have been gen- Wavelength (μm) erated with Ti:sapphire (τp = 19 fs) [76], Yb:YAG τ ≈ τ = ( p 500 fs) [77], diode-pumped Nd : glass ( p Figure 8: Basic principle of the A-FPSA concept. With the top reflector, we can control τ = 60–100 fs) [6], [54], [63], and Cr:LiSAF ( p the incident intensity to the saturable absorber section. The thickness of this absorber 45–100 fs) [52], [72], [81], [82] lasers. In the section is adjusted for antiresonance. The typical reflectivity (dashed line) and group picosecond regime, the A-FPSA acts as a fast sat- delay (solid line) is shown as a function of wavelength. At antiresonance, we have high- urable absorber [29], and in the femtosecond regime, broad-band reflection and minimal group delay dispersion.

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seem surprising initially, because the phase shift from the first inter- For a center wavelength around 800 nm, we typically use an face from the MQW absorber layer to GaAs is zero due to the fact AlGaAs–AlAs Bragg mirror with a small enough Ga content to that n¯ > n (GaAs). However, all the other layers from the introduce no significant absorption. These mirrors have less reflec- AlAs–GaAs Bragg mirror add constructively with a phase shift of tion bandwidth than the GaAs–AlAs Bragg mirrors because of the π at the beginning of the absorber layer. Therefore, this zero-phase lower refractive index difference. However, we have demonstrated reflection is negligible. We also could have chosen to stop the Bragg pulses as short as 19 fs from Ti:sapphire laser [76] with such a reflector with the AlAs layer instead of the GaAs layer. However, device. In this case, the bandwidth of the mode-locked pulse we typically grow the Bragg reflector during a separate growth run, extends slightly beyond the bandwidth of the lower AlGaAs–AlAs followed by a regrowth for the rest of the structure. For this reason, mirror, because the much broader SiO2/TiO2 Bragg mirror on top we chose to finish the Bragg reflector with the GaAs layer to reduce reduces bandwidth limiting effects of the lower mirror. Reducing oxidation effects before the regrowth. the top mirror reflectivity increases the minimum attainable The saturable absorber layer inside the high-finesse A-FPSA pulsewidth due to the lower mirror bandwidth. (Fig. 9) is typically extended over several periods of the standing wave pattern of the incident electromagnetic wave. This results in AR-Coated SESAM about a factor of 2 increase of the saturation fluence and intensity The other limit of the A-FPSA design is a zero top reflector i.e., an compared to the material value measured without standing-wave AR-coating (Fig. 7) [74], [76]. Such device designs are shown in effects. We typically measure a saturation fluence of ≈ 60 μJ/cm2 Fig. 10 for a Ti:sapphire laser. The thickness of the absorber layer [8] for an AR-coated (i.e., Rt = 0%) LT grown InGaAs–GaAs has to be smaller than d to reduce the nonsaturable insertion loss of device. With a top reflector the effective saturation fluence is these intracavity saturable absorber devices. To obtain broad-band increased as given by (13) and (14) of [8]. For a relatively high top performance with no resonance effects, we add transparent AlAs or reflector >95%, the effective saturation fluence is typically AlGaAs spacer layers. The limitations of this device include the increased by about two orders of magnitude. bandwidth of the lower AlAs–AlGaAs Bragg mirror, and the potentially higher insertion loss compared to the high-finesse A-FPSA. Bottom Bragg Mirror 95% These AR-coated SESAM’s have start- 16x AIAs/GaAs Top Mirror ed and stabilized a soliton mode-locked Ti:sapphire laser achieving pulses as short as 34 fs [for device in Fig. 10(a)] [74] and MQW- 13 fs [for device in Fig. 10(b)] [33] with a GaAs- Saturable mode-locking build-up time of only Substrate Absorber ≈ 3 μs and ≈ 200 μs, respectively. As mentioned before, stable mode-locking was achieved over the full stability regime of the laser cavity. The measured maximum modulation depth R was ≈5% 4 with a bitemporal impulse response of 230 fs and 5 ps [for the device in Fig. 10(a)] ≈ 3 and R 6% with a bitemporal impulse response of 60 fs and 700 fs [for device in LT-MQW TiO2 Saturable Absorber Fig. 10(b)] measured at the same pulse AIAs 2 GaAs GaAs 50x InGaAs/GaAs energy density and pulse duration as SiO

Refractive Index 2 SiO2 inside the Ti:sapphire laser. For the first device [Fig. 10(a)] we were limited in 1 pulsewidth by the bandwidth of the lower 0.0 0.5 1.0 AlAs–AlGaAs Bragg mirror [74], which z [μm] was then replaced by a broad-band silver mirror [Fig. 10(b)]. In addition, the position of the thin saturable absorber layer d Bottom Reflector Top Reflector within the spacer layer was adjusted with (AIAs/GaAs Bragg Mirror) (SiO2/TiO2 Bragg Mirror) respect to the standing wave intensity pat- ϕ ϕ Rt exp(i t) Rb exp(i b) Absorber Layer tern to adjust the effective saturation fluence, or to partially compensate bandgap- induced wavelength dependence in the Figure 9: High-finesse A-FPSA: A specific design for a ≈1.05 μm center wavelength laser. latter case. The enlarged section also shows the calculated standing-wave intensity pattern of an incident The AR-coated SESAM device can be electromagnetic wave centered at 1.05 μm. The Fabry–Perot is formed by the lower AlAs- viewed as one design limit of the A-FPSA = · λ/ GaAs Bragg reflector, the absorber layer of thickness nd¯ 4 2 and a top SiO2/TiO2 with a ≈0% top reflector [74], [76]. Bragg reflector, where n¯ is the average refractive index of the absorber layer. Fig. 10(a) shows a simple AlAs–AlGaAs

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GaAs Substrate Bottom Bragg Mirror Etch-Stop 1 18x AIAs/AIGaAs Si- Ag, 5μm Substrate Etch-Stop 2 Epoxy GaAs- Substrate

LT GaAs, 10nm 4 Saturable Absorber 4 Absorber Layers 3 15 nm GaAs 3 2 AIAs AIAs AIAs 2 Ag AIGaAs AIGaAs AIGaAs AR-Coating AIO 50nm

2 AIAs

1 50nm AIGaAs AR 1 AIAs Refractive Index 0 Coating −0.2 0 0.2 Refractive Index 80nm z [μm] 0 Bottom Reflector −0.1 0.0 0.1 0.2 0.3 d (AIAs/AIGaAs Bragg Mirror) Z (μm) (a) (b)

100 AIAs/AIGaAs Bragg Mirror (Without GaAs Quantum Well) Bragg Mirror 80 (with GaAs Quantum Well) )

% 60 AR-Coated Bragg Mirror (with GaAs Quantum Well) 40 Reflectivity (

0 750 800 850 900 950 Wavelength (nm) (c)

Figure 10: AR-coated SESAM : Two specific designs for a ≈800-nm center wavelength laser such as Ti:sapphire or Cr:LiSAF. (a) The basic structure is a AlAs–AlGaAs Bragg reflector with a single GaAs quantum well as the saturable absorber. The additional AR-coating is required to prevent Fabry–Perot effects [see Fig. 10(c)]. The bandwidth is limited to ≈ 30 fs pulses by the lower AlGaAs–AlAs Bragg mirror. (b) Broad bandwidth for sub-10-fs pulse generation is obtained by replacing the Bragg mirror with a silver mirror. This device, however, requires post-growth etching to remove the GaAs substrate and etch-stop layers from the absorber-spacer layer. (c) Low-intensity reflectivity of the AlAs–AlGaAs Bragg reflector without a GaAs quantum-well absorber, with a GaAs absorber and with both a GaAs absorber and the AR-coating [according to Fig. 10(a)].

Bragg reflector with a single-GaAs quantum-well absorber in the in Fig. 10(a) that explains this resonance dip is formed by the lower last quarter-wavelength thick AlAs layer of the Bragg reflector. The part of the AlAs–AlGaAs Bragg reflector, the transparent AlAs additional AR-coating is required to prevent Fabry–Perot effects layer with the GaAs absorber quantum-well layer of total thickness [74]. The need for this additional AR-coating is maybe not obvi- nd¯ = λ/4 and the Fresnel reflection of the last semiconductor/air ous but can be seen in low-intensity reflectivity measurements of interface (without AR-coating), where n¯ is the average refractive this device with and without an AR-coating [Fig. 10(c)]. The index of the last AlAs and GaAs layer. This Fabry–Perot is at reso- reflectivity dip in Fig. 10(c) at ≈850 nm is due to the absorption nance because the round-trip phase shift ϕrt is according to (7): in the GaAs quantum-well and corresponds to a Fabry–Perot resonance. This strong wavelength dependent reflectivity prevents ϕ = ϕ + ϕ + 2knd¯ short pulse generation and pushes the lasing wavelength of the rt b t = π + + π Ti:sapphire laser to the high-reflectivity of the device at shorter 0 wavelength at the edge of the Bragg mirror [74]. The Fabry–Perot = 2π. (9)

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A ϕrt of 2π allows for constructive interference and therefore ful- then incorporates the GaAs quantum-well. In this case, the phase fills the resonance condition of the Fabry–Perot. No AR-coating shift of the lower part of the Bragg mirror is ϕb = 0 instead of π would be required if the AlAs–AlGaAs Bragg reflector in Fig. 10(a) (9) and, therefore, ϕrt = π, the condition for antiresonance (7). would end with the quarter-wavelength-thick AlGaAs layer that This design would correspond to a specific low-finesse A-FPSA or also referred to as the saturable Bragg reflector [see next Section III-E and Fig. 11(b)]. An additional Bottom Bragg Mirror AR-coating, however, increases the modulation 25x AIAs/GaAs depth of this device and acts as a passivation layer for the semiconductor surface that can improve long-term reliability of this SESAM device. GaAs- Substrate Low-Finesse A-FPSA The two design limits of the A-FPSA are the high- finesse A-FPSA [Fig. 7(a)] with a relatively high top reflector (i.e., >95%) and the AR-coated 4 LT InGaAs Absorber Layer 30 nm (20 nm) SESAM [Fig. 7(b)] with no top reflection (i.e., Rt ≈ 0%) [74]. Using the incident laser mode 3 area as an adjustable parameter, the incident pulse energy density Ep can be adapted to the saturation 2 Fresnel Reflection at fluence Esat of both SESAM’s for stable mode-lock- AIAs AIAs GaAs GaAs GaAs Air-GaAs-Interface Refractive Index ing by choosing Ep a few times Esat (see Section II) 1 [76]. A specific intermediate design is the low- − 0.2 0.0 0.2 0.4 0.6 finesse A-FPSA [75]–[77], where the top reflector z (μm) is formed by the ≈30% Fresnel-reflection of the d Bottom Reflector Top Reflector semiconductor/air interface [Fig. 7(c) and Fig. 11]. (AIAs/GaAs Bragg Mirror) (Air-GaAs Interface) Reducing the top reflector typically requires a thin- ϕ R exp(iϕ ) Rb exp(i b) t t ner saturable absorber and a higher bottom reflector to minimize nonsaturable insertion loss. (a) Fig. 11(a) shows a specific design for a wave- AIAs/AIGaAs Bragg Mirror length ≈1.05 μm. Similar to the high-finesse A- FPSA (Fig. 9), the bottom mirror is a Bragg mirror formed by 25 pairs of AlAs–GaAs quarter- iϕb wave layers with a complex reflectivity of Rb e GaAs- with R > 99%. The thickness d of the spacer Substrate b and absorber layers are adjusted for antiresonance (7), with ϕb = π [83] and ϕt = 0 which gives a minimal thickness of λ/2n¯ for m = 1 (8). The residual reflection from the different spacer and 4 Saturable Absorber absorber layers is negligible in comparison to the GaAs Quantum Well 3 accumulated reflection from the lower multilayer AIGaAs 2 Bragg reflector and the semiconductor-air interface. This is also confirmed by the calculated 1 AIAs AIAs AIAs AIGaAs AIGaAs standing wave intensity pattern shown in Fig.

Refractive Index 0 11(a). Because there is no special surface passiva- −0.2 0 0.2 z [μm] tion layer, it is advantageous for a higher damage Bottom Reflector d threshold to have a node of the standing wave (AIAs/AIGaAs Bragg Mirror) intensity pattern at the surface of the device [78]. (b) Independently, a similar low-finesse A-FPSA device for a center wavelength ≈860 nm [Fig. − − Figure 11: Low ﬁnesse A FPSA: (a) A specific design for a 11(b)] was introduced, termed the saturable ≈ μ 1.05 m center wavelength laser. In contrast to the high-finesse Bragg reflector (SBR) [75]. This device is very A-FPSA in Fig. 9, here the Fabry–Perot is formed by the lower AlAs–GaAs Bragg reflec- similar to the previously introduced AR-coated tor, the absorber-spacer layer of thickness nd¯ = λ/2 and the Fresnel reflection from the semiconductor-air interface. Again, the thickness d is adjusted for antiresonance (7). (b) SESAM device [74] shown in Fig. 10(a). In this Another specific design for a ≈ 860 nm center wavelength laser. This device was also called case, however, no AR-coating is required on the saturable Bragg reflector (SBR) [75] and corresponds to a low-finesse A-FPSA, where the AlAs–AlGaAs Bragg reflector. This can be Fabry–Perot is formed by the lower AlAs–AlGaAs Bragg reflector, the absorber-spacer explained with the A-FPSA design concept: We layer of thickness nd¯ = λ/4 and the Fresnel reflection from the semiconductor-air inter- can also describe this SBR device as a low-finesse face. Again, the thickness d is adjusted for antiresonance (7). A-FPSA [Fig. 11(b)], consisting of a lower

18 IEEE LEOS NEWSLETTER August 2007 21leos04.qxd 8/29/07 4:56 PM Page 19

AlAs/AlGaAs Bragg mirror plus a quarter-wave thick [88]. Chirped mirrors [42], [89], [90], mentioned earlier, are com- Fabry–Perot cavity at anti-resonance (the lowest possible order pact dispersion compensation elements, but typically require mul- and thickness). The thickness d of the spacer/absorber layer is tiple reflections to achieve sufficient dispersion compensation. A adjusted for antiresonance (7), with ϕb = 0 [83] and ϕt = 0, Gires–Tournois mirror [91] is also a compact dispersion compensa- which gives a minimal thickness of λ/4n¯ for m = 0 (8). A sat- tion technique, but has a tradeoff in terms of bandwidth and tun- urable absorber is then located inside this Fabry–Perot. With this ability. device pulses as short as 90 fs have been reported with a Ti:sap- Recently, we combined both saturable absorption and disper- phire laser [84], which are significantly longer than the 34 fs sion compensation in a semiconductor Gires–Tournois-like struc- pulses obtained with the similar AR-coated SESAM device [Fig. ture, called a dispersion-compensating saturable absorber mirror 10(a)]. This is most likely due to the lower modulation depth of (D-SAM) [Fig. 7(d)] [78]. By replacing one end mirror of a diode- this device. It is important to realize that the Bragg reflector does pumped Cr:LiSAF laser with this device, we achieved 160-fs puls- not play a key role in its operation and does not actually saturate. es without further dispersion compensation or special cavity design. For example, the Bragg reflector can be replaced by a metal reflec- This is the first time that both saturable absorption and dispersion tor [Fig. 12(a)] as discussed above to obtain larger bandwidth. compensation have been combined within one integrated device. An earlier version of a nonlinear or saturable AlAs–AlGaAs The D-SAM, in contrast to the A-FPSA, is operated close to the Bragg reflector design was introduced by Kim et al. in 1989 [85]. In this case, the nonlinear Bragg reflector operates on saturable absorption due to GaAs Substrate band filling in the narrower bandgap material of the 1. Etch Stop Bragg reflector. This results in a distributed absorp- Si- Ag, 5μm tion over many layers. This device, however, would Substrate 2. Etch Stop introduce too much loss inside a solid-state laser. Epoxy Therefore, only one or a few thin absorbing sections inside the quarter-wave layers of the Bragg reflector are required. The effective saturation fluence of the device can then be varied by changing the position 4 of the buried absorber section within the Bragg LT GaAs, 15nm Absorber Layer reflector or simply within the last quarter-wave 3 layer of the Bragg reflector, taking into account that 2 Ag AIAs

a very thin absorber layer at the node of a standing 70nm wave does not introduce any absorption. 1 Refractive Index The limitations of these SESAM devices include 0 AIGaAs, 20nm the bandwidth of the lower Bragg mirror, and poten- −0.1 0.0 0.1 0.2 0.3 tially higher insertion loss than in the high-finesse z [μm] A-FPSA. Pulses as short as 19 fs have been generat- (a) ed with the high-finesse A-FPSA compared to 34 fs with the low-finesse A-FPSA using the same lower 8 Bragg mirror, for example [76]. Replacing the lower Experiment Bragg mirror with a broad-band silver mirror [Fig. Ideal Sech2 6.5 fs 12(a)] resulted in self-starting 10-fs pulses [56] and 6 more recently pulses as short as 6.5 fs [44] [Fig. 12(b)] with a KLM-assisted Ti:sapphire.

D-SAM 4 Many applications require more compact and simpler femtosecond sources with a minimum number

of components. Intracavity prism pairs for dispersion Interferometric Autocorrelation 2 compensation typically limit the minimum size of femtosecond laser resonators. Alternative approaches have been investigated for replacing the prism pairs 0 by special cavity resonator designs incorporating −30 −20 −10 0 10 20 30 more compact angular dispersive element. For Delay, fs example, a prismatic output coupler [86], or simi- (b) larly only one prism [87], has supported pulses as short as 110 fs with a Ti:sapphire laser, or 200-fs Figure 12: Shortest pulses achieved with an intracavity SESAM device: (a) broad- pulses with a diode-pumped Nd:glass laser, respec- band low-finesse A-FPSA device used for sub-10-fs pulse generation and (b) interfer- tively. In both cases, the basic idea can be traced back ometric autocorrelation of 6.5-fs pulses from a Ti:sapphire laser. The shortest pulses to the prism dispersion compensation technique ever produced directly out of a laser without any further pulse compression techniques.

August 2007 IEEE LEOS NEWSLETTER 19 21leos04.qxd 8/29/07 4:56 PM Page 20

LiSAF, 5 mm, 3% Cr 3+

ROC ROC Pump II 10 cm Pump I 10 cm 40 cm R=10 cm

1.0 Fused Silica 0.8 Prism#1 50 cm 0.6 45 fs 0.4

0.2

Autocorrelation Fused Silica Prism#2 0.0 −100 0 100 % Time Delay, fs 2 Output Coupler

Figure 13: Diode-pumped Cr:LiSAF laser cavity setup that generated pulses as short as 45 fs with soliton mode-locking.

MQW modulator inside a Fabry–Perot structure, which we called 1.6 antiresonant Fabry–Perot Modulator (A-FPMod) [97]. We then Diode-Pumped Cr:LiSAF 1.4 actively mode-locked a diode-pumped Nd: YLF laser. 1.2 One advantage of quantum-well modulators compared to other modulators such as acoustooptic modulators or phase modulators is 1.0 that they also can act as saturable absorbers leading to passive 0.8 mode-locking with much shorter pulses. Combining the effects of 0.6 saturable absorption and absorption modulation within one single device, we have demonstrated the possibility to synchronize pas- 0.4 CW Output Power, W CW Output Power, sively mode-locked pulses to an external RF signal [97]. At higher 0.2 Slope eff. = 18% output powers we were limited by the increased saturation of the 0.0 active modulator. 0246810 Absorbed Pump Power, W An All-Solid-State Ultrafast Figure 14: 1.4-W CW output power from a diode-pumped Laser Technology: Passively Modelocked Cr:LiSAF laser. Diode-Pumped Solid-State Lasers In the last few years, we have seen first demonstrations of Fabry–Perot resonance, which tends to limit the available band- potentially practical ultrafast solid-state lasers. Our approach width of the device. In the future, chirped mirror designs that for practical or “real-world” ultrafast lasers is as follows: For incorporate saturable absorber layers could also potentially provide simplicity, reliability, and robustness, we only consider diode- both saturable absorption and negative dispersion, but with poten- pumped solid-state lasers with passive mode-locking or tially more bandwidth. Q-switching techniques, where we use SESAM’s to provide efficient pulse formation and stabilization. In addition, we do A-FPMod not want to rely on critical cavity alignment and therefore use We do not have to rely only on passive saturable absorption with fast saturable absorber mode-locking in the picosecond semiconductors. Multiple-quantum-well (MQW) modulators regime and soliton mode-locking in the femtosecond regime. based on the quantum-confined Stark effect [92]–[94] are promis- The general goal is to develop a compact, reliable, easy-to- ing as active modulation devices for solid-state lasers, sharing the use, “hands-off” all-solid-state ultrafast laser technology. same advantages of passive SESAM’s: they are compact, inexpen- sive, fast, and can cover a wide wavelength range from the visible Cr:LiSAF to the infrared. In addition, they only require a few volts of drive Diode-pumped broad-band lasers are of special interest for num- voltage or several hundred milliwatts of RF power. In general, how- ber of practical applications. Ti:sapphire is probably the best ever, semiconductor MQW modulators would normally introduce known of the ultrafast lasers, but must be pumped in the green excessive insertion losses inside a solid state laser cavity and would spectral region, were no high-power diode lasers yet exist. also saturate at relatively low intensities [95], [96]. We extended However, the fairly newly developed Cr:LiSAF family of crystals the antiresonarit Fabry–Perot principle by integrating an active (Cr:LiSAF [98], Cr:LiCAF [99], Cr:LiSCAF [100], and

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Cr:LiSGAF [101]) have fluorescence linewidths similar to Ti:sap- pumping femtosecond optical parametric oscillators, and ultrafast phire and can be pumped at wavelengths near 670 nm where spectroscopy. The absorption band of Nd:glasses at ≈800 nm commercial high-brightness high-power (i.e., <0.5 W) diode allows for diode pumping [113], resulting in a compact, wall-plug arrays are available. However, these crystals have a stronger ten- driven setup, which does not require water cooling. Typical dency for upperstate lifetime quenching [102] and suffer from Nd:glasses have a fluorescence bandwidth of 20–30 nm FWHM, lower thermal conductivity, resulting in nonideal performance supporting sub-100-fs pulse generation at a wavelength of ≈1.06 (limited average power) at relative low pump powers. μm. Initially, the shortest pulses from a (bulk) Nd:phosphate laser The output power of a diode-pumped Cr:LiSAF laser was [114], 88 fs, were produced by additive pulse mode-locking initially limited to <10 mW [103]–[105]. French et al. used (APM). High-finesse A-FPSA’s in various Nd:glass lasers [6], [54] an MQW SESAM inside a coupled cavity for RPM [103], have supported pulses as short as 130 fs for diode pumping and 90 [104] and inside the main cavity producing pulses as short as fs for Ti:sapphire pumping. 220 fs in 1994 [105]. However, their device introduced too much fixed losses. Shortly afterwards, we demonstrated [106], [107] significantly higher average output power of 140-mW CW and 50-mW mode-locked with pulses as short as 98 fs Nd:Glass using two 0.4-W high-brightness diode arrays, improved pump mode matching, and a low-loss, high-finesse A-FPSA. Within a year, we improved the pulse duration to 45 fs with a mode-locked average output power of 60 mW and later then 80 mW [71], [72], [81] (Fig. 13). Briefly afterwards, Tsuda et al. [75] used a low-finesse A-FPSA design (they termed it 60 fs “SBR,” see Section III-E) inside a diode-pumped Cr:LiSAF laser and demonstrated 100-fs pulses with 11-mW average output

power. Recently, they improved this result with a MOPA diode Autocorrelation laser, which provides a near-diffraction-limited 0.5-W pump, achieving 70-fs pulses with 100-mW output power [108]. Significant efforts by many groups around the world are directed toward shorter pulses and more output power. Presently, the shortest pulses of ≈20 fs have been obtained with KLM Cr:LiSAF [109], [110] with an average output power in the regime of 1 mW. Higher output power has been achieved −100 0 100 only at the expense of longer pulses. For example, Time Delay, fs 40-fs pulses with an average output power of 70 mW have been (a) recently obtained [110] in a KLM Cr:LiSAF system. The limited average output power of femtosecond diode- pumped Cr:LiSAF lasers is their main drawback in comparison to Ti:sapphire lasers. Novel diode pumping techniques can address (Dotted Line) Fluorescence Spectrum this problem, and we have achieved 400 mW [111] and more recently as much as 1.4-W CW output power from a diode- pumped Cr:LiSAF laser (Fig. 14) [82], [112]. We have passively mode-locked this laser with a low-finesse A-FPSA [82] and obtained pulses as short as 50 fs with an average output power of 21.6 nm 340 mW. Higher average output power of 500 mW was achieved with 110-fs pulses. These are the highest average power levels ever achieved to date with femtosecond diode-pumped solid-state lasers. Furthermore, the results show no fundamental limitations for fur- Optical Spectrum (Solid Line) ther improvements in both shorter pulses and higher output powers. In contrast to Kerr-lens mode-locking, soliton mode-locking with SESAM’s has the advantage that the laser cavity mode is decoupled from mode-locking dynamics. This is important in our case, because the cavity design can be more easily optimized for 1.02 1.04 1.06 1.08 1.10 1.12 high-power without having to take Kerr-lensing effects into Wavelength, μm account as well. (b)

Nd:glass Figure 15: Soliton mode-locked Nd:glass (fluorophosphate LG 810) Diode-pumped femtosecond Nd:glass lasers offer a cost-effective laser using a low-finesse A-FPSA. (a) noncollinear autocorrela- and compact alternative to Ti:sapphire lasers operated near 1.06 tion, (b) optical spectrum (solid line) and fluorescence spectrum of μm, with applications such as seeding high-power amplifiers, Nd:glass laser (dotted line).

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Recently, we have demonstrated 60-fs pulses [Fig. 15(a)] We have also extended the designs of SESAM’s to longer wave- with an average output power of ≈80 mW from an optimized length such as 1.3 μm [80] and 1.5 μm [123]. To benefit from the diode-pumped Nd:glass (Nd:fluorophoshate, LG-810, 3% Nd) good quality of AlAs–GaAs Bragg mirrors, we chose to grow the laser using a low-finesse A-FPSA with a larger modulation 1.3-μm saturable absorber layer on a GaAs substrate. To achieve depth of ≈1%, but at the expense of higher intracavity loss (at saturable absorption at 1.3 μm, however, the indium concentration low intensities) of 2% [63]. We measured a bitemporal in the InGaAs absorber material must be increased to approxi- impulse response of the A-FPSA with a fast recovery time of mately 40%, which results in a significant lattice mismatch to the 200 fs and a slow recovery time of 25 ps. The pulse energy den- GaAs substrate. This lattice mismatch reduces the surface quality, sity incident on the saturable absorber was typically a few times resulting in higher insertion losses, and reduced laser power. In above the saturation fluence of ≈100 μJ/cm2, limited by the addition, these devices exhibit more bulk defects than a 1-μm onset of multiple pulsing instabilities. The laser cavity is sim- SESAM, also increasing insertion loss. This is even more enhanced ilar to the diode-pumped Cr:LiSAF laser shown in for the 1.5-μm saturable absorber. In addition to the short picosec- Fig. 13. The mode-locked spectrum of the 60-fs pulses is 21.6- ond recovery time of the saturable absorber, the low-temperature nm wide (FWHM) and spreads over most of the available MBE growth partially relieves the lattice mismatch, resulting in Nd:glass fluorescence bandwidth [Fig. 15(b)]. The mode-lock- improved optical quality of the absorber layer. ing is self-starting and is well-discribed by our soliton mode- locking model. Passively Q-Switched Microchip Lasers In a Q-switched laser, the pulse duration generally decreases with Yb:YAG shorter cavities and with higher pump power (increased small-sig- Yb:YAG is interesting as a high-power diode-pumped laser source nal gain). For solid-state lasers, typically Q-switched pulsewidths due to its small quantum defect, resulting in a potentially very effi- range from nanoseconds to microseconds. Pulsewidths less than a cient laser with low thermal loading, and its wide absorption band nanosecond (“ultrafast” by Q-switching standards) have recently at 940 nm [115], [116]. Additionally, Yb:YAG has a broad emis- been achieved by using diode-pumping and very short cavity sion spectrum supporting tunability [117], [118] and femtosecond lengths. Extremely short cavity length, typically less than 1 mm, pulse generation in the few 100-fs regime. Using a low-finesse A- allows for single-frequency Q-switched operation with pulsewidths FPSA, we have demonstrated a passively mode-locked Yb:YAG well below a nanosecond. These Q-switched “microchip” lasers are laser, generating stable and self-starting pulses as short as 540 fs compact and simple solid-state lasers which can provide high peak with typical average output powers of 150 mW [77]. Yb:YAG has power with a diffraction limited output beam. Pulse durations of never been passively mode-locked before, because the upper state 337 ps and more recently 218 ps have been demonstrated with a lifetime of the laser is relatively long, ≈1 ms. Previously, only active passively Q-switched microchip laser consisting of a Nd:YAG crys- mode-locking in Yb:YAG has been demonstrated with a pulse tal bonded to a thin piece of Cr4+:YAG [126], [127]. With a duration of 80 ps[119]. monolithic Cr4+ co-doped Nd:YAG laser, pulses of 290 ps have Recently, we further optimized the pulse duration with a high- been obtained [128]. Using active Q-switching, pulses as short as er modulation depth of ≈1% for the low-finesse A-FPSA and 115 ps have been reported [129]. obtained pulses as short as 340 fs (Fig. 16) at a center wavelength From the discussion in Section II, we see that the regime of pure of 1.033 μm and with a spectral width of 3.2-nm FWHM. The passive Q-switching requires that we reduce the cavity length sub- measured bitemporal impulse response showed a fast component of stantially (4). Following this, we have passively Q-switched a diode- 460 fs and a slow recovery time of ≈7 ps. The cavity setup is oth- pumped Nd:LSB and Nd:YVO4 microchip laser with an A-FPSA erwise the same as in [77]. at a center wavelength ≈1.06 μm and achieved pulses as short as 180 ps [65] and 56 ps [66] (Fig. 17), respectively. By changing the Nd:YAG, YLF, LSB, and YVO4 design parameters of the saturable absorber, such as the top reflec- In the picosecond pulse regime, we 0use SESAM’s as fast saturable tor, we can vary the pulsewidth from picoseconds to nanoseconds; absorbers, controlling the recovery time by low-temperature MBE grown semiconductors (Fig. 5). A more detailed review with regard to Nd:YAG and Nd:YLF using a high-finesse A-FPSA is given in [7]. The results for the various laser crystals are summa- Yb:YAG Measured Ideal Sech2 rized in Table I. With picosecond lasers, we achieve significantly shorter pulses if we use the gain material at the end of a linear cavity. This “gain-at-the-end” leads to enhanced special hole burning 340 fs (SHB) that effectively inhomogenously broadens the gain bandwidth, flattening the saturated gain profile and allowing for a larg- Autocorrelation er lasing bandwidth [120]–[122], resulting in shorter pulses. However, we typically observe a time-bandwidth product that is between 1.2–2 times as large as for ideal transform-limited −2 −10 1 2 Gaussian or sech2 pulse. This large time-bandwidth product is Time Delay, ps mainly due to the flat gain produced by SHB, which produces a non-Gaussian pulse shape, and is not due to a chirp on the pulse Figure 16: Soliton mode-locked Yb:YAG laser using a low-finesse that could be compensated externally [122]. A-FPSA.

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by changing the pump power, we can vary the pulse repetition rate from the kilohertz to megahertz regime. Because the optical 1.0 penetration depth into our typical intracavity SESAM devices is extremely short (< 1μm) [83], we can maintain a very short laser cavity, allowing for minimum 0.5 56 ps pulsewidths. To date, the 56-ps pulsewidths are the shortest ever produced from a Q-switched solid-state laser. Sampling Oscilloscope Our approach can also be extended to Nd:YVO4 Microchip Laser (3% Doped) 0.0 other wavelengths using different semi- −200 0 200 Delay, ps conductor materials. Recently, we have A-FPSA demonstrated a passively Q-switched 10% μ Output Coupler 1.34- m diode-pumped Nd:YVO4- Output microchip (200-μm thick) laser. We @ 1064 nm achieved single frequency, 230-ps pulses with 100-nJ pulse energy at a repetition rate of 50 kHz, resulting in a peak power Diode Pump Laser of about 450 W at an average power of 5 @ 808 nm mW [130], [131]. As a passive Q-switching device, we used an MOCVD grown In-GaAsP–InP A-FPSA. Shorter pulses Dichroic Beamsplitter are expected with further improvements of HT @ 808 nm the A-FPSA. This is the first demonstra- 200 μm HR @ 1064 nm Cavity Length tion to our knowledge of a passively Q-

switched microchip laser at a wavelength Figure 17: Passively Q-switched diode-pumped Nd:YVO4 microchip laser producing pulses as short longer than ≈1 μm. In contrast to as 56 ps. The shortest pulses ever produced from a Q-switched solid-state laser. Cr4+:YAG saturable absorbers, our saturable absorber devices can be adapted to longer wavelengths using Soliton mode-locking provides us a new, useful model of different semiconductor materials. Recently, we have extended this femtosecond pulse generation. By showing that we do not approach to passively Q-switched Er:Yb:glass microchip lasers at a need a saturable absorber with a response as fast as the wavelength ≈1.5 μm [131], which is important for sensing and pulsewidth, we have demonstrated that pulses as short as LIDAR application where “eye-safe” wavelengths are required. ≈6.5 fs can be supported with SESAM’s, and we have a new mechanism to assist in obtaining pulses below the 10-fs level Conclusion directly from the laser. During the last six years, we observed a tremendous progress in We also expect significant continued progress in the next ultrashort pulsed laser sources. Compact “real-world” picosecond few years to make these lasers more compact and simpler. Bulk and femtosecond laser systems are now a reality. This review has scale optical devices will be supplanted in some applications by only considered free-space laser systems, but it is important to hybrid, quasimonolithic structures (e.g., diode-pumped Q- note that there also has been tremendous progress in mode- switched microchip lasers). In addition, many applications such locked fiber lasers, and there are also similar applications of as material processing and surgery require higher average pow- SESAM’s in this area. ers (>10-W CW) and pulse energies (>1 mJ). Novel approach- Rapid progress in ultrashort pulse generation has been based on es are still needed to make such pulsed laser systems more com- novel broad-band solid-state laser materials, and novel designs of pact. One of our key remaining research challenges will be scal- saturable absorption and dispersion compensation. Early attempts ing of the SESAM devices to these higher pulse energies and to passively mode-lock solid-state lasers with long upper state life- average powers. times consistently resulted in Q-switched mode-locking. The In general, the capability to control both the linear and non- main reason for this was that the absorber response time was typ- linear optical properties beyond the “natural” material properties ically in the range of ≈1 ns, which reduced the saturation intensi- has turned out to be extremely successful, and we can only expect ty to the point that self-Q-switching could not be prevented. more progress in this direction. This design trend is also reflected Stable mode-locking with intracavity saturable absorbers has been in other fields, for example, quasiphase matching in nonlinear achieved by varying the parameters such as response time and optics, bandgap engineering in semiconductor technology, and absorption cross section through special growth and design tech- sliding filters in optical communications. In the future, similar niques. With SESAM’s, we can benefit from control of both mate- new developments would be desirable for solid state laser materi- rial and device parameters to determine the performance of the sat- als (bandgap engineering for solid-state crystals), because ultrafast urable absorber. We can view these as basic optoelectronic devices laser sources will ultimately become limited by the available mate- for ultrafast laser systems. rial characteristics.

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Ferguson, Chai, “Investigation of the laser properties of Cr3+:LiSrGaF6,” “Passively mode-locked diode-pumped solid-state lasers using an IEEE J. Quantum Electron., vol. 28, pp. 2612–2618, 1992. antiresonant Fabry–Perot saturable absorber,” Opt. Lett., vol. 18, pp. [102] M. Stalder, M. Bass, and B. H. T. Chai, “Thermal quenching of fluorescence 640–642, 1993. in chromium-doped fluoride laser crystals,” J. Opt. Soc. Amer. B, vol. 9, pp. [125] Prof. R. Wallenstein, University of Kaiserslautern, Germany, private com- 2271– 2273, 1992. munication, 1996. [103] P. M. W. French, R. Mellish, J. R. Taylor, P. J. Delfyett, and L. T. Florez, [126] J. J. Zayhowski and C. Dill III, “Diode-pumped passively Q-switched “All-solid-state diode-pumped modelocked Cr:LiSAF laser,” Electron. Lett., picosecond microchip lasers,” Opt. Lett., vol. 19, pp. 1427–1429, 1994. vol. 29, pp. 1262–1263, 1993. [127] J. J. Zayhowski, J. Ochoa, and C. Dill, III, “UV generation with passively [104] P. M. W. French, R. Mellish, and J. R. Taylor, “Modelocked all- Q-switched picosecond microchip lasers,” presented at CLEO 1995, p. 139, solid-state diode-pumped Cr:LiSAF laser,” Opt. Lett. vol. 18, pp. paper CTuM2. 1934–1936, 1993. [128] P. Wang, S.-H. Zhou, K. K. Lee, and Y. C. Chen, “Picosecond laser [105] P. M. Mellish, P. M. W. French, J. R. Taylor, P. J. Delfyett, and L. T. Florez, pulse generation in a monolithic self-Q-switched solid-state laser,” “All-sond-state femtosecond diode-pumped Cr:LiSAF laser,” Electron. Lett., Opt. Commun., vol. 114, pp. 439–441, 1995. vol. 30, pp. 223–224, 1994. [129] J. J. Zayhowski and C. Dill III, “Coupled cavity electro-optical- [106] D. Kopf, K. J. Weingarten, L. Brovelli, M. Kamp, and U. Keller, “Diode- ly Q-switched Nd:YVO4 microchip lasers,” Opt. Lett., vol. 20, pumped sub-100-fs passively mode-locked Cr:LiSAF using an A-FPSA,” pp. 716–718, 1995. presented at CLEO 1994, paper CPD22. [130] R. Fluck, B. Braun, U. Keller, E. Gini, and H. Melchior, “Passively Q- [107] D. Kopf, K. J. Weingarten, L. Brovelli, M. Kamp, and U. Keller, “Diode- switched 1.34 μm Nd:YVO4 microchip laser,” Advanced Solid-State pumped 100-fs passively mode-locked Cr:LiSAF using an A-FPSA,” Opt. Lasers 1997, paper WD5. Lett., vol. 19, pp. 2143–2145, 1994. [131] R. Fluck, B. Braun, U. Keller, E. Gini, and H. Melchior, “Passively [108] S. Tsuda, W. H. Knox, and S. T. Cundiff, “High efficiency diode pumping Q-switched microchip lasers at 1.3 μm and 1.5 μm,” presented at of a saturable Bragg reflector-mode-locked Cr:LiSAF femtosecond laser,” CLEO 1997.

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“Discovering” the SESAM Commentary by Ursula Keller

Passive mode-locking with an intracavity saturable absorber was ded into a mirror structure. In the simplest case this is a Bragg first demonstrated in 1966 [1], six years after the first laser was mirror (which was first demonstrated with a single quantum well discovered. However, passive modelocking in solid-state lasers absorber in 1995 [6]). However, the SESAM concept is much suffered from a fundamental problem: the so-called Q-switched broader and can be extended to any other mirror structure. A more modelocking behavior, where an overlying large Q-switched recent review of SESAM design concepts is given in Ref. 7. pulse modulated the train of clean mode-locked pulses. This meant that the laser would turn itself off at regular intervals. The References underlying mode-locked pulses could only be used in special cir- [1] A. J. D. Maria, D. A. Stetser, and H. Heynau, “Self mode-locking of lasers cumstances and with very limited applications. The semiconduc- with saturable absorbers,” Appl. Phys. Lett., vol. 8, pp. 174-176, 1966. tor saturable absorber mirror (SESAM) solved this problem more [2] U. Keller, D. A. B. Miller, G. D. Boyd, T. H. Chiu, J. F. Ferguson, and M. T. Asom, “Solid-state low-loss intracavity saturable absorber for Nd:YLF than 25 years later in 1992 for the first time for diode-pumped lasers: an antiresonant semiconductor Fabry-Perot saturable absorber,” Opt. solid-state lasers [2]. Lett., vol. 17, pp. 505-507, 1992. I invented and led the development of SESAMs, which are a [3] U. Keller, “Recent developments in compact ultrafast lasers,” Nature, vol. novel family of optical devices that allow for very simple, self- 424, pp. 831-838, 14.08.2003 2003. starting, passive modelocking of ultrafast solid-state and semi- [4] U. Keller, “Ultrafast solid-state lasers,” Progress in Optics, vol. 46, pp. 1- conductor lasers. I invented the first member of the SESAM fam- 115, April 2004. ily in 1992 when I was still at Bell Labs in New Jersey, USA [2]. [5] S. V. Marchese, S. Hashimoto, C. R. E. Bär, M. S. Ruosch, R. Grange, M. Golling, T. Südmeyer, U. Keller, G. Lépine, G. Gingras, B. Witzel, This breakthrough device allowed the first demonstration of a “Passively modelocked thin disk laser reach 10 µJ pulse energy at mega- passively mode-locked neodymium:YLF laser - without Q- hertz repetition rate and drive high field physics experiments”, CLEO switching. After moving to ETH Zurich in Switzerland, we Europe 2007, Munich, Germany, June 17-22, 2007 developed the theoretical underpinnings of the performance of [6] L. R. Brovelli, I. D. Jung, D. Kopf, M. Kamp, M. Moser, F. X. Kärtner, and U. SESAMs in solid-state lasers, worked out design guidelines for Keller, “Self-starting soliton modelocked Ti:sapphire laser using a thin semi- application to practical laser systems, and took this know-how to conductor saturable absorber,” Electron. Lett., vol. 31, pp. 287-289, 1995. demonstrate unprecedented laser performance improvements in [7] G. J. Spühler, R. Grange, L. Krainer, M. Haiml, V. Liverini, M. Golling, S. Schön, K. J. Weingarten, and U. Keller, “Semiconductor saturable absorber several key directions: shortest pulse widths (in the 5-fs regime mirror structures with low saturation fluence,” Appl. Phys. B, vol. 81, pp. with only about two optical cycles), highest average and peak 27-32, July 2005. power from a passively mode-locked laser (nanojoules extended to more than 10 microjoules), and highest pulse repetition rate to (~1 GHz extend to >160 GHz) [3]. Today, SESAM modelocked Biography: Ursula Keller solid-state lasers fulfill the requirements for industrial applica- Ursula Keller joined ETH as an associate professor in 1993 and has tions and are being used for many different applications (see for been a full professor of physics since 1997. She received the Ph.D. example overview Table 2 in Ref [4]). in Applied Physics from Stanford University in 1989 and the With the help of Dr. Thomas Südmeyer (senior postdoc in my Physics “Diplom” from ETH in 1984. She was a Member of group) and Sergio Marchese (graduate student), we were able to Technical Staff (MTS) at AT&T Bell Laboratories in New Jersey increase the pulse energy of passively modelocked solid-state from 1989 to 1993. Her research interests are exploring and push- lasers by more than four orders of magnitude: We demonstrated ing the frontiers in ultrafast science and technology: ultrafast solid- more than 10 µJ directly out of a SESAM modelocked diode- state and semiconductor lasers, ultrashort pulse generation in the pumped solid-state laser oscillator [5], and we believe that we can one to two optical cycle regime, frequency comb generation and scale this concept even further, well above the 100 µJ regime. stabilization, reliable and functional instrumentation for extreme This pulse energy will enable material processing and high field ultraviolet (EUV) to X-ray generation, attosecond experiments laser physics at very high pulse repetition rates (1 – 100 MHz). using high harmonic generation, and attosecond science. She has Previously, such pulse energies were only obtained with mode- published more than 260 peer-reviewed journal papers and 11 book locked oscillators, followed by one or several amplifier stages at chapters and she holds or has applied for 18 patents. She was a much lower pulse repetition rates of ~ 1 kHz. A higher pulse rep- “Visiting Miller Professor” at UC Berkeley in 2006 and a visiting etition rate increases the possible scan rate in material processing professor at the Lund Institute of Technologies in 2001. She (e.g. marking, waveguide writing etc.) and increases the signal- received the Philip Morris Research Award in 2005, the first-placed to-noise ratio in high field physics experiments. This feature will award of the Berthold Leibinger Innovation Prize in 2004, and the enable many new measurements that were not previously possible Carl Zeiss Research Award in 1998. She was the “2006 Ångström because space charge effects smeared out the important dynamics. lecturer” supported by the Royal Swedish Academy of Sciences and This JSTQE publication was the first longer review paper the LEOS Distinguished Lecturer for modelocked solid-state lasers describing SESAMs and also introduced the acronym which con- in 2000. The Thomson Citation Index highlighted her as the third- tains all relevant information: first it is based on a semiconductor place top-cited researcher during a decade (1991-1999) in the field saturable absorber material (which is ideally suited for this appli- of optoelectronics in 2000. She is an OSA Fellow and an elected for- cation) and second the semiconductor saturable absorber is embed- eign member of the Royal Swedish Academy of Sciences.

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Industry Research Highlights High-Power Vertical-Cavity Surface-Emitting Laser Pump Sources Jean-Francois Seurin, Guoyang Xu, James D. Wynn, Dennis Tishinin, Qing Wang, Viktor Khalfin, Alex Miglo, Prachi Pradhan, L. Arthur D’Asaro, and Chuni Ghosh

Abstract the disadvantages of low array reliability, an elliptical beam This work presents recent results on high-power, high-effi- profile, and poor wavelength stability. Furthermore, upward ciency two-dimensional vertical-cavity surface-emitting laser scaling of the output power requires complex and costly assem- (VCSEL) arrays emitting around 980 nm. More than 230 W bly of edge-emitting bars into stacks. In the case where colli- of continuous-wave (CW) power is demonstrated from a ~5 mating and/or wavelength stabilizing optics are required, the mm x 5 mm chip. In quasi-CW mode, smaller chips exhibit complexity of the assembly process increases further. 100 W output power, corresponding to more than 3.5 The vertical-cavity surface-emitting laser (VCSEL) technology kW/cm2 of power density. We show that many of the advan- presents an attractive alternative as a high-power (several hundred tages of low-power single VCSEL devices such as reliability, Watts) semiconductor laser source because it can be easily wavelength stability, low-divergence circular beam, and low- processed in 2D arrays to scale up the power {1}. Ironically, cost manufacturing are preserved for these high-power arrays. VCSELs’ rise to fame originated in “low-power” (sub-milliwatt) VCSELs thus offer an attractive alternative to the dominant applications during the mid-90s {2}. This success was mainly due edge-emitter technology for high-power pumping. to the lower manufacturing costs and higher reliability of VCSELs compared to edge-emitters. This work presents recent advances on Introduction high-power VCSEL 2-D arrays and shows that many key features Compact and robust high-power semiconductor lasers are of single VCSEL devices are preserved for these high-power arrays. needed in a variety of industrial, medical, and defense applica- First, we go over the design and fabrication details. We then tions, foremost among them the pumping of solid-state and present recent results: using a relatively small VCSEL 2-D array fiber lasers. Currently, the dominant technology is that of edge- (~0.22 cm2 area), we have demonstrated more than 230 W of emitting semiconductor lasers. However, this technology has continuous-wave (CW) output power, corresponding to more than 1 kW/cm2 power density. We have also demonstrated 100 W from quasi-CW P-Contact (QCW) small arrays (~0.028 P-AlGaAs DBR (R>99.9%) cm2 area), corresponding to Oxide Aperture more than 3.5 kW/cm2 ( 5~25µm) Epitaxial Growth power density. These arrays (~10µm-Thick) InGaAs Quantum Wells emit at around 980 nm. To the best of our knowledge, N-AlGaAs DBR these CW and QCW power N-GaAs Substrate levels represent record results (100~200µm-Thick) for 2-D VCSEL arrays. Finally, we go over some of N-Contact the main advantages of these

Light Output AR Coating (Si3N4) high-power VCSEL arrays in terms of reliability, and spectral and beam properties. Figure 1: Schematic cross-section (not to scale) of a bottom-emitting 980 nm vertical-cavity surface-emitting laser structure. The active region typically comprises 1~4 InGaAs quantum wells, embedded in a Structure design one-wavelength-thick AlGaAs cavity. The cavity is sandwiched between two distributed Bragg reflector stacks (DBRs) consisting of alternating high and low refractive index quarter-wavelength layers. The The design of high-power 2- reflectivity of these DBRs is in the range 99.5 - 99.9% for the output mirror and >99.9% for the back D VCSEL arrays derives from mirror. Current and/or mode confinement can be achieved by selective oxidation of an Aluminum-rich the design of efficient single- layer located near the active region. Light emission is through the transparent GaAs substrate. A Si3N4 devices. Our single device anti-reflection (AR) coating is deposited at the substrate/air interface. VCSEL structure is based on a selectively oxidized, bottom-emitting design as PRINCETON OPTRONICS, INC.,1 ELECTRONICS DRIVE, MERCERVILLE, NJ 08619 shown in Figure 1.

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The selective oxidation process {3} provides efficient in a record 51% conversion efficiency for bottom-emit- optical mode and injection current confinement and ting 980 nm single VCSEL devices, as shown in Figure 2. greatly improves the performance of the device since the This level of performance is comparable to that already oxidized material is electrically insulating and has a achieved for top-emitting 980 nm VCSELs {9}. lower refractive index than the semiconductor material {4}. The bottom-emitting configuration (also referred to as “junction- down”) is necessary for efficient heat-sinking and more uniform current injection {5}. 12 60 17µm-Diam Indeed, using such mounting configuration, CW, Room Temp. 51.2% we have previously achieved record 3 W out- 10 50 put power at room temperature under CW operation from single devices with large CE (%) ) apertures {6}. The devices used in large 8 40 % arrays have lower output powers (10- 100mW, depending on aperture size) with high conversion efficiency. By combining 6 30 1000~10 000 such devices in parallel in a two-dimensional array, very high powers can be achieved while maintaining high conver- 4 20 Power (mW), Voltage (V) sion efficiency. Power (mW) Conversion Efficiency ( The epitaxial structure is grown using Voltage (V) MOCVD on 3” GaAs n-doped (Silicon) sub- 2 10 strates. The structure consists of InGaAs quantum wells in a one-wavelength cavity, 0 0 sandwiched between Carbon-doped and 024681012 Silicon-doped AlGaAs distributed Bragg Current (mA) reflectors (DBRs). A thin, Aluminum-rich AlGaAs layer is placed at the bottom of the p-DBR near the active region for subsequent Figure 2: Room-temperature continuous-wave (CW) power, voltage, and conversion oxidation to form an optical and electrical efficiency of an electrically injected bottom-emitting 980 nm VCSEL device. The aperture. aperture is defined by selective oxidation and is 17 μm in diameter. The conversion The structure is optimized for high wall- efficiency reaches a maximum of 51.2% at 8 mA. plug efficiency in mainly two ways. First, the doping profiles in the DBRs are carefully designed to minimize optical absorption while maintaining Au-Plating Passivation and Isolation satisfactory electrical conduc- Layer (Si3N4) tivity {7}. Second, the reflec- Bonding-Pad Metals tivity of the output n-DBR is optimized to obtain the high- P-Metal Disks for Contact est slope efficiency while and Mesa Etching maintaining a reasonable Oxidation thru Exposed threshold current. Decreasing N-GaAs Substrate (Thinned and Polished) Mesa Side-Walls the output reflectivity will increase the slope efficiency Light Output but will also increase the threshold current. At some point heating at threshold will be excessive and the slope effi- Figure 3: Cross-section schematic of the processed VCSEL array. The GaAs substrate is thinned and polished to an optical finish, followed by the deposition of a quarter-wavelength Si N layer (AR ciency will cease to increase 3 4 coating) and patterned evaporated metals to form the n-contacts and emission windows. Individual (and even start to decrease) devices are defined by reactive-ion etching (RIE) of individual mesas followed by selective oxidation with further reduction of the to form electrically conducting apertures. The p-contact disks serve as the RIE mask. The epitaxial output mirror reflectivity {8}. surface is then passivated with a Si3N4 layer. Windows are then opened by patterned etching of the Implementation of these nitride, followed by e-beam evaporation of Ti/Pt/Au metals to form the bonding pad. Both the n-met- design optimizations resulted als and bonding pad are electro-plated with Gold to improve current distribution across the array.

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Array fabrication application the arrays can be linear (1D), rectangular or Processing of the epitaxial material into 2-D VCSEL arrays square (2D). Furthermore, since the position of the indi- follows the same standard, well-establish processing tech- vidual elements in a VCSEL array is defined by photoli- niques that have been used for selectively oxidized single thography, arbitrary design layouts of the elements with VCSEL devices for some time now (see {1} for example). A placement accuracy at the micron level are possible. cross-section schematic of the processed sample is shown After cleaving and sorting, individual arrays are sol- in Figure 3. Plating of the n- and p-contacts is required for dered onto metallized high-thermal-conductivity sub- uniform current distribution within the array. mounts such as diamond or BeO. Then, the chip-on-sub- We found that our selective oxidation process was mount can be packaged onto a micro-channel cooler to extremely uniform within an array and among arrays increase the heat removal capacity, especially for CW within the same sample. Thus, we believe the selective operation. Figure 4 shows a ~5 mm x 5 mm VCSEL array oxidation process is well suited for the production of chip fully packaged on a micro-channel-cooler. VCSEL arrays even larger than 5mm x 5mm. These arrays are tested at the wafer level (before cleav- Array results ing and separation) to check for performance and excessive Figure 5(a) shows the CW LIV characteristics of an array “dead pixels” for example. It is noteworthy that VCSEL packaged on a micro-channel-cooler similar to the one shown array fabrication is identical to the well-established, low- in Figure 4. This array has an emission area of ~0.22 cm2 cost silicon integrated-circuit planar processing. Since and was operated at a constant heat-sink temperature (15°C). VCSELs are grown, processed and tested while still in the A record 231W output power was reached with 320 A drive wafer form, there is significant economy of scale resulting current, limited by thermal roll-over. This corresponds to a from the ability to conduct parallel device processing, power density of 1 kW/cm2, similar to that achieved by CW whereby equipment utilization and yields are maximized high-power edge-emitter stacks. This array has a peak con- and set-up times and labor content are minimized. version efficiency >44%. Lower-power arrays (90 W maxi- Furthermore, the wafers can be diced into single devices mum power) have been recently fabricated with conversion or arrays of different shapes and sizes. Depending on the efficiencies at 51%, identical to that of single devices. Smaller arrays were soldered on submounts and tested in QCW mode. The chip-on-submounts were not packaged on a micro-channel- Voltage Monitor Wires cooler. Instead, they were tested on a TEC-controlled stage maintained at 20°C. Figure 5(b) shows the LIV characteristics of a 0.028 cm2 array. Pulse-width and duty-factor were 100μs and 0.3%, respectively. Maximum power reached was 100 W, limited by the QCW current driver (125A), although some early signs of rollover are evident. This corresponds to a power density of 3.5 kW/cm2, also similar to what is achieved with edge-emitter stacks. Higher power levels can be achieved by connecting several chips in series. We also examined the spectral and beam properties of the array of Figure 5(a) at a 100W CW output power. Figure 6(a) shows the emission spectrum of this array. The spectral full-width half-maximum (FWHM) is only 0.8 nm, about one- Wirebonds Submount VCSEL Array Temperature fifth that of edge-emitter bars or Sensor stacks (typically in the 3 to 5 nm range). We also measured the wavelength shift as a function of the Figure 4: Photograph of a fully assembled 2-D VCSEL array on a diamond submount and heat-sink temperature to be 0.065 micro-channel-cooler for CW testing. The VCSEL chip size is approximately 5 mm x 5 mm. nm/K, identical to the value for sin-

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250 5 100

200 4 80 100µsec/0.3%

150 3 60 Power (W) Power (W)

100 2 Voltage (V) 40

50 1 20

2 Array Area: 0.22 cm2 Array Area: 0.028 cm 0 0 0 0 40 80 120 160 200 240 280 320 0 20 40 60 80 100 120 Current (A) Current (A) (a) (b)

Figure 5: (a) CW output power and voltage of a 2-D VCSEL array assembled on a micro-channel-cooler. The VCSEL chip size is approximately 5 mm x 5 mm and is maintained at a constant heat-sink temperature of 15°C. A maximum power of 231 W at 320 A is reached, limited by thermal roll-over. This power level corresponds to a power density of 1 kW/cm2 for this array. (b) QCW output power of a smaller (~0.028cm2 area) 2-D VCSEL array. The pulse width and duty cycle are 100 μs and 0.3%, respectively. The array is tested on a TEC- controlled stage maintained at 20°C. The array reaches a maximum power of 100 W, limited by the current driver (125A max). This power level corresponds to a power density of 3.5kW/cm2.

2.5

2.0

1.5 W) μ FWHM~0.8nm

1.0 Signal (

0.5

0.0 962 964 966 968 970 972 974 Wavelength (nm) (a) (b)

Figure 6: (a) Emission spectrum and (b) far-field beam distribution at a 100 W CW output power (~120A) for the array tested in Figure 5(a). At 100W, the array lases around 969 nm and has a spectral full-width half-maximum of 0.8 nm. The output beam is circular, with a “quasi-top-hat” distribution, and a 1/e2 full-width divergence angle of 17°.

gle devices. This value is also one-fifth that of edge-emit- Figure 6(b) shows the intrinsic far-field beam profile of ters (typically 0.33 nm/K). Therefore, similarly to single the array. The beam is circular, with a quasi-top-hat pro- devices, high-power VCSEL arrays benefit from an intrin- file. The 1/e2 full-width divergence angle is 17°. Since sically narrow spectrum and stable emission wavelength. such beam characteristics can be achieved without any This is useful for many pumping applications where the optics, VCSEL arrays present a cost-effective solution for medium has a narrow absorption band. end-pumping applications.

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VCSEL reliability Acknowledgements In terms of reliability, VCSELs have an inherent advan- The authors are grateful for the support from the tage over edge-emitters because they are not subject to DARPA Super High Efficiency Diode Source program catastrophic optical damage (COD). For edge-emitters, (SHEDS). this failure mechanism is very sensitive to the quality of the emission facet coating as well as junction tempera- References ture. This problem of sensitivity to surface conditions for [1] M. Grabherr et al, “Bottom-emitting VCSEL’s for high-CW opti- edge-emitters is not present in VCSELs because the gain cal output power,” IEEE Photon. Technol. Lett., Vol. 10, No. 8, region is embedded in the epitaxial-structure and does not interact with the emission surface, and because the pp. 1061-1063 (August 1998). power densities involved are much smaller. Over the [2] K. S. Giboney et al, “The ideal light source for datanets,” IEEE years, several reliability studies for VCSELs have yielded Spectrum, Vol. 35, No. 2, pp. 43-53 (February 1998). failures-in-time (FIT) rates (number of failures in one bil- [3] J. M. Dallesasse et al, “Hydrolyzation oxidation of AlxGa1-xAs- lion device-hours) on the order of 10 or less {10}, where- AlAs-GaAs quantum well heterostructures and superlattices,” as FIT rates for telecom-grade edge-emitters is on the order of 500 {11}. The failure rate for industry-grade Appl. Phys. Lett., Vol. 57, No. 26, pp. 2844-2846 (December high-power edge-emitter bars or stacks is generally worse. 1990). This VCSEL reliability advantage is very important for [4] D. L. Huffaker et al, “Native-oxide defined ring contact for low laser systems, where the end-of-life and field failures are threshold vertical-cavity lasers,” Appl. Phys. Lett., Vol. 65, No. overwhelmingly dominated by pump-laser failure. This 1, pp. 97-99 (July 1994). also means that VCSELs can be reliably operated at higher temperatures {12}. This advantage is significant [5] R. Michalzik et al, “High-power VCSELs: modeling and experi- because the requirements for refrigeration become much mental characterization,” Proc. SPIE, Vol. 3286, pp. 206-219 less for VCSELs, resulting in a more compact laser system (April 1998). with higher overall efficiency. [6] L. A. D’Asaro et al, “High-power, high efficiency VCSELs pursue Conclusions the goal,” Photonics Spectra, pp. 64-66 (February 2005). We have shown that it is possible to use VCSEL tech- [7] M. G. Peters et al, “Growth of beryllium doped AlxGa1- nology to make compact high-power pump sources. xAs/GaAs mirrors for vertical-cavity surface-emitting lasers,” J. Record CW (230 W) and QCW (100 W) power levels Vac. Sci. Technol. B, Vol. 12, No. 6, pp. 3075-3083 (Nov/Dec have been demonstrated from 2-D VCSEL arrays. 1994). Moreover, power density levels are comparable to those of edge-emitter stacks. [8] G. M. Yang et al, “Influence of mirror reflectivity on laser per- It was also shown that many of the advantages on formance of very-low-threshold vertical-cavity surface-emitting which low-power single VCSEL devices built their success lasers,” IEEE Photon. Technol. Lett., Vol. 7, No. 11, pp. 1228- are preserved for these high-power VCSEL arrays. These 1230 (November 1995). advantages include low manufacturing costs, spectral sta- [9] K. L. Lear et al, “Selectively oxidized vertical cavity surface emit- bility and beam quality. Although the conversion efficiency of VCSELs has improved significantly in recent ting lasers with 50% power conversion efficiency,” Electron. years (~51%), it still lags a bit behind that of edge-emit- Lett., Vol. 31, No. 3, pp. 208-209 (February 1995). ters (55~60% for commercial products). Still, since [10] J. A. Tatum et al, “Commercialization of Honeywell’s VCSEL VCSELs can operate reliably at high temperature, the Technology,” Proc. SPIE, Vol. 3946, pp. 2-13 (May 2000). overall system efficiency could be higher using VCSELs [11] H.-U. Pfeiffer et al, “Reliability of 980 nm pump lasers for sub- since a refrigeration apparatus would not be needed. Therefore, because of their significant and unique marine, long-haul terrestrial, and low cost metro applications,” advantages in terms of costs, reliability, and performance Optical Fiber Communication Conference and Exhibit, OFC VCSELs could become the next technology of choice for 2002, pp. 483-484 (March 2002). compact and efficient high-power semiconductor laser [12] R. A. Morgan et al, “200∞C, 96-nm wavelength range, continu- sources. In the near future, we plan to increase the VCSEL ous-wave lasing from unbonded GaAs MOVPE-grown vertical array CW and QCW power densities to 2 kW/cm2 and 6 kW/cm2, respectively. Work to improve the conversion cavity surface-emitting lasers,” IEEE Photon. Technol. Lett., Vol. efficiency is also ongoing. 7, No. 5, pp. 441-443 (May 1995).

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Industry Research Highlights High Power Laser Diodes: From Telecom to Industrial Applications Norbert Lichtenstein

Abstract We present an overview of latest developments in high-power laser diode devices for G08 (2004) industrial applications. We review several 1600 P-Side up Mounted CW-Operation @ 25°C devices enabling display applications as well 1400 as material processing. G07 (2002) 1200

Introduction 1000 High-power laser diodes have seen tremendous G06 (2000) development in the last two decades, fueled by 800 the demand for highly reliable pump lasers for G05 (1999) use in erbium doped fiber amplifiers (EDFAs) 600 in telecom applications. From the first field 400 G03 (1996) deployment of 980-nm narrow stripe laser diodes between Sacramento and Chicago in 200 Ex-Facet Light Output Power (mW) 1993 using Bookham’s first generation of 0 pump lasers, to the latest evolution of a highly reliable module delivering 750 mW of fiber- −200 coupled single-spatial mode output power, a 0 500 1000 1500 2000 2500 roughly ten-fold increase in the available out- Injected Current (mA) put power has been achieved [1]. The ex-facet output power characteristics from various chip Figure 1: Ex-facet output power characteristics of single spatial mode pump generations are depicted in Figure 1. laser generations.

330

310

290

270

250 Current (mA)

230

210

190 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Time (Years)

Figure 2: Accelerated aging test under stress conditions demonstrating a low failure rate of 37 FIT.

BOOKHAM SWITZERLAND AG, BINZSTRASSE 17, 8045 ZURICH, SWITZERLAND

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High reliability has been maintained using key technologies such Laser Diode Low Reflectivity Front Facet Fiber Bragg Grating as the AlGa(In)As-based single (Reflectivity < 10%) quantum well (SQW) material system providing best electrical, optical and thermal properties, E2 facet passivation to effectively prevent catastrophic optical mirror damage, a temperature insensitive ridge- High Reflectivity Chip Grating Distance (1-2m) waveguide design for both electri- Rear Facet cal and optical confinement, and the use of high-temperature AuSn Figure 3: Wavelength stabilized laser diode using an external fiber Bragg grating. solder technology for high power operation. The reliability of such devices is illustrated in Figure 2 by a 16-year 0 long-term reliability test under 980.8 accelerated conditions (150- 980.6 250 mW, 30-75°C case tempera- −10 ture). A low failure rate of 37 FIT 980.4 (failures in 109 cumulative operating hours) for the 130 mW ex- 980.2 facet operating condition of these −20 Wavelength (nm) devices has been obtained. 980.0 0 20406080100 Today, Bookham is offering Temperature (°C) these key technologies in a com- −30 plete portfolio of broad area single

Optical Power (dB) 35 nm emitters, narrow stripe single emitters, and laser diode bars in the wavelength range between 780 −40 and 1060 nm. T=80°C In this article, an overview of − ° T= 10 C our latest developments on laser −50 960 970 980 990 1000 1010 1020 diode components for use in industrial applications is given. Wavelength (nm) Industrial Applications Figure 4: Temperature range for wavelength stabilized operation. Wavelength stabilization: From EDFA to Display Currently significant industrial development 400 0.5 work is focusing on light sources for display and projection, which may become the next killer 350 application. The field of deployment for such 0.4 300 devices might range from high-end cinema pro-

) jection with rear-projection TV to hand-held 250 0.3 % projectors to be integrated in mobile phones and 200 notebooks. Today, arc lamps are predominantly used for illumination with red, green, and blue 0.2 150 (RGB) light generated by color wheels, for RMS Noise ( 100 example. While the lack of efficiency and the Optical Power Ex-Fiber 0.1 short lifetime of arc lamps can be managed in 50 high-end systems, consumer products require 0 0.0 improved performance. High power LEDs pro- 0 100 200 300 400 500 600 700 800 vide large benefits in terms of efficiency, com- Laser Forward Current (mA) pactness, and reliability, and are increasingly being used in displays. Laser diodes might offer Figure 5: Ex-fiber output power and RMS noise from a narrow band wavelength the ultimate solution due to the superior color stabilized 976 nm narrow stripe laser diode. gamut of the resulting image. However, direct

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laser diodes in the required wavelengths around 620 nm, Material Processing: 440 nm, and (especially) 560 nm still suffer from difficul- Fiber Laser or Direct Diode? ties in achieving the required output power. Second har- CO2 gas lasers and diode pumped rod and disk lasers offer monic generation (SHG) from high power infrared laser beam quality superior to that of direct diode lasers today. diodes using non-linear materials like periodically poled However, most applications do not require zero order lithium niobate (PPLN) can overcome this problem. A mode beam profiles. In Figure 6 the required optical basic requirement here is a high brightness beam, prefer- power and beam quality for some typical applications of ably in the fundamental spatial mode like that emitted by high power laser systems is plotted. For comparison, the narrow-stripe laser diodes. To obtain a narrow spectrum beam parameter product for various fibers with core and the required coherence length of a few centimeters, diameters between 50 μm and 1500 μm with a numeri- wavelength stabilization of the laser diodes is required. cal aperture (NA) of 0.22 is overlaid in the graph. Today, Stabilization using a narrow bandwidth external fiber the required output powers between tens of Watts and Bragg grating (FBG) fiber as shown in Figure 3 has origi- several kilowatts are achieved from both fiber lasers as nally been developed for stable operation of EDFAs. The well as collimated laser diodes. This level has been feedback provided by the grating locks the laser to wave- enabled by the continuous improvement of power and lengths in a narrow band around the FBG center (Bragg) brightness of the pump sources. In the following section, wavelength if the spectral detuning from the gain maximum is not excessive. In a typical configuration, the FBG has a maximum reflectivity of a 1000 few percent and is positioned at a dis- Hardening tance of 1-2 m from the front facet of Polymer Welding Brazing 1500um,0.22 NA the laser chip [3]. This forces the Cladding laser to run for all injection currents 100 600um,0.22 NA Soldering Welding, Sintering and heat-sink temperatures in the Deep-Penetration 200um,0.22 NA coherence-collapse (multi-mode) Welding Non-Metal state of emission [4]. This is the pre- Printing Cutting ferred mode of operation for pump- 10 50um,0.22 NA Metal Cutting

ing applications since low-frequency (mm-Mrad) noise (sub-MHz) is strongly sup-

pressed if several longitudinal laser Beam Parameter Product Marking modes are oscillating simultaneously. 1 Drilling Using appropriate FBG designs, wavelength stabilization to After P. Loosen, Fraunhofer Inst. Fuer Lasertechnik, Aachen <0.01 nm/K over a temperature 1 10 100 1000 10000 range of 90°C can be achieved, while Laser Output Power (W) the gain peak of the laser diode shifts at 0.3 nm/K (Figure 4). To obtain a coherence length of Figure 6: Comparison of typical beam quality and power requirements for laser applications several cm as required for effective in the industry. SHG, FBGs with very narrow bandwidth have been used. In Figure 5 the light output characteristic for a narrow band CP-Active Fiber FBG configuration is plotted show- Multi-Mode Pumps ing a kink-free pattern related to stable locking over the whole oper- Filter ating range. Furthermore, an RMS noise value (DC to 2 MHz) of less than 0.1% has been achieved. Measurements of the spectral width Fiber Laser Single Mode Pump reveal <25 pm FWHM line width Output Optic “Seed Laser”~1064nm making this device ideally suitable for second harmonic generation. While the initial wavelength obtained from SHG is 488 nm, the whole range from blue, green to yel- low can be covered. Figure 7: Schematic fiber laser in master-oscillator (seed) power-amplifier configuration.

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the state-of-the-art for the key laser 2.5 diode components will be discussed. CW, T= 25°C Pulsed, 200ns, 0.2%, T= 25°C Fiber Laser 2.0 Typical fiber lasers are based on double-clad fibers doped with rare earth elements like Ytterbium or Erbium and 1.5 absorption peaks in the wavelength range between 910 nm and 980 nm. Pump light is injected into the outer 1.2 1.0 2.0 A f = 10 kHz core of the double-clad fiber, and is 1.0 τ = 200 ns p effectively absorbed in the inner core 0.8

Power (Ex-Fiber) (W) while propagating along the fiber. 0.6 0.8 A 0.5 0.4 The architecture of the fiber laser 0.2 might be based on either a cavity Output Power (W) 0.0 formed by wavelength selective FBGs 0.0 −100 0 100 200 300 400 adapted to the emission wavelength of Time (ns) the active fiber material or, as shown in 012345Figure 7, a master-oscillator power- Current (A) amplifier architecture. In the latter case, a seed laser diode emitting light Figure 8: Output power characteristics of a seed laser at 1064 nm in CW and pulsed operat the fiber laser emission wavelength ation mode. Inset: Shape of the optical pulse at 0.8 A and 2.0 A drive current. around 1064 nm is injected into the single-mode core of the double-clad fiber. In CW operation, these seed laser diodes show similar performance to that of the 980 nm pumps described earlier. In pulsed 20 80 mode, the seed lasers are driven with large signal modu- 70 lation. Typical pulse duty cycles are on the order of 1 %, )

15 60 % with a pulse length in the tens or hundreds of nanoseconds. For these conditions more than 2 W of peak output 50 power has been obtained as shown in Figure 8, effectively 10 40 saturating the amplifier to obtain stable fiber laser operation. The shape of the optical signal nicely follows the 30 electrical signal with a rise time of several nanoseconds, Output Power (W) 5 20 limited by the inductance of the conventional 14-pin but- Wall Plug Efficiency ( 10 terfly package, which is not optimized for high-speed operation. 0 0 0 5 10 15 20 Due to the architecture of fiber lasers, the number of Current (A) pump ports is limited. Therefore, the maximum power level that can be obtained from one amplifier stage of a Figure 9: Optical output power reaching 19 W with maximum elec- fiber laser scales with the pump power coupled into the tro-optical conversion efficiency of 70% from a 90-mm-wide broad double-clad fiber. A cost effective and highly reliable way area laser diode in CW operation mode at 15°C case temperature. to pump fiber lasers is to use discrete multi-mode high power pump laser modules. To maximize the output power, the chip has been optimized for highest efficiency to minimize the heat load. Figure 9 shows a maximum of 19 W output power in CW mode while the electro-optical conversion efficiency exceeds 65 % up to current levels of 11 A. The peak conversion efficiency amounts to > 70%. In addition, the far field profile of the diode in both directions has been designed for best overlap with the mode field of the fiber. Coupling efficiencies larger than 90% can be achieved using an optimized fiber lens design integrated in the multi-mode fiber tip. Today, the available output power per module is up to 10 W from multi- Figure 10: Combination of the output beams of several collimated mode fibers with a diameter of 105 µm, enabling fiber vertical stacks of laser diode bars taking into account spatial, laser systems with output power levels in the kW range. polarization and wavelength multiplexing [7][8]. To assess the long-term reliability of the pump lasers,

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highly accelerated multi-cell life test studies have been architectures can provide output powers approaching the performed using hundreds of devices. From these tests, 1 kW level due to improved cooling [7]. acceleration factors for current, power, and junction tem- However, for commercial use in laser systems, drive perature have been derived indicating less than 2% annu- currents beyond 200 A cause practical problems. To al failure rate of the modules for industrial operation con- address this problem, we have developed the ditions and telecom-grade reliability for de-rated opera- VeryHighBrightness (VHB) bar, a new type of device [9]. tion conditions around 4 W.

Direct Diode The competitiveness of systems directly using the collimated 500

beams of diode lasers without the THeatsink = RT use of brightness converters like solid-state lasers is inevitably 400 linked to the availability of high- power, high-brightness laser diode bars. A major breakthrough has 300 been the introduction of our stable AuSn solder technology, effectively eliminating degradation modes associated with intermittent opera- 200 Bar Geometries tion [5]. Since the introduction of 1.2 mm x 10 mm this solder, the available power lev- Light Output Power (W) els have been continuously 100 2.4 mm x 10 mm increased by improving the robustness and efficiency of the devices. 3.6 mm x 10 mm Today, bars with standard 10 mm 0 emitting aperture on water-cooled 0 100 200 300 400 500 600 micro-channel coolers (MCC) with Injection Current (A) a rated CW output power of 120 W [6] are used in commercial systems in high volume. Power degradation Figure 11: Maximum output power from various generations of laser diode bars with increased rates (wear-out) extrapolated from cavity length and efficiency. long-term testing of the bars have shown lifetimes in excess of 30,000 hours with only marginal increase of the drive current. 2.0 For commercial use in high 160 CW Room Temperature power laser systems, the bars are 60 arranged in vertical stacks, deliver- 140 ing for example 2.4 kW of output 1.5 50 power from a 20-bar arrangement. 120 ) % Combination of several stacks by 40 spatial beam shaping and polariza- 100 tion and wavelength multiplexing, Power 80 W Theatsink 1.0 as schematically demonstrated in 80 25°C 30 Figure 10, can deliver fiber-cou- Voltage (V) 60 pled output power of more than 20 Wallplug Efficiency ( μ Light Output Power (W) 850 W and 4 kW in a 400 m 40 0.5 and 1000 μm fiber core, respec- Intensity (a.u.) 10 tively [7]. Free space delivery of up 20 to 10 kW has been reported. 970 980 990 Wavelength (nm) Further power scaling of the 0 0.0 0 laser bars has been demonstrated in 0 50 100 150 200 the lab resulting in 425 W CW Injection Current (A) output power using standard MCC technology (Figure 11) More sophisticated double-side cooling Figure 12: Very high brightness bar (VHB)

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[5] N. Lichtenstein, B. Schmidt, A. Fily, S. Weiß, S. Arlt, S. Pawlik, B. Sverdlov, J. Müller and C. Harder, “DPSSL and FL 5 Pumps Based on 980nm-Telecom Pump Lifetest Laser Technology: Changing the 1.3 Hz full on/off 4.5 VAC80C-9xx-01 Industry”, Photonics West 2004, Proc. 50% d.c. ° of SPIE 5336-31, San Jose, California, Theatsink = 25 C 4 2004 [6] N. Lichtenstein, Y. Manz, P. Mauron, A. 3.5 Fily, B. Schmidt, J. Müller, S. Arlt, S. m

μ BAC120C-9xx-01 Weiß, A. Thies, J. Troger, C. Harder, “325 Watt from 1-cm wide 9xx Laser 3 Bars for DPSSL- and FL-applications”, Photonics West 2005 (Invited), Proc. of P (W)/100 2.5 SPIE 5711-01, San Jose, California, 2005. 2 [7] H. Li, I. Chyr, D. Brown, X. Jin, F. BAC80C-9xx-01 Reinhardt, T. Towe, T. Nguyen, R. 1.5 Srinivasan, M. Berube, R. Miller, K. Kuppuswamy, Y. Hu, T. Crum, T. 1 0 1000 2000 3000 4000 5000 6000 Truchan, J. Harrison “Ongoing Time (h) Development of High-Efficiency and High-Reliability Laser Diodes at Spectra-Physics”, Photonics West 2007, Proc. of SPIE 6456-9 , San Jose, Figure 13: Reliability tests at various normalized power levels for three generations of laser bars. California, 2007. [8] Andre Eltze, “Diode lasers enter new To target a convenient operating backbone of modern fiber optical dimensions for metal welding”, point of 80 W, a reduced emitting telecommunication. We demonstrat- Proceedings of the 1st Pacific aperture of only 3.2 mm has been ed the leveraging of this technology International Conference on Application chosen. With a maximum output for applications like second harmon- of Lasers and Optics 2004 power of 150 W as shown in Figure ic generation or as seed sources for [9] Y. Manz, M. Krejci, S. Weiss, A. Thies, 12, the performance scales nicely fiber lasers. Scaling the power levels D. Schulz and N. Lichtenstein, with the results obtained from the for both multi-mode emitters and “Brightness Scaling of High Power 10-mm-wide bars. Most importantly, laser diode bars enables fiber lasers Laser Diode Bars”, Proceedings of the the divergence angles in the lateral and direct laser diodes to take over Conference on Lasers and Electro-Optics direction have been kept constant from traditional gas and solid-state Europe, 2007 and contain 90% of the power with- lasers. in 7°. Therefore, while keeping the Biography: drive currents around 100 A, a 3-fold References Norbert Lichtenstein increase in the brightness over con- [1] Bookham Press Release, “Bookham to Dr. Norbert Lichtenstein is currently ventional 80 W laser diode bars has unveil world’s most powerful telecoms the Director of R&D at Bookham been achieved. 980nm pump laser module at Switzerland AG where he oversees Preliminary life test data for the OFC/NFOEC 2007”, March 26th 2007 new product development of high- VHB bar is plotted in Figure 13, [2] A. Oosenbrug, “Reliability Aspects of power laser diodes for telecom and indicating similar wear-out reliabili- 980-nm Pump Lasers in EDFA industrial applications. He joined ty at power levels of up to 4.6w nor- Applications”, Proc. of SPIE, San Jose, Uniphase Laser Enterprise in 1998 malized to 100 μm aperture size, as California, 1998, pp. 20-27. leading a program for broad-area has been demonstrated for earlier [3] T. Pliska, N. Matuschek, S. Mohrdiek, laser device development. At the site A. Hardy, Ch. Harder, “External feed- generations of laser diode bars. in Zurich, he has held different posi- back optimization by means of polariza- tions in development and manage- tion control in fiber Bragg grating sta- Conclusion bilized 980-nm pump lasers,” IEEE ment within JDS Uniphase, Nortel We have demonstrated the perform- Photon. Tech. Lett., vol. 13, no. 10, pp. Networks as well as Bookham, ance of laser diodes used in various 1061-1063, 2001. including responsibility for develop- applications ranging from the tradi- [4] M. Achtenhagen, S. Mohrdiek, T. ment of 980 nm telecom lasers, 1480 tional usage in EDFAs to material Pliska, N. Matuschek, Ch. Harder, and nm pump lasers, and laser diode processing. Thanks to continuous A. Hardy, “L-I characteristics of fiber bars. He holds a PhD from the improvement in the performance and Bragg grating stabilized 980-nm pump University of Stuttgart and has pub- reliability of single mode laser lasers,” IEEE Photon. Tech. Lett., vol. lished more than 35 publications as diodes, these devices today form the 13, no. 5, pp. 415-417, 2001. well as several patents.

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LEOS and the Growth of Photonics Fred Leonberger

As I think back on the advances in al processing are enabled by integrat- tems applications are an important photonics over the past two decades, ed photonics components and fiber part of the technical conference and the progress in technology develop- lasers. The widespread deployment of course have been one of the key ment and commercialization has of new optical broadband systems, elements of the trade show. I am sure been monumental and the time for example GPON, is driving the that recent and current LEOS initia- seems very compressed. Throughout manufacture of devices such as DFB tives will likewise play a key role in this period, and especially by some lasers and APDs in volumes at least the further advancement of our field. early initiatives, LEOS has played a an order of magnitude larger than It has been an exciting two significant role in facilitating this those previously seen, and at very decades for photonics and LEOS has rapid growth. low price points. These types of had a key role. I suspect the next There is no doubt that the com- advances will have ramifications decade will be at least as exciting munity of people who were engaged back into core telecom markets. and fast paced! in photonics research in the late Meanwhile, photonic research 1980s, and those that joined the advances in areas such as photonic- Biography: field in the next few years, through crystal devices and silicon photonics, Fred Leonberger their creativity and dedication, were and convergence at the system level, Fred Leonberger is the Principal of key enablers for the commercial promises generations of new prod- EOvation Technologies LLC, a tech- photonics revolution a decade later ucts and networks for provision of nology and business advisory firm he in communications, consumer prod- enhanced services. founded in 2003, and serves on the ucts, and other areas. In DWDM I was fortunate to be part of the Board of Directors and/or Advisors optical communications especially LEOS leadership team for a number for a number of photonics compa- it was amazing to see the speed with of years at the beginning of this nies. He is also a Senior Advisor at which technology that was in the period when the society launched the MIT Center for Integrated research lab one year could be, in several key initiatives in publica- Photonic Systems (CIPS), where he just a few years, brought to a level tions and meetings that have facili- chairs a joint MIT/industry working of reliability and manufacturability tated the growth of photonics by group focused on advanced FTTx that new systems utilizing that providing key forums for informa- technologies. technology could be quickly and tion exchange and professional inter- Dr. Leonberger previously served widely deployed. Likewise, in opti- actions. LEOS held its first Annual as Senior Vice President and Chief cal datacom, the rapid progress Meeting in 1988, and over the years, Technology Officer of JDS Uniphase towards deploying optical trans- this meeting has helped LEOS estab- Corporation (JDSU). He was also a ceivers by the millions was very lish a strong identity and became a co-founder and General Manager of impressive. key forum for reporting optoelec- UTP, an optical modulator company, The subsequent collapse of the tronic advances. LEOS launched and has held management positions optical communications markets in Photonics Technology Letters in at MIT Lincoln Laboratory and 2001 and the “dark years” that fol- 1989, and PTL quickly became the United Technologies Research lowed in no way detracts from these journal of choice for rapid publica- Center. He is a member of the technical achievements. Many of tion of photonics advances. In 1990, National Academy of Engineering, these advances are now finding new after a few years of discussions, the and a recipient of the IEEE applications even as the core telecom IEEE Communications Society Photonics Award. He has been markets are recovering. Numerous joined with LEOS and OSA as a full involved in LEOS activities for over applications in sensing and industri- sponsor of the OFC. To this day, sys- 20 years.

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News Awards and Recognition at LEOS 2007

A special Awards Banquet will be held on Tuesday evening advanced ultra-high speed photodetectors.” This year’s at the LEOS Annual Meeting. The following awards will LEOS Distinguished Service Award will be presented to be presented during the ceremony: Mary Y. Lanzerotti, “for dedicated service as Editor of the The IEEE/LEOS William Streifer Scientific Achievement LEOS Newsletter from 2001 through 2006, resulting in Award will be presented to Shun-Lien Chuang, “for con- outstanding changes in this publication.” tributions to the development of the fundamental theories The 2006-2007 Distinguished Lecturers Servio D. of strained quantum-well lasers and the physics of opto- Cova, Bishnu P. Pal, David V. Plant, and M. Selim Unlu electronics devices.” will be recognized for their service to the Society. The Aron Kressel Award will be presented to Rodney Recognition as elected members of the LEOS Board of S. Tucker, “for contributions to the modeling and analysis Governors will be given to Filbert J. Bartoli, Christopher and applications of high-speed semiconductor lasers.” R. Doerr, Silvano Donati, and Diana Huffaker. Andreas Umbach, Gunter Unterborsch and Dirk Trommer Recognition as Vice President of Publications will be will be presented with the Engineering Achievement given to Jens Buus and Nan M. Jokerst. Award, “for the research, development and fabrication of Our congratulations to all the recipients!

Call for Nominations: The IEEE LEOS Young Investigator Award

The IEEE LEOS Young Investigator Award was estab- Nomination packages consist of a nomination cover lished to honor an individual who has made outstanding page, a statement of the nominee’s research achievements technical contributions to photonics (broadly defined) in photonics, the nominee’s curriculum vitae, and three to prior to his or her 35th birthday. five reference letters (to be received at the LEOS office The award shall consist of a certificate of recognition prior to the deadline). and an honorarium of $1,000. The funding for this award Please consider nominating an under-age-35 colleague is being sponsored by General Photonics Corporation. for the inaugural cycle of this award! Nomination packages will be due at the LEOS execu- For full information about the LEOS awards program tive office by 30 September. Nominees must be under 35 look under the “Awards” tab on the LEOS web site years of age on Sept. 30th of the year in which the nomi- (http://www.i-leos.org/ ) nation is made. The award may be presented either at the Nomination form can be found on page 41 Optical Fiber Communications Conference (OFC) or the Conference on Lasers and Electro-Optics (CLEO), to be Andrew Weiner selected by the recipient. The first award was CLEO 2007. LEOS Awards Chair

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News (cont’d)

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Career Section

IEEE/LEOS 2006-2007 Distinguished Lecturers Toshihiko Baba

Short summary of my talk: and superlens effects of negative refractive Photonic nanostructures, i.e. photonic optics in a SOI PC slab by optimizing I/O crystals (PCs) and high index contrast interfaces for low reflection and diffraction structures (HICs), have attracted attention losses. The superlens is unique because it in this decade. They strongly control light focuses light at the flat surface, and forms emission and propagation, and so allow a real image inside the PC. This means novel phenomena and applications. that the focusing characteristics are inde- Recently, they are particularly discussed pendent of the input position of light. A with other topics such as nanolaser, slow compact wavelength demultiplexer and light, negative refraction, and Si photon- parallel optical coupler were demonstrated ics. My presentation reviewed our research as applications of this lens. activities on these topics. In parallel with PCs, the Si photonics is The PC nanolaser has been expected as a increasing importance for intra-chip opti- high-efficiency and high-speed light source in a pho- cal interconnects and low-cost and compact photonic tonic chip. Two key issues are the small modal volume lightwave circuits. The HIC Si wire waveguide allows and high Q. We employed a point-shift nanocavity sharp bends and micro-optic components due to the consisting of only the shift of two neighboring holes in strong optical confinement. In addition to these com- a PC slab. Its cavity mode has an extremely small vol- ponents, we have demonstrated a very compact H-tree ume of 0.2 times the cubic wavelength, which is close optical signal distribution circuit and AWG demulti- to the optical diffraction limit. We fabricated it into plexer whose footprint was less than 100 micron 1.55-μm-GaInAsP QW slab and obtained the room square. By carefully optimizing the connection temperature cw lasing by photopumping with an effec- between elements, the sidelobe level and device loss tive threshold power of nearly 1 micro-watts. For this were reduced to less than -20 dB and 1.5 dB, respec- cavity, we also observed the spontaneous emission tively, and the polarization-insensitive characteristics enhancement due to the Purcell effect and the nearly were also obtained. It will be a practical device for thresholdless operation. In the photonic chip, such course WDM. nanolasers must be integrated with passive components. We integrated them with 1.30-μm-GaInAsP Short summary of my travel: passive PC waveguides using MOVPE buttjoint I visited all the LEOS chapters that invited me, as long regrowth process, and obtained a practical value of as their meeting plan fit to my schedule. The follow- external quantum efficiency of 8%. ing summarizes the date, chapter, city, place of meet- Slow light in the PC waveguide is of great interest ing, organizer of meeting, special note of my trips. due to its potential for optical buffering and enhance- (1) 08/09/06, Japan, Tokyo, Kozai Kaikan, Y. ment of the light-matter interaction. However, a nar- Yoshikuni, I started my year term from Japan with row bandwidth and large group velocity dispersion are other two DL lecturers Prof. M. Selim Ünlü and serious problems that disturb its practical use. We Dr. M. Notomi. have proposed some modified PC waveguides for wide- (2) 13/10/06, Ottawa, Ottawa, National Research band and dispersion-free operation. For example, dis- Council, K. Liu, It was just after OSA annual persion-compensated slow light with a group index (= meeting at Rochester. The audience showed par- slowdown factor) of 35 - 40 in a wide wavelength ticular interest on Si photonics. range of 35 nm was obtained a directional coupler of (3) 08/11/06, Ukraine, Guanajuato (Mexico), chirped PC waveguides with opposite dispersions. The Guanajuato University, I. Sukhoivanov, My talk zero-dispersion slowlight was also demonstrated by was scheduled as a formal opening lecture of some minute control of waveguide structure. Multiconference on Electronics and Photonics. Negative refractive optics has become a hot topic (4) 13/11/06, Italy, Turin, Avago Technology, T. with metamaterials. However, those based on PCs are Tambosso, I was impressed by the steadiness of advantageous for lightwaves as they are free from their research on semiconductor devices, while sur- absorption loss. We successfully observed superprism prised at hearing the low percentage of students in

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Career Section (cont’d)

the science and engineering majors in Italy. enjoyed seeing some research activities on bio- (5) 15/11/06, Italy, Rome, Universitate degli Studi di sensing and cavity QED of the University. Roma, A. d’Alessandro, I gave a lecture in a spe- (9) 21/03/07, Rochester, Corning, Corning Research cial class of the university. Center, S. Garner, I saw the history of Corning and (6) 15/03/07, Poland, Lodz, Technical Institute of recent research on bio-sensing applications. Lodz, W. Nakwaski, I also met Prof. M. Marciniak (10)14/05/07, Albuquerque, Albuquerque, University who invited me to International Conference on of New Mexico, Y. D. Sharma, An image sensor in Transparent Optical Network on June. the wavelength range from 3 to 10 microns and (7) 19/03/07, Hampton Roads, Norfork, Old auto-tracking system of laser beam were demon- Dominion University, A. Dharamsi, I gave a lec- strated at the visit. ture for undergraduate and younger graduate stu- (11)16/05/07, Central New England, Boston, Boston dents of the university. University, M. Cabodi, A one day symposium on (8) 20/03/07, Washington/Northern VA, Washington nanophotonics was held, and I and Dr. M. Notomi DC., University of Merryland, M. Dagenais, I gave lectures.

Bishnu P. Pal

In continuation of my report as a ing me a nice tour of different laboratories, he Distinguished Lecturer for 2005-06 pub- organized the talk at the campus of Old lished in June 2006 issue, I have great pleas- Dominion University, which was attended ure in reporting my experience of participa- mostly by students of that university. They tion in this great program during my extend- raised several questions and queries, which ed term as DL for 2006-07. During the sum- were very interesting for me. I met one of our mer months of June-July, we are entitled to former students Amir from IIT Delhi, who is ask for vacation leave of 60 days at my currently a Graduate student with Amin; Amir Institute, namely Indian Institute of and another fresh Graduate student Karan con- Technology Delhi. Our Institute Administration tinued their queries during the dinner at a is very generous in approving such a request Thai restaurant. Dr. Sean Garner of Corning unless there is any compelling administrative Inc. organized the next DL on the evening of reason for denying so. Dr. Lucy Zheng and June 8th at Sullivan Park campus of Corning Dr. George Simonis of the Washington/Northern VA LEOS Inc. for the Rochester LEOS chapter. It was my close friend chapter organized the first talk on June 5th 2006 at the Prof. Govind Agrawal (Govind and I were graduate stu- College Park campus of University of Maryland. It was dents at IIT Delhi about 30 years ago) of Inst. of Optics, organized as an evening seminar. The attendance was rather who motivated me to accept this invitation. It allowed me thin, as the campus students had already finished their a wonderful opportunity to visit some of Corning’s fiber semester end examinations by then and left the campus. optics related research laboratories and also to interact with Nevertheless there were interesting questions and discus- some of its outstanding researchers during the day begin- sions during the talk, which was attended by attendees from ning post-lunch. Sean arranged to video record the lecture companies like Ciena. Post-talk, my hosts treated Nilotpol for Corning’s technical library, which was indeed an honor. Kundagrami, a former student of mine (now working with He took pains to publicize the talk announcement for local an American Company in Maryland) and me to a sumptu- residents, who could have been interested to listen to the ous dinner at a Korean restaurant near the campus. This talk as a public lecture. I had explicitly chosen this invita- visit to the campus also gave me an opportunity to interact tion to get an opportunity to deliver the talk at a famous with two of College Park campuses well-known researchers industry like Corning in Fiber Optics and propagate namely, Professors Rajarshi Roy and Chris Davis. They were research interest of academics like us on exotic fibers like kind enough to give their time for discussing their work microstructured fibers, which happened to be an area of and I’m thankful to Chris for showing his laboratories to some substantial interest to Corning itself. I have had some Nilotpol and I. My next stop was at Norfolk, VA for the follow-up interactions with Dr. Karl Koch of the company, Hampton Roads chapter, which was organized by the chap- who is involved with R & D on photonic bandgap fibers ter chair Prof. Amin Dharamsi. Amin is an extremely jovial there. The next stop was at CREOL at University of Central person, who can make you laugh all the time! Besides giv- Florida, where my invitation came from the LEOS student

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Career Section (cont’d)

chapter. The chapter chair Dr. Yung-Hsun Wu coordinated Mundas Foundation of European Community to spend it with a great deal of enthusiasm and interest. Ozharar some time at the Heriot Watt University in Edinburgh, Sarper, the new student chapter chair received me on arrival. Scotland as a Photonics Scholar. I reached here at Edinburgh The number of attendees was very large and I received some on May 29th. On the way I delivered one of my last DLs at very nice compliments from the audience. Dr. Wu also the Technical University of Eindhoven, Netherlands at the arranged visits to a few of the laboratories, which were of invitation of Dr. Fouad Karouta, who had actually invited great interest to me. Meeting Professors Peter Delfyett and me to deliver my DL at the Annual Workshop of the B. Zeldovich was a bonus; I met Peter first at the Korean Benelux Student chapter held there on May 25th. This year Optical Society’s annual meeting early in the year 2006, the theme of the workshop was “Progress in Optical Devices where he was the second plenary speaker besides me. The and Materials”. I was delighted to get this invitation, as I excitement with which Prof. Zeldovich demonstrated some always look forward to meeting young students all over the of his experiments, which he has devised to explain the fun- world. It was once again a very interesting experience filled damental concepts underlying basic physical phenomena with interactions with attendees from several Benelux coun- such as coupled pendulums for example, through optics, tries. The workshop was held for the whole day and I stayed were unique, highly motivating and fascinating. His words on listening to the students’ presentations. One interesting of appreciation (e.g. I do not have the words to describe my feature of this annual LEOS workshop is that the chapter fascination by the breadth of your fiber work…..) through decides to invite a Ph.D. student, who would have made an email later were very motivating and inspiring. Dr. Wu significant research contributions during his Graduate and her husband (also a Graduate student there at CLEO) work. This year Dr. Guenther Roelkens from Ghent made sure that I catch my return flight by driving me to the University presented this invited talk. Thereafter, I deliv- airport in their own car. It was indeed fascinating to meet ered my last DL at City University London on June 26th. several outstanding students at CREOL. Some of the work Prof. B.M.A. Rahman of City U organized the lecture under on Liquid crystal displays was also very interesting. The fol- the auspices of IEEE MTT/ED/AP/LEOS joint chapter in lowing week I drove from Rochester, NY (where I stayed for the form of a half-day seminar addressed by Prof. Brian a couple of months with my daughter Parama, who is a Culshaw of University of Strathclyde (on possible optical Graduate student at the Institute of Optics there with Prof. fibers sensors based on Photonic crystal fibers) and Prof. Wayne Knox) to Ottawa’s NRC for my next talk. The invi- David Richardson from University of Southampton (on tation was kindly extended to me by local LEOS chapter high power fiber lasers) besides me. A large number of lis- Chair Dr. Kexing Liu 9of Ciena), who was helped by Dr. teners from other Institutes and laboratories from London John Alcock of NRC Ottawa in organizing my talk on the formed the audience. This has been my last official DL talk. afternoon of June 22nd at NRC. I had indicated my inter- However I have another DL talk to deliver for the Scottish est to John beforehand to visit some of the Photonics relat- chapter of LEOS at the behest of Dr. Ajoy Kar, its chair, who ed laboratories at NRC, which he had kindly agreed. In par- insisted that I deliver a talk at their half-day LEOS chapter ticular I was impressed by the laboratory and infrastructure DL seminar on “Guided Wave Devices: New Dimensions” on active semiconductor fabrication during my visit to these being held on July 11th this year at Heriot Watt University laboratories. I had very absorbing discussions with Kexing in Edinburgh even though my term as DL would be over by later on that day over dinner. Finally, I visited the Toronto’s June 30th. Since I am staying on in Scotland for the next LEOS chapter on June 29th. Dr. Emanuel Istrate, the chap- two months, I accepted his invitation. ter Chair organized the lecture on the afternoon of that day, Overall visits to different LEOS chapters have been a which was well attended. I could also meet one of our for- great rewarding and stimulating experience for me. I mer students from IIT Delhi there. Before the lecture I had learnt a lot from the scientists and students I met during a quick run through some of the very interesting laborato- these visits to different campuses. Within the time and ries on various aspects of Photonics, which included work budgetary constraints (which is important since I live in on photonic crystal structures. Emanuel even spent time India – distance-wise far from the western world) I have post-lecture to show me his own work. Emanuel and his tried to maximize visits to chapters, which were geo- Chapter’s Treasurer Dr. Jianzhao Li continued discussions graphically wide spread as per the original suggestion in during the dinner. I was very impressed with their depth of my DL offer letter. I wish I had more time in hand to visit knowledge. That summarizes my visits to different LEOS the other 6-7 chapters, whose kind invitations I could not chapters in North America. In 2007, as I have had difficul- oblige. Those of you who could have some interest to the ty getting leave of absence until this summer, I had to content of my talk, a video of my talk should now be regretfully decline several other LEOS chapter requests to available on LEOS portal: Education tab/LEOS visit them. In the meanwhile I was invited by Erasmus University/Distinguished Lecturers. In all, I could visit a

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Career Section (cont’d)

total of 13 LEOS chapters during my two-year term these fibers as photonic bandgap guided optical fibers. In 1987 (2005-07) as a DL, which I greatly enjoyed. I feel this is in the same issue of Physical Review Letters, Eli Yablonovitch a fantastic program of LEOS, which aims to network sci- and Sameer John independently proposed for the first time the entists in different countries, and provides great opportu- possibility of controlling properties of light through the pho- nity for exposing to students to various facets of contem- tonic bandgap effect in man-made photonic crystals. porary research on Photonics in general. Microstructured optical fibers have been a fall out of that research. The talk would focus on basic functional principle of “Microstructured Optical Fibre: optical wave guidance in such fibers vis-a-vis conventional An Emerging Technology fibers. Details of propagation and design & technology of 1D and its Potentials” photonic band gap Bragg fibers would be described, in which Consequent to the mind boggling progress in high-speed opti- we have recently made some research contributions and our cal telecommunication witnessed in late 1990s, it appeared collaborators in Russian Academy of Science have succeeded in that it would only be a matter of time before the huge theoret- fabricating some of our designed fibers. Discussions on appli- ical bandwidth of 53 THz, offered by low-loss transmission cations would include designs of dispersion compensating windows in low water peak high-silica optical fibers would be fibers, fibers for metro networks, nonlinear spectral broadening tapped for telecommunication through dense wavelength divi- in them and generation of supercontinuum light. sion multiplexing techniques! In spite of this possibility, there has been a considerable resurgence of interest amongst Bishnu P. Pal obtained M.Sc. and Ph.D. degrees in Physics researchers to develop application-specific specialty fibers, e.g. from Jadavpur University (Kolkata) and IIT Delhi in 1970 fibers in which transmission loss of the material would not be and 1975, respectively as a National Science Talent Search a limiting factor and in which nonlinearity and dispersion Scholar. In late 1977 he joined the academic staff of IIT properties could be conveniently tailored to achieve transmis- Delhi as a specialist on Fiber Optics, where he is a Professor sion characteristics that are otherwise almost impossible to real- of Physics since 1990. He has worked as Visiting Professor ize in conventional high-silica fibers. Research targeted at such at the Norwegian Institute of Technology, Trondheim fiber designs in the early 1990s gave rise to a new class of fibers, (Norway), University of Strathclyde, Glasgow (UK), known as microstructured optical fibers (MOFs), which are Optoelectronics Research Center at City University of characterized with wavelength scale periodic refractive index Hong Kong, and Universities at Nice and Limoges features across its cross-section. The periodicity could be real- (France), the Fraunhofer Institute für Physikalische ized by having a two-dimensional periodic array of low and Messtechnik, Freiburg (Germany) as Alexander von high refractive index regions e.g. air holes embedded in a solid Humboldt Fellow, and the National Institute of Standards dielectric like fused silica glass. These structures exhibit pho- and Technology, Boulder (USA) as a Fulbright Scholar, for tonic bandgaps i.e. they forbid propagation of a certain band of various periods. He has been a founding member of wavelengths within them. If the frequency of incident light International Journal of Optoelectronics (Taylor & Francis, happens to fall within the photonic bandgap, which is charac- UK) and he is currently a Member of the Editorial teristic of these fibers, then propagation of light is forbidden Advisory Boards of the journals: J. Elect. Engg. & Tech. inside it. In contrast to the electronic bandgap, which is the (Korea), Optoelectron. Letts. (China), and IETE Students consequence of a periodic arrangement of atoms/molecules in a Journal (India). Prof. Pal has extensive teaching, research, semiconductor crystal lattice, a photonic bandgap arises due to sponsored R&D, and consulting (for Indian and US indus- a periodic distribution of refractive index in a PCF. However by tries) experience on various aspects of Fiber Optics and introducing in the central region a defect to an otherwise peri- related components and he has published and reported over odic structure, light (within the bandgap) could be localized in 130 research papers and research reviews in international the defect region thereby mimicking a fiber core. The defect journals and conferences and has coauthored one each region could be a medium of refractive index higher or lower Indian and US patents. He is co-author of the book entitled (e.g. air) than the average refractive index of the surrounding Fiber Optics and Instrumentation (in Russian, layers. In the former case, light is guided by modified total Mashinostroenie Publishing House, Leningrad, 1987) and internal reflection due to the average refractive index of the has edited the books: Fundamentals of Fiber Optics in cladding being lower than the central defect region. In case of Telecommunication and Sensor Systems (New Age lower refractive index defect, the corresponding MOFs are Publications, New Delhi and John Wiley, New York, known as photonic bandgap fibers (PBGFs). In contrast to a 1992, 4th reprint 2006) and more recently “Guided Wave conventional optical fiber, in which light is guided by total Optical Components and Devices: Basics, Technology, and internal reflection, Bragg scattering is responsible for effective Applications (Academic Press/Elsevier, Burlington, 2006). wave guidance in such fibers, which led to the christening of He has also contributed 11 chapters in other books and

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Career Section (cont’d)

monographs. He has been deeply involved with the con- components for optical networks and also the Gowri ception and development of the Fiber Optics Laboratory at Memorial Award for the year 1991 of IETE (India) for his IIT Delhi in late 1970s. Prof. Pal is a Fellow of IETE paper (co-author B.D. Gupta) on fiber optic biosensors. (India) and Optical Society of India, Foreign Member of He is currently also a Traveling Lecturer of OSA for his the Royal Norwegian Society of Sciences and Letters significant contributions to Guided Wave Optical Academy (Norway), and is a Member of the Optical Components and Devices. His current research interests Society of America and IEEE/Laser and Electrooptics concern guided wave optical components for DWDM Society (USA). He has been an invited speaker at over 24 and optical networks, gain flattening in EDFAs, special- international conferences and he has been a member of ty fibers like dispersion compensating fibers, and the Technical/Advisory Committees of over 15 microstructured optical fibers, and also fiber optic sen- International Conferences. He is a co-recipient (with K. sors, optrodes, and near field fiber probes. Prof. Pal is Thyagarajan) of the First Fiber Optic Person of the Year running a 3-year term as a Member of the International award in 1997 instituted by Lucent Technology in India Council of the Optical Society of America effective for his significant contributions in all-fiber branching January 2007.

David V. Plant

It has been an honor and pleasure to serve as a Bio: David V. Plant received the Ph.D. degree LEOS Distinguished Lecturer for my second in electrical engineering from Brown term in 2006 – 2007. I have had the pleasure of University, Providence, RI, in 1989. From visiting several LEOS chapters throughout 1989 to 1993, he was a Research Engineer Canada and the Europe and I plan to continue with the Department of Electrical and visiting as an Emeritus Distinguished Lecturer. I Computer Engineering at UCLA. He has been have visited and given talks at the following a Professor and Member of the Photonic Chapters: Benelux Chapter to participate in the Systems Group, the Department of Electrical Annual Symposium of LEOS which is held every and Computer Engineering, McGill year, Vancouver, British Columbia as part of the University, Montreal, QC, Canada, since 1993. Canadian Conference of Electrical and Computer Since September 1, 2006, he has been the Chair Engineers, and Ottawa as part of the Agile All- of the Department of Electrical and Computer Photonic Networks Annual Research Review. Engineering. During the 2000 to 2001 academic years, he took a leave of absence from McGill Summary of Lecture University to become the Director of Optical Integration at Agile All-Photonic Networks Accelight Networks, Pittsburgh, PA. He is the Director and Abstract: Recent advances in fiber optic technology have Principal Investigator of the Centre for Advanced Systems prompted researchers to envision a future all-photonic network and Technologies Communications at McGill University that is capable of supporting multiple access and services at very (www.sytacom.mcgill.ca). He is also Scientific Director and high bit rates. The confluence of optical transmission and optical Principal Investigator of the Agile All-Photonics Networks network functions opens up new paradigms for network archi- Research Network (www.aapn.mcgill.ca). Hi research inter- tectures that are enabled by emerging photonic technologies. ests include optoelectronic-VLSI, analog circuits for commu- Characteristics of these architectures and technologies that dis- nications, electro-optic switching devices, and optical net- tinguish them from existing ones include: (1) networks in which work design including OCDMA, radio-over-fiber, and agile the transmission of information is based on optical packets packet switched networks. Dr. Plant has received five teach- (burst-switched or packet-switched networks, with and without ing awards from McGill University, including most recently all-optical header recognition), (2) optical code-division multi- the Principal’s Prize for Teaching Excellence (2006). He was plexing for allocating bandwidth-on-demand in bursty, asyn- named an inaugural James McGill Professor, an IEEE chronous traffic environments, and (3) practical implementations Distinguished Lecturer, was the recipient of the R.A. for optical generation, shaping, and processing. The bursty Fessenden Medal and the Outstanding Educator Award, nature of these networks imposes new design constraints on both from IEEE Canada, and received a NSERC Synergy transmitters, receivers, and optical components. We review vari- Award for Innovation. He is a member of Sigma Xi, a Fellow ous system and technology considerations for such networks. of Optical Society of America and a Fellow of the IEEE.

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Career Section (cont’d) Indium Phosphide & Related Materials (IPRM 2006) Best Student Paper Award Recipient

Kale J. Franz received a B.S. degree in has been a graduate student at Princeton University, pursuing engineering physics from the Colorado research as a member of Prof. Claire Gmachl’s research group School of Mines in 2004, and an M.A. and in association with Prof. Stephen R. Forrest. degree in electrical engineering from His graduate research focuses on mid-infrared quantum cas- Princeton University in 2006. He is cade lasers. Mr. Franz divides his research activities into two currently working towards a Ph.D. tracks. The first concerns novel designs and new ideas in quan- degree, also in electrical engineering. tum cascade laser development. The topic of his IPRM paper, During the summers of 2001 and related to dual-wavelength quantum cascade lasers, falls under 2002, Mr. Franz worked for the U.S. this category. The second track is achieving high performance in Department of Energy, Office of quantum cascade lasers. Science on science policy-related matters. During the summers Mr. Franz is a student member of the IEEE/LEOS society. of 2002 and 2003, he worked at the National Renewable His graduate work is sponsored by a National Science Energy Laboratory on carbon nanotube synthesis. Since 2004, he Foundation Graduate Research Fellowship. 2006 LEOS-Newport/Spectra-Physics Research Annual Student Paper Award Recipients At the 2006 LEOS Annual Meeting, Program Chair, the Program Chair from this student awards activity. Further Newport Corporation and its Spectra- the previous meeting, and a representa- information may be found on the LEOS Physics Lasers Division joined LEOS in tive from Newport Corporation. and Newport web sites. sponsoring the Student Paper Awards Recipients are chosen based on the qual- The results for the 2006 LEOS- program. Given annually by LEOS, the ity of the original research described in Newport/Spectra-Physics Research awards are open to students from univer- the submission(s) and on the presenta- Student Paper Award are as follows: sities whose papers have been accepted tion to the judges. The finalists receive 1st Place - Tomoyuki Takahata for presentation. The Newport Student certificates of recognition and monetary 2nd Place - Ali K. Okyay Paper Award recognizes the top five awards ranging from $250 up to $1,000 3rd Place - Hans C. Hansen finalists whose submissions are judged for first place. LEOS is very pleased with Runner Up - Maxim Abashin by the Annual Meeting General Chair, Newport’s offer to expand and sponsor Runner Up - Haltice Altug

Tomoyuki Takahata received his B.S. cal systems). In 2004, he demonstrated that the transmittance of and M.S. degrees in mechano-informatics a PCW increased with a cantilever of an atomic force microscope from the University of Tokyo in 2002 in one of air holes of the PCW. In 2005, he designed a single- and 2004, respectively. He is now a chip optical modulator composed of a flexible PCW and nano- Ph.D. student at Department of rod array fixed on a wafer. Tomoyuki Takahata is a student mem- Mechano-informatics, the University of ber of the Japanese Society of Applied Physics (JSPS). Tokyo. His current research is focused on “It was such an honor to receive the 2005 LEOS/ mechanically tunable photonic crystal Newport/Spectra-Physics Annual Student Paper Award. It waveguides (PCWs) actuated by MEMS (microelectromechani- encourages me to build bridges between photonics and MEMS.”

Ali K. Okyay (S’97) was born in Tarsus, PhD student, expected to graduate in 2007. He is working Turkey. He received his Bachelor of with Prof. Krishna Saraswat, and Okyay’s current research Science Degree in Electrical and focuses on optoelectronic device design and process integra- Electronics Engineering in 2001 with tion of SiGe photodetection technologies. It includes an high honor from Middle East Technical extremely compact and fully integrated semiconductor University (METU), Turkey. In 2001, he optoelectronic switch based on Si MOSFETs, in addition to started his MS degree in Electrical traditional Ge-based photodetectors. He is an author and Engineering in Stanford University where he is currently a coauthor of over 20 journal and conference papers. 21leos04.qxd 8/29/07 4:56 PM Page 48

Career Section (cont’d)

Hans Christian Hansen Mulvad His Ph.D. project concerns the stabilization of fiber-based received the M.Sc. degree in physics switches for high-speed optical time-division multiplexed from the University of Copenhagen in (OTDM) communication systems. His PhD research focuses 2004. He is currently a Ph.D. student mainly on demultiplexing, add-drop multiplexing, as well as in Prof. Palle Jeppesen’s group at clock recovery at bitrates from 160Gb/s and above. The work COM•DTU, The Department of presented at the LEOS annual meeting 2006 in Montreal was Communications, Optics, and Materials on 160 Gb/s simultaneous add-drop multiplexing in a non- at the Technical University of Denmark. linear optical loop mirror.

Maxim Abashin received his B. S. and Generation in Periodically Polled Crystals was done in collabo- M. S. with honor degrees in Applied ration with Polyus R&D Institute under the supervision of Physics and Mathematics in 2001 and Prof. V. G. Dmitriev. 2003 from Moscow Institute of Physics In 2003 he entered Ph. D. program at University of and Technology. During his undergrad- California San Diego, where he works in the Prof. Fainman’s uate years, he was working for IPG group on the near-field characterization of nanophotonic Photonics where he was doing research devices. His research interests include characterization of of nonlinear optics phenomena and char- Silicon photonics, Photonic Crystal and Metamaterial devices acterization of fiber optics devices. His as well as the improvements in Near-field Scanning Optical M. S. thesis on theoretical investigation of Second Harmonic Microscopy itself.

Haltice Altug joined Electrical and magazines. Her work on nano-cavity array lasers Computer Engineering Department received Best Paper and Research Excellence award in of Boston University as an Assistant IEEE LEOS Conference in 2005. She was the co-recipi- Professor in 2007 after receiving ent of first place award in the Inventors’ Challenge her Ph.D. degree in Applied competition of Silicon Valley with micron scale all- Physics from Stanford University. optical switch work. Prior to working on nano-photon- Her research involves design and ic devices, she worked on multiple quantum well elec- implementation of high perform- tro-absorption modulators for optical interconnects, ance and ultra-compact nano-pho- three dimensional metallic photonic crystals, micro- tonic devices including lasers and all-photonic switches scopic theory of vortex states in superconductivity, and their large-scale on-chip integration for communi- phase transition in superconducting NbTi wires, and cation systems and on-chip optical interconnects. She electron conductance quantization in metal nano-con- also works on the development of on-chip biosensors tacts. During my PhD, she won Intel and LEOS operating from visible to infrared frequencies and inte- Fellowship. She also served as the president of Stanford grated with microfludic platforms for proteomincs and Student Chapter of Optical Society of America in 2005. genomics applications. She is author and co-author of Ms. Altug will like to take this opportunity to thank over 20 journals and conference papers. She recently LEOS and Newport/Spectra-Physics for offering the demonstrated ultra-fast photonic crystal nanocavity Graduate Fellowship program and Best Student Paper lasers, which has been featured on the cover of Nature awards. She is grateful for receiving both awards in 2005 in Physics, and highlighted in Nature Photonics and Laser Sydney. IEEE-LEOS annual meetings are great occasions to Focus World magazines. Her work on slow light and exchange ideas and to establish new collaborators and nano-cavity lasers has been featured on the cover of friends. She is very glad to be part of the Nano-photonics Applied Physics Letters and highlighted in several Technical program committee for IEEE-LEOS 2007.

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Membership Section

Benefits of IEEE Senior Membership

There are many benefits to becoming an IEEE Senior Member: • The professional recognition of your peers for technical and professional excellence • An attractive fine wood and bronze engraved Senior Member plaque to proudly display. • Up to $25 gift certificate toward one new Society membership. • A letter of commendation to your employer on the achievement of Senior member grade (upon the request of the newly elected Senior Member.) • Announcement of elevation in Section/Society and/or local newsletters, newspapers and notices. • Eligibility to hold executive IEEE volunteer positions. • Can serve as Reference for Senior Member applicants. • Invited to be on the panel to review Senior Member applications.

The requirements to qualify for Senior Member elevation are, a candidate shall be an engineer, scientist, educator, technical executive or originator in IEEE-designated fields. The candidate shall have been in professional practice for at least ten years and shall have shown significant performance over a period of at least five of those years.” To apply, the Senior Member application form is available in 3 formats: Online, downloadable, and electronic version. For more information or to apply for Senior Membership, please see the IEEE Senior Member Program website: http://www.ieee.org/organizations/rab/md/smprogram.html

New Senior Members The following individuals were elevated to Senior Membership Grade thru May - July:

Keren Bergman Marc Currie Lawrence Chen Pietro Ferraro Isabelle Huynen Lan Fu Mani Sundaram Daan Lenstra Diaa Abdel-Muguid Khalil Jeffry M Bulson John Mazurowski Richard Lytel Robin K. Huang Anna K. Swan Valluri R. Rao Franco Zappa Pierre S. Berini Soon F. Yoon Richard Desalvo Daniel P. Foty Jian Chen San-Liang Lee Xiaomin Jin Janice A. Hudgings John M. Dudley Guifang Li Eric Larkins Michal Lipson Avery L. McIntosh Lijun Ma Farhad Akhavan Lianshan Yan Sarun Sumriddetchkajorn

Visit the LEOS web site for more information: www.i-LEOS.org

August 2007 IEEE LEOS NEWSLETTER 49 21leos04.qxd 8/29/07 4:56 PM Page 50

Membership Section (cont’d)

The Santa Clara Valley Chapter of IEEE LEOS and a Celebration of the 30th Anniversary of LEOS with Dr. Charles H. Townes, Nobel Laureate Profile for the IEEE LEOS Newsletter Brent K. Whitlock, Chair and William E. Murray, Treasurer Santa Clara Valley chapter of IEEE LEOS July 1, 2007

The Santa Clara Valley chapter of facility, the glass blowing lab at San bring their whole family to for an IEEE LEOS (SCV LEOS) is part of Jose State University, the Schott entertaining and inspiring scientific the Santa Clara Valley Section of the glass sculpture exhibit with a talk by presentation. In addition, this is an IEEE, the largest IEEE Section in the glass sculpture artist Dr. Christopher opportunity for members to socialize world. SCV LEOS is organized as a Ries at SPIE’s Photonics West con- with each other and their friends and joint chapter also including LEOS ference, and a tour of the Stanford families at a celebration of the season members in the San Francisco Linear Accelerator Center (SLAC). with festive food and refreshments. Section and the Oakland/East Bay Prof. Tony Siegman of Stanford Section, the other two Sections in the University first suggested this pro- San Francisco Bay Area Council. gram and was the first speaker in this Altogether, SCV LEOS has about series in 2003. Since then, the series 470 members. The SCV LEOS web has also featured Dr. Robert E. site is http://www.ieee.org/scv/leos. Fischer, President of OPTICS 1 (also SCV LEOS was the recipient of a member of the Magic Castle in the 1997 IEEE LEOS Most Improved Hollywood), and Prof. Ken Pedrotti Chapter Award. Since then, SCV of University of California Santa LEOS has strived to continue to serve Cruz. In 2006, SCV LEOS combined its members and the community a field trip/facility tour with the hol- with interesting and relevant pro- iday lecture by having a behind the grams. SCV LEOS typically holds 12 scenes tour and presentation about to 15 technical programs per year. the Chabot Planetarium at the Figure. 1: Dr. Brent K. Whitlock (IEEE We have a monthly technical pro- SCV LEOS Chair) presents Dr. Townes Chabot Space and Science Center gram, usually in the evening on the with a plaque commemorating and thank- museum in Oakland, California. Jim first Tuesday of every month. ing him for his participation in the IEEE Kosinksi II, Planetarium and Speakers include university profes- LEOS 30th Anniversary Program. Theater Manager at Chabot Space sors, industry researchers, engineers, and Science Center, gave the SCV business executives, entrepreneurs, Every year SCV LEOS holds a hol- LEOS group a unique presentation and LEOS Distinguished Lecturers. iday lecture modeled after the on the Planetarium’s technology and Several times per year, we have a field famous Faraday Lecture Series of the capabilities, as well as a behind the trip or facility tour, typically with a Royal Society of London. The holi- scenes tour of the facility including picnic. The tours are an opportunity day lecture is designed as a fun talk the computing and imaging equip- for our members to visit and learn about some aspect of science related ment as well as the planetarium about places of technical interest in to lasers and electro-optics that apparatus. The event included a full the area with guided tours and pre- would be enjoyed by the general day admission to the Chabot Space sentations not generally available to population, even children. It is and Science Center museum, a the public. Some recent tours have intended to be an event that would catered lunch, and several planetari- included visits to the Lick educate people in an entertaining um and MegaDome Theatre shows. Observatory on Mount Hamilton, manner and inspire people to learn SCV LEOS from time to time the Exploratorium museum in San more about optics and photonics and also holds special workshops, sym- Francisco, the Naval Postgraduate even choose scientific careers in the posiums, and seminars. For exam- School in Monterey, LumiLEDs’ field. It is an event that people can ple, in 2004, SCV LEOS held a 2 21leos04.qxd 8/29/07 4:56 PM Page 51

Membership Section (cont’d)

day symposium on SBIR funding bench at his alma mater – Furman ping, plasma breakdown of air, and programs that included speakers University – that commemorates Raman scattering. Dr. Townes from various government funding where Townes had the conceptual commented that one of the lessons agencies. People attending learned breakthrough that led to the we have learned from research is about the various sources of SBIR invention of the MASER, the “how much fun it is to find new funding and other forms of federal microwave forerunner of the laser. things.” R&D funding, as well as how to pre- Dr. Siegman interviewed Dr. Dr. Townes also discussed some pare and submit successful funding Townes on his career from his col- of his contributions to public serv- proposals. This program also lege days up to the late 1950’s at ice in various forms over his career. included a venture capitalist (VC) He commented that “I think we all panel discussion. Presentation mate- have a duty to try to advise the rials from this two-day symposium government, whether we agree were collected together onto CD- with it or not. If they’re willing to ROM for the attendees and made listen to you, then do it. That’s available to others after the program very important.” for a nominal fee. The program concluded with a To celebrate the 30th comment by Dr. Townes on “the Anniversary of the IEEE Lasers and importance of being open minded Electro-Optics Society, SCV LEOS to new ideas, and basic research, invited Dr. Charles H. Townes and where… we’re trying to find out his wife Frances to be guests for a new things. That’s very important gala event at the Palo Alto to our industry, and just look what Research Center (PARC) on May lasers have done… We’ve got to be 22, 2007: “IEEE LEOS 30th open minded and think about new Anniversary Program: The Laser — Figure. 2: Dr. Elsa Garmire (left), Dr. things… We can’t predict these Origin, Development, and Future.” Charles Townes (center), Dr. Tony things. We must allow other peo- The event was presented jointly Siegman (right). ple to explore. We must be open with the MIT Club of Northern minded about new ideas. That’s California and the Optical Society Bell Laboratories and Columbia very important to our society.” of Northern California. Dr. Townes University, culminating in his Following that were some ques- shared the 1964 Nobel Prize in famous 1958 Physical Review tions from the audience and a pres- Physics for “Fundamental work in paper with Dr. Arthur Schawlow entation of commemorative IEEE the field of quantum electronics, that predicted the feasibility of LEOS plaques to Drs. Townes, which has led to the construction of “optical masers,” which are now Siegman, and Garmire along with oscillators and amplifiers based on known as “lasers.” Dr. Townes gifts of green wavelength laser the maser-laser principle.” The commented on the importance of pointers. program followed a “talk show” sharing ideas within the scientific You can read Dr. Towne’s own interview format with Dr. Anthony community for advancement of words in his memoirs How the Siegman (McMurtry Professor of technology, saying that the com- Laser Happened: Adventures of a Engineering Emeritus, Stanford munity of scientists and engineers Scientist, as well as a series of inter- University) and Dr. Elsa Garmire are really the ones who made this views in 1991 and 1992, archived (Sydney E. Junkins 1887 Professor field grow. “Science and engineer- at http://www.calisphere.universi- of Engineering, Dartmouth) as the ing, they’re a community thing.” tyofcalifornia.edu. interviewers. Dr. Elsa Garmire, who was a The hour long interview was graduate student of Dr. Townes at Acknowledgements: illustrated by photographs and MIT, interviewed Dr. Townes on IEEE SCV LEOS 2007 Officers: exhibits drawn from Dr Siegman’s the explosion of fundamental Brent K. Whitlock, Chair and Dr. Garmire’s archives. The research that followed the first Robert Herrick, Vice-Chair program opened with a photo- demonstration of the laser in 1960 William E. Murray, Treasurer graph of Dr. Townes and his wife by Ted Maiman at Hughes Min Hua, Secretary Frances seated on a replica of the Research. This included stimulat- Ram Sivaraman, Franklin Park (Washington, DC) ed Brillouin scattering, self-trap- Past Chair/Program Chair

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Conference Section

Recognition at CLEO/QELS 2006

Alan Willner, LEOS President, presented the 2006 IEEE/LEOS Randy A. Bartels (left) received the IEEE/LEOS Young Quantum Electronics Award to Sajeev John (left), “For the Investigator Award, “For pioneering contributions to ultra- invention and development of light-trapping crystals and the fast molecular photonics and photonic reagent control of quan- elucidation of their properties and applications. The award is tum systems on an unprecedented time-scale.” The award is given to honor an individual (or group of individuals) for out- given to honor an individual who has made outstanding tech- standing technical contributions to quantum electronics, either nical contributions to photonics prior to his or her 35th birth- in fundamentals or applications, or both. day. Funding is provided by General Photonics Corporation.

Alan Willner recognized David Plant, Katsumi Midorikawa, Selim Unlu, Markus Amann and Nabeel Riza, newly elected to the grade of IEEE Fellow.

52 IEEE LEOS NEWSLETTER August 2007 21leos04.qxd 8/29/07 4:56 PM Page 53

Conference Section (cont’d)

Conferences through 31 December 2007 For further information please see the LEOS conference calendar at www.ieee.org/leos

2007 IEEE/LEOS International Conference Conference URL: http://www.i-LEOS.org 02-Oct-2007 to 05-Oct-2007 on Optical MEMS and Their Applications Conference E-mail: [email protected] Ghe Fairmont Empress, Victoria, (MEMS 2007) British Columbia, Canada Conference Dates: 4th International Conference on Conference URL: http://www.i-LEOS.org 12-Aug-2007 to 16-Aug-2007 Group IV Photonics (GFP 2007) Conference E-mail: [email protected] Hualien Farglory Hotel, Hualien, Taiwan Conference Dates: Conference URL: http://www.i-LEOS.org 19-Sept-2007 to 21-Sept-2007 IEEE LEOS 20th Annual Meeting Conference E-mail: Radisson Miyak Hotel, Tokyo, Japan (LEOS 2007) [email protected] Conference URL: http://www.i-LEOS.org Conference Dates: Conference E-mail: 21-Oct-2007 to 25-Oct-2007 7th Pacific Rim Conference on Lasers [email protected] Wyndham Palace Resort & Spa, Lake and Electro-Optics (CLEO Pac Rim 2007) Buena Vista, FL USA Conference Dates: Avionics, Fiber-Optics Photonics Conference URL: http://www.i-LEOS.org 26-Aug-2007 to 31-Aug-2007 Conference (AVFOP 2007) Conference E-mail: COEX, Seoul, Korea Conference Dates: [email protected]

President’s Column (continued from page 3)

activities help shape people’s opinions of LEOS awards, journals papers, confer- somebody say, “Gee, I invited that per- you in these two critical areas. ence invitations, and committee son before but he was always so hard to In an “ideal” world, you would be chairmanships. Will Dr. D partially track down”? Maybe even, “That person judged simply on your objective merits, base his judgment on the LEOS stamp would make a great editor, but unfortu- and everyone would form an opinion based of high quality? Maybe, maybe not. nately he is not a LEOS member”. on fair criteria that they would meticu- 3. People tend to be risk averse. The last You’ll probably never know the answers lously evaluate. Many times, unfortunate- thing Dr. D wants is to be criticized to these and similar questions. ly, this is probably not true. People’s opin- for inviting a speaker who turns out to In some respect, this whole argu- ions tend to form based on many subjec- be a “dud” or too contentious. ment is somewhat contrary to our web- tive factors. Let’s take a scenario of a “Dr. Therefore, will he invite a researcher based culture. However, just like sales- D” trying to decide which researcher that he has personally heard present, people still value seeing the customer should be invited to speak at a conference. or will he invite someone just based face-to-face, the same is probably true The guiding principles are that Dr. D on their published results? (if not more so) when you are trying to wants: (i) the highest quality technical sell yourself and your ideas. In many work, (ii) that is presented in a clear and You can effectively market yourself ways, there is incredible value in a sit- professional manner, (iii) by someone who through active participation in LEOS- down or hallway chat at a conference. will be relatively straightforward to inter- sponsored activities. I want to emphasize one final point. act with. Let’s examine a few general char- Being a volunteer on a LEOS committee acteristics of our decision maker, Dr. D: Membership: Avoiding is a wonderful opportunity to interact “Out of sight, out of mind” with others towards a common goal. A 1. People are busy and time is at a premium. LEOS activities help the decision mak- unique feature of a LEOS committee If Dr. D is already personally familiar ers pick you. Even in our Web-based is that it provides a neutral and non- (through LEOS-related interactions) world, “face time” (e.g., conferences) threatening forum to showcase your with a very good choice, will he complemented by “quality assurance” organizational skills without any hid- expend much more time to search for (e.g., peer-reviewed journals) is crucial. den agenda (hopefully). a choice that might be a little better? There’s an old expression that “it’s The bottom line is that being an Maybe, maybe not. what you don’t know that can hurt active LEOS member puts you in a posi- 2. There is too much information at our fin- you.” Was your name mentioned as a tion to see and be seen. gertips, we rarely know all the facts, and potential candidate for a position, invi- we need help to judge the best quality. Dr. tation or award? Was there anybody Respectfully submitted, D has narrowed his decision to two around the table who could say, “I know Alan E. Willner choices, but one choice has many that person and she is dynamite”? Did University of Southern California 21leos04.qxd 8/29/07 4:56 PM Page 54

COME CELEBRATE 30 YEARS WITH US IN SUNNY FLORIDA LEOS 2007 Buena Vista Resort & Spa Lake Buena Vista, Florida October 21st-October 25th, 2007

LEOS 2007 is the 20th Annual Meeting of the IEEE Lasers and Electro-Optics Society. This year the conference will be held from 21 to 25 October in Lake Buena Vista, Florida, and includes short courses, invited papers, topical symposia and contributed technical papers addressing a broad spectrum of topics of interest to LEOS members and the lasers and electro-optics community.

This conference has established itself as an important forum for the latest developments in lasers and electro-optic technologies. With a strong core of invited talks given by preeminent speakers from around the globe, LEOS 2007 will provide an ideal plat- form to learn about new fields and understand technological trends. The program subcommittees are seeking papers that show novel and innovative work in the topic areas listed.

Following are just some of the added bonuses that attendees can expect at the conference: Careers and Research Forum!

The 2007 LEOS Annual Meeting is pleased to announce the 3rd "Careers in Research" Forum. The Forum will take place on Sunday, October 21st, and will include a welcome reception. The Forum's charter is to promote career awareness among students and young researchers in photonics and related fields.

Attendees will have the opportunity to: • Listen to the invited presentations from academia, industry, and entrepreneurs highlighting milestones for achieving success. • Interact with the highlighted speakers • Present poster papers of their research. Student Opportunities

To recognize outstanding student contributions, the Newport Student Paper Awards are open to students from universities whose papers have been accepted for presentation at the LEOS Annual Meeting. The top five finalists receive certificates of recognition and monetary awards ranging from $100 up to $1000 for first place. To further encourage students to participate in the conference, non- member students will be able to register for a free one year IEEE/LEOS student membership after paying the non-member student registration fee. Free Short Courses for LEOS Members

As an additional benefit for all LEOS members, the cost of the short courses will be waived at this year's annual meeting, though there will be a $20.00 charge for each set of short course notes. Special Session: Photonics- Creative Teaching Methods

This special session will focus on teaching optics, because there's more to being a professor than great research. Experienced teach- ers will share their wisdom on course development, and other aspects of teaching.

This session will be held on Sunday, October 29, from 15.30-17.00 (between the short courses and the Careers in Research Forum).

For More information please visit : www.i-leos.com

Or contact: Mary S. Hendrickx Senior Conference Administrator Phone + 1 732-562-3897 Fax + 1 732-562-8434 [email protected]

54 IEEE LEOS NEWSLETTER August 2007 21leos04.qxd 8/29/07 4:56 PM Page 55

Publication Section Call for Papers

Announcing an Issue of the IEEE JOUR- beginning of 2008. Please send a .pdf or Word file of NAL OF SELECTED TOPICS IN QUANTUM each manuscript, including keywords, all author ELECTRONICS on THz Materials, biographies and IEEE Copyright Form to Chin Tan- Devices and Applications yan at [email protected]. IEEE Copyright Form is Submission Deadline: available online at: http://www.ieee.org/about/docu- September 1st, 2007 mentation/copyright/cfrmlink.htm. Additional infor- The THz region of the electromagnetic spectrum with mation for authors regarding manuscript format may the corresponding frequencies between 0.2 THz and be found on the inside back cover of any issue of the 10 THz has not been completely explored. Such an IEEE Journal of Selected Topics in Quantum emerging field has been rapidly advanced, especially Electronic or online at http://www.ieee.org/organiza- within the last decade. IEEE Journal of Selected Topics tions/pubs/transactions/information.htm All submis- in Quantum Electronics invites manuscript submissions will be reviewed in accordance with the normal sions in the area of THz materials, devices, and appli- procedures of the Journal. cations. The purpose of this issue of JSTQE is to highlight the recent progress and trends in developing Email your manuscript and supporting documents to: novel materials, devices, and applications in the THz Chin Tan-yan domain. Broad technical areas include (but are not Publications Coordinator limited to): JSTQE Editorial Office IEEE/LEOS • novel materials and structures 445 Hoes Lane •bulk Piscataway, NJ 08854 USA • thin films • heterostructures Contact [email protected] for any questions about • engineered materials this issue. For all papers published in JSTQE, there are • sources voluntary page charges of $110.00 per page for each • quantum cascade lasers page up to eight pages. Invited papers can be twelve • difference-frequency generation, parametric generation, pages and Contributed papers should be 8 pages in and parametric amplification length before overlength page charges of $220.00 per • optical rectification page are levied. The length of each paper is estimated • photoconduction when it is received. Authors of papers that appear to be • electronics approach overlength are notified and given the option to short- • applications en the paper. Additional charges will apply if color • spectroscopy figures are required. • sensing [email protected] • imaging • advances in THz communication concepts and systems Announcing an Issue of the IEEE JOUR- • new measurement techniques and instrumentation NAL OF SELECTED TOPICS IN QUANTUM • advances in imaging configurations ELECTRONICS on Nonlinear-Optical • detection schemes Signal Processing • components, waveguides, and integrated circuits Submission Deadline: • measurements using surface plasmons, near-field November 1, 2007 effects, photonic crystals, metamaterials, and non- The IEEE Journal of Selected Topics in Quantum linear optics. Electronics invites contributions of original papers in the area of nonlinear-optical signal processing. The Guest Editors for this issue are Prof. Yujie J. All-optical processing of high-speed signals, Ding, Lehigh University, USA; Prof. Qing Hu, enabled by nonlinear-optical devices, represents criti- Massachusetts Institute of Technology, USA; Prof. Dr. cal technology for future optical communications, Martin Koch, Technische Universität Braunschweig, computing, and military applications. Originally Germany. demanding pulsed high-power lasers, nonlinear-opti- The deadline for submission of manuscripts is cal signal processing can now be achieved with low, September 1st, 2007; publication is scheduled for the semiconductor-laser compatible, powers and in small

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Publication Section (cont’d)

device sizes, owing to the progress in quasi-phase- JSTQE Editorial Office - Chin Tan-yan matched materials, semiconductor optical amplifiers, Nonlinear-Optical Signal Processing Issue photonic-crystal materials, highly-nonlinear fibers, IEEE/LEOS and silicon photonics. This, in turn, facilitates the 445 Hoes Lane development of novel optical signal processing archi- Piscataway, NJ 08854 USA tectures and schemes, which bring nonlinear-optical signal processing devices and systems to the verge of Contact [email protected] for any questions about practical deployment. The purpose of this issue of this issue. For all papers published in JSTQE, there JSTQE is to document the state of the art and recent are voluntary page charges of $110.00 per page for developments in the field from both experimental and each page up to eight pages. Invited papers can be theoretical perspectives. Solicitations topics include twelve pages and Contributed papers should be 8 (but are not limited to): pages in length before overlength page charges of $220.00 per page are levied. The length of each paper Wavelength conversion is estimated when it is received. Authors of papers Optical limiting that appear to be overlength are notified and given the Frequency translation option to shorten the paper. Additional charges will 2R and 3R regeneration apply if color figures are required. Phase conjugation Optical data format conversion Announcing an Issue of the IEEE JOUR- Phase-sensitive amplification NAL OF SELECTED TOPICS IN QUANTUM All-optical clock recovery ELECTRONICS on Semiconductor Quantum information processing Photonic Materials All-optical switching Submission Deadline: Frequency comb generation December 1, 2007 Parallel multi-wavelength processing The IEEE Journal of Selected Topics in Quantum Nonlinear pulse shaping Electronics invites contributions of original papers in Optical buffering the area of Semiconductor Photonic Materials. The Optical bi-stability purpose of this issue of JSTQE is to document the state Optical burst / packet switching of the art and recent developments in the field from Micro- and nano-scale nonlinear-optical devices both experimental and theoretical perspectives. The All-optical OTDM multiplexing/demultiplexing clear emphasis of this special issue is the materials and the processing methods for semiconductor photonic The Guest Editors for this issue are: Michael Vasilyev devices, rather than devices. Contributions are, howev- (University of Texas at Arlington, USA), Yikai Su er, encouraged in which device results are presented to (Shanghai Jiao Tong University, China), and Colin J. highlight materials and processing advances. McKinstrie (Alcatel-Lucent Bell Labs, USA). The deadline for submission of manuscripts is Solicitations topics include (but are not limited to): November 1, 2007; publication is scheduled for the Spring of 2008. Please send a .pdf or Word file of • Epitaxial growth, e.g. MBE, MOCVD, CBE each manuscript, including keywords, all author • Heterostructures biographies and IEEE Copyright Form to Chin Tan- • Quantum wells, wires, dots yan at [email protected]. IEEE Copyright Form is • Selective Area Epitaxy available online at: http://www.ieee.org/about/docu- • QW/QD intermixing mentation/copyright/cfrmlink.htm. Additional infor- • Processing techniques mation for authors regarding manuscript format may • Micro and nanofabrication be found on the inside back cover of any issue of the • Wet/Dry etching IEEE Journal of Selected Topics in Quantum • Ohmic and Schottky contacts Electronic or online at http://www.ieee.org/organiza- • Ion implantation, plasma processes tions/pubs/transactions/information.htm All submis- • Optical characterization sions will be reviewed in accordance with the normal • Electrical characterisation procedures of the Journal. • Non-linear optical materials and devices • Silicon and Group IV photonics Email your manuscript and supporting documents to: • Wide Bandgap materials, e.g. nitrides, oxides

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Publication Section (cont’d)

• Narrow bandgap semiconductors option to shorten the paper. Additional charges will • Materials for THz photonics apply if color figures are required. • Photonic Crystals • Nanophotonic structures IEEE Sensors Journal Special Issue • Nanowires, nanotubes on Optical Fiber Sensors. The deadline for submitting manuscripts has been The Guest Editors for this issue are: Catrina Bryce extended to September 1, 2007 (University of Glasgow, UK), Chennupati Jagadish The IEEE Sensors Journal announces a special issue on (Australian National University, Australia) and James optical fiber sensors in 2008. Coleman (University of Illinois, USA) Fiber optic sensing technology continues to be the subject of significant research endeavor investigating Justification both the phenomena which can be utilized in sensing Semiconductor materials played a crucial role in the and the applications of techniques established within development of photonics technologies. This issue is the laboratory. The ongoing interest is stimulated at dedicated to all semiconductor materials and process- the basic level by an ever increasing portfolio of tech- ing of relevance to photonics applications. Purpose of nologies through which light may be caused to inter- this special issue is to highlight recent developments act with the physical, chemical or biological condi- in the field covering a broad range of materials, tions which surround it. In parallel the applications processes and techniques developed for a variety of oriented research, in areas from bioscience to struc- device applications. tural monitoring to environmental assessment, has The deadline for submission of manuscripts is specifically highlighted one or more of the unique December 1, 2007; publication is scheduled for the benefits which fiber sensor technology offer. These Summer of 2008. Please send a .pdf or Word file of include the ability to operate over long interrogation each manuscript, including keywords, all author distances, complete immunity to electro magnetic biographies and IEEE Copyright Form to Chin Tan- interference, intrinsic safety and a very versatile range yan at [email protected]. IEEE Copyright Form is of measurand to lightwave transduction techniques. available online at: http://www.ieee.org/about/docu- Further, as the technology enters application the mentation/copyright/cfrmlink.htm. Additional infor- research becomes ever more interdisciplinary, embrac- mation for authors regarding manuscript format may ing issues such as self diagnosis and recalibration, sen- be found on the inside back cover of any issue of the sor integration and data fusion, network architec- IEEE Journal of Selected Topics in Quantum tures, packaging, system robustness and long term Electronic or online at http://www.ieee.org/organiza- reliability. tions/pubs/transactions/information.htm All submis- This special issue on optical fiber sensors will con- sions will be reviewed in accordance with the normal tribute toward encapsulating recent exciting develop- procedures of the Journal. ments in the incorporation of new transduction mechanisms to the guided wave format whilst in parallel Email your manuscript and supporting documents to: covering the continually expanding world of field tri- Chin Tan-yan – Publications Coordinator als and application assessments. Optical fiber sensors JSTQE Editorial continue to represent the core of the Special Issue, but Semiconductor Photonic Materials Issue the scope has been expanded to reflect growing new IEEE/LEOS applications, new techniques and material interactions 445 Hoes Lane of fiber optic technology especially in the life sciences Piscataway, NJ 08854 USA and nanotechnology. Relevant topics include, but are not limited by: Contact [email protected] for any questions about this issue. For all papers published in JSTQE, there • Physical and Mechanical Sensors: are voluntary page charges of $110.00 per page for Temperature, Pressure, Strain, Vibration, each page up to eight pages. Invited papers can be Acceleration, Flow, Rotation, Displacement. twelve pages and Contributed papers should be 8 • Sensors for Electromagnetic Phenomena: pages in length before overlength page charges of Magnetic field, Electric field, Current, Voltage. $220.00 per page are levied. The length of each paper • Chemical, Environmental, Biochemical and is estimated when it is received. Authors of papers Medical Sensors: that appear to be overlength are notified and given the Spectroscopic techniques, Environmental monitoring,

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Publication Section (cont’d)

Instrumentation for the life-sciences, Biophotonics, In Deadlines: vivo applications, OCT.. • Manuscript Submission: September 1, 2007 • Interferometric & Polarimetric Sensors: • Notification of Acceptance: February, 2008 Gyroscopes, Hydrophones, Geophones, • Final Manuscript due: May, 2008 Magnetometers, Acoustic Sensor Arrays. • Tentative publication date: September, 2008 • Distributed Sensing: Time, Frequency and coherence domain reflectome- Guest Editors: try, Rayleigh, Raman and Brillouin detection tech- • Prof. B. Culshaw, University of Strathclyde, b.cul- niques, Sensing cable designs. [email protected] • Multiplexing and Sensor Networking: • Prof. J.M. Lopez Higuera, University of Cantabria, Topologies and theories, Multiplexing techniques, [email protected] System applications studies. • Prof. A. Cusano, University of Sannio, • Passive & Active Devices for Photonic Sensing: [email protected] Sources, Detectors, Modulators, Specialty fibers, • Prof. I.R. Matias, Public University of Navarra, Integrated optics devices, Fiber gratings, MEMS, [email protected] Micro-optic components. • New Concepts for Photonic Sensing: IEEE Sensors Journal Special Issue 2D and 3D photonic crystals, Hollow core fibers. on Optical Fiber Sensors Photonic crystal fibers The deadline for submitting manuscripts has been • Nanophotonics: extended to September 1, 2007 Nanomaterials and nano-optical devices, IEEE Sensors Journal will publish a special issue on Metamaterials, Diffractive optics. optical fiber sensors early in 2008. • Signal Processing applied to Optical Fiber Fiber Optic Sensing technology continues to be the Sensors: subject of significant research endeavor investigating Genetic Algorithms, Neural Networks, Data both the phenomena which can be utilized in sensing fusion, Pattern Recognition, Classification analysis, and the applications of techniques established within Statistical Methods. the laboratory. The ongoing interest is stimulated at • System Applications and Field Trials: the basic level by an ever increasing portfolio of tech- Including metrology; Process control, Avionics, nologies through which light may be caused to inter- Condition and environmental monitoring. act with the physical, chemical or biological condi- • Smart Structures and Smart Materials: tions which surround it. In parallel the applications Surveillance networks for buildings, Monitoring for oriented research, in areas from bioscience to structur- material fatigue, Stress monitoring of aircrafts, al monitoring to environmental assessment, has specif- Smart materials. ically highlighted one or more of the unique benefits which fiber sensor technology offer. These include the We are inviting specialists in sensing from academia and ability to operate over long interrogation distances, industry to submit their latest research results as high complete immunity to electro magnetic interference, quality journal paper manuscripts. Solicited and invited intrinsic safety and a very versatile range of measurand papers shall undergo the standard IEEE Sensors Journal to lightwave transduction techniques. Further, as the peer review process. All manuscripts must be submitted technology enters application the research becomes on-line, via the IEEE Manuscript CentralTM, see ever more interdisciplinary, embracing issues such as http://sensors-ieee.manuscriptcentral.com. Upon sub- self diagnosis and recalibration, sensor integration and mission, authors should select the "2008 Optical Fiber data fusion, network architectures, packaging, system Sensors Special Issue" Manuscript Type instead of robustness and long term reliability. "Regular Paper" as well as indicate in the Author This special issue on optical fiber sensors will con- Comments section that it is intended for the special tribute toward encapsulating recent exciting develop- issue. Authors for this Special Issue are encouraged to ments in the incorporation of new transduction mech- suggest names of potential reviewers for their manu- anisms to the guided wave format whilst in parallel scripts in the space provided for these recommen-dations covering the continually expanding world of field tri- in Manuscript Central. For manuscript preparation and als and application assessments. submission, please follow the guidelines in the Optical fiber sensors continue to represent the core Information for Authors at the IEEE Sensors Journal web of the Special Issue, but the scope has been expanded page, http://www.ieee.org/sensors. to reflect growing new applications, new techniques

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Publication Section (cont’d)

and material interactions of fiber optic technology Manuscript CentralTM, see http://sensors-ieee.manu- especially in the life sciences. Relevant topics include, scriptcentral.com. Upon submission, authors should but are not limited by: select the "2008 Optical Fiber Sensors Special Issue" Manuscript Type instead of "Regular Paper" as well as • Physical and Mechanical Sensors: indicate in the Author Comments section that it is Temperature, Pressure, Strain, Vibration, intended for the special issue. Authors for this Special Acceleration, Flow, Rotation, Displacement. Issue are encouraged to suggest names of potential • Sensors for Electromagnetic Phenomena: reviewers for their manuscripts in the space provided Magnetic field, Electric field, Current, Voltage. for these recommen-dations in Manuscript Central. • Chemical, Environmental, Biochemical and For manuscript preparation and submission, please fol- Medical Sensors: low the guidelines in the Information for Authors at Spectroscopic techniques, Environmental monitor- the IEEE Sensors Journal web page, ing, Instrumentation for the life-sciences, http://www.ieee.org/sensors. Biophotonics, In vivo applications, OCT. • Interferometric & Polarimetric Sensors: Deadlines: Gyroscopes, Hydrophones, Geophones, • Manuscript Submission: September, 2007 Magnetometers, Acoustic Sensor Arrays. • Notification of Acceptance: February, 2008 • Distributed Sensing: • Final Manuscript due: May, 2008 Time, Frequency and coherence domain reflectome- • Tentative publication date: September, 2008 try, Rayleigh, Raman and Brillouin detection techniques, Sensing cable designs. Guest Editors: • Multiplexing and Sensor Networking: Prof. B. Culshaw Topologies and theories, Multiplexing techniques, Optoelectronic Sensors System applications studies. and Systems Group • Passive & Active Devices for Photonic Sensing: Department of Electronic Sources, Detectors, Modulators, Specialty fibers, and Electrical Engineering Integrated optics devices, Fiber gratings, MEMS, Micro-optic components. University of Strathclyde • New Concepts for Photonic Sensing: Glasgow, Scotland 2D and 3D photonic crystals, Hollow core fibers. [email protected] Photonic crystal fibers • Nanophotonics: Prof. J. M. Lopez Higuera Nanomaterials and nano-optical devices, Photonics Engineering Group Metamaterials, Diffractive optics. Electronics Technology, Systems • Signal Processing applied to Optical Fiber and Automation Engineering Department Sensors: University of Cantabria Genetic Algorithms, Neural Networks, Data Santander, Spain fusion, Pattern Recognition, Classification analysis, Statistical Methods. [email protected] • System Applications and Field Trials: Including metrology; Process control, Avionics, Prof. A. Cusano Condition and environmental monitoring. Optoelectronic Division • Smart Structures and Smart Materials: Engineering Department Surveillance networks for buildings, Monitoring for University of Sannio material fatigue, Stress monitoring of aircrafts, Benevento, Italy Smart materials. [email protected] We are inviting specialists in sensing from academia and industry to submit their latest research results as Prof. I. R. Matias high quality journal paper manuscripts. Electrical & Electronic Engineering Solicited and invited papers shall undergo the stan- Public University of Navarra dard IEEE Sensors Journal peer review process. All Pamplona, Spain manuscripts must be submitted on-line, via the IEEE [email protected]

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