Yunfei ZHAO

Glass Waveguide Fabrication by Ion Implantation for Optical Communication Applications

Mémoire présenté à la Faculté des études supérieures de l'Université Laval pour l'obtention du grade de Maître ès Sciences(M. Sc.)

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Yunfei Zhao

ABSTRACT

PIanar and channel waveguides in fused silica were fabncated by 3.0 MeV proton implantation. At 0.6338 pn wavelength, the pIanar waveguide is single mode with optical loss of 2.57 dB/cm. The channel waveguide is mukirnode with optical loss of 2.07 dB/cm at 0.6328 pmwavelength, and a single-mode at 1.55 p.

Planar waveguides in ZBLAN glas were fabricated in two samples, the first one with 4 MeV and the second one with 2 and 4 MeV double He+ implantation. The induced index change was observed to be negative. The first plana waveguide had a fundamenta1 mode up to 0.98 p,but it was cut off at 1.3 p.The optical loss was 1.8 dB/cm at 0.633 p.The second planar waveguide had a double guiding Iayer structure up to 0.98 p,but the top guide was cut-off at 1.06 pm. The index change was reconstructed by near-field mode propagation method, having a peak value of -5x10? Fabrication de guides d'onde en verre par implantation ionique pour applications aux communications optiques

Yudei Zhao

Des guides d'onde plans et canaux dans la silice ont été fabriqués par implantation de protons avec des énergies de 3 MeV. À la longueur d'onde de 0.6328 pm, le guide d'onde plan est monomode avec une perte optique de 2.57 dB/cm. Le guide d'onde canai est multimode avec une perte optique de 2.07 dIB/cm à 0.6328 p.II est cependant mononode à 1.55 p.

Deux guides d'onde plans en verre de ZBLAN ont été fabriqués par implantation ionique d'helium, avec une énergie de 4 MeV dans un premier échantillon et des énergies des 2 et 4 MeV dans un deuxième cas. Le changement d'indice de réfraction observé est négatif. Le premier guide d'onde est monomode jusqu' à 0.98 prn mais il ne guide pas à 1.3 pm. La perte optique est de 1.77 dB/cm. Le second échantillon a une structure comportant deux ré>= ons guidantes jusqu'à 0.98 p.m. Le mode du guide supérieur est cependant coupé à 1.06 p. Le profil de l'indice de réfraction a été reconstruit en utilisant la méthode de propagation du champ proche. La valeur maximale du changement de l'indice est de - 5x10~~.

(ii) Acknowedgements

For many years when 1 was Young, 1 dreamed to be an optical scientist. 1 had the chance to entre the optics world in 1997 when 1 came to Centre d'Optique, Photonique et Laser (COPL) at the Electrical Engineering Department, Université Laval. After studying and working in such an excellent academic environment of COPUGEL, 1 have built up knowledge in this field, and 1 have a deep understanding of fiber opticai communication as a whole. 1 would first like to acknowledge my research director, Professor Sophie LaRochelie: and equally 1 thank my research CO-director,Professor Emile J. Knystautas, for introducing me to this exciting guided-wave optics world. 1 would next Iike to acknowledge Professor Alain Villeneuve from COPUPHY, who gave me many insights on my research topic from the be-oinning to the end during my study. Mr. André Fer1and, technicien-expert in COPUGEL, continuously gave me technicd assistance, and never hesitated to answer my questions. Mr. Nicolas Bélanger, doctoral student in COPLPHY, helped me to measure optical guided-mode profde and cut-off condition evaiuation for planar waveguide in ZBLAN glass at wavelena& around 0.8 - 0.98 pn. 1 also want to thank Mr. Mohammed Saad, optical scientist in INO(Institut National d'optique), who supplied ZBLAN glasses that we used in Our experiments. 1 would like to thank Martin Cloutier and Daniel Gagnon, techniciens in the accelerator laboratory; Marc D'Auteuil, technicien-expert in COPUPHY; Ali Salirnania, doctoral student in COPUPHY; Christine Latrasse, professionelle in COPUGEL, and Karine Le Foulgoc, post-doctoral fellow in COPWPHY, for their invaluable assistance. 1also thank dl the professors and students in our COPUGEL group. 1 felt a fnendship and exciting academic environment during my study. Special thaaks was given to Professor Michel Tetu for his course, encouragement, help and understanding to a foreign student. I would Iike to thank my wife, Hong Zhou for her support and understanding. By accom- panying me to Quebec, she lost herself a lot. And my daughter, Yvonne, she always brightens my life with her open and optimistic nature, vigour of childhood, and her active living style. Finally 1 acknowledge the NSERC, FCAR, Bell Canada and Québec Telephone for their financial support.

(iii) Table of Contents

Abstract ...... i .. Résumé ...... II ... Ackno wledgements ...... LU Table of contents ...... iv List of figures ...... viï List of tables ...... x

Introduction...... I

Chapter I Optical vvnveguides: Theory. fabrication and application in optical communications ...... 7 1.1 Theory of optical waveguides...... 7 1.1.1 Electromagnetic mode theory for optical propagation ...... 7 1.1.2 Planar waveguide ...... 9 1.1.2.1 The basic three-layer planar waveguide...... 11 1.1.2.2 The gaded-index planar waveguide...... 13 1.1 -3 Cylindrical waveguide --- optical fiber...... 14 1.1.4 Rectangular waveguide --- charme1 waveguide ...... 15 Waveguide fabrication...... 17 Waveguide charactenzation ...... 17 1-3 . 1 Mode-profile characterization ...... 18 . . 1-3 -3 Index-profile detemunation ...... 19 1.3.3 Loss measurernent ...... 19 Application of waveguides in optical communication ...... 20

4.2.1 Proton implantation...... 52 4.2.2 TRIM calculation...... 53 . . 4.3 Planar waveguide fabrication...... 53 4.3.1 Near-field mode-profile measurement ...... 53 4.3.2 Optical loss evaluation ...... 55 4.3.3 Refractive-index reconstruction ...... 56

* . 4.4 Channel waveguide fabncabon ...... 57 4.4.1 Near-field mode-proflle characterization...... 57 4.4.2 Optical loss evaluation ...... 60 4.4.3 Refractive-index reconstruction ...... 61 4.5 BPM-CAD simulation ...... -63 Reference ...... 63 Chapter 5 Waveguide Fabrication in ZBLAN by He+ Implantation ....64

ZBLAN glas...... 65 MeV helium ion implantation into ZBLAN ...... 66 5.2.1 He f-implantation ...... 66 5.2.2 TMcalculation ...... 67 Planar waveguide fabncated by 4 MeV Het implantation...... 68 5.3. L Near-field mode-profile characterization...... 68 5.3 -2 Optical loss evaluation...... 70 Planar double waveguide structure induced by 2 MeV and 4 MeV Hef implantation... 7 1 5.4.1 Near-field mode-profile evaluation ...... 71 5.4.2 Refractive-index profile reconstruction ...... 73 5.5 BPM-CAD simulation and discussion ...... 75 Reference...... 80 Conclusion...... 81 LIST OF FIGURES Chapter 1

Fip. 1.1. Basic step-index three-layer planar waveguide, in which nî > ni and n3 ...... 10 Fig. 1.2. Cross-section view of a rectangular dielectric waveguide bounded by regions with - - lower refractwe mdex...... 15 Fig. 1.3. Sketch of a typicd EYoomode, (a) transverse mode, and @) lateral mode...... 16 Fig .1 -4. Structure of an Array Waveguide Grating (AWG), w hich can provide wavelength division multiplexing/demultip~eXingor routing ...... 2 1 Fig. 1.5. An integrated laser/rnodulator redized using selective area gowth...... 23 Chapter 2 Fig.2.1. Pear-shape nuclear collision cascade process of ion implanted into soiid material... 26 Fig.2.2. TRIM calculation results of 2 MeV proton implantation into fused silica :(a) Longi-

tudinal Range, (b) Phonon distribution...... , ...... 28-29 Fig.2.3. (a) Planar and (b) channel waveguide fabrication processes in the case of positive refractive-index changes induced by ion implantation...... 35 FÏg.2.4. PIanar(a) and channel (a and b)waveguide fabrication processes in the case of negative refractive-index change induced by ion implantation., ...... 35 Chapter 3 Fig.3.1.The diagram of the Van de Graaff accelerator at Universityé Laval ...... 40 Fig.3.2. Schematic of the expenmentai setup for coupling laser into the waveguides and measuring the mode profiles and optical loss...... 43 Fig.3.3. Beam waist--- He-Ne Laser coupling into channel waveguide in fused silica...... 45 Chapter 4 Fig.4.1. The attenuation spectrum for an ultra-low-loss single-mode fiber...... 5 1 Fig.4.2. Norrnalized range, ionization and vacancy of 3 MeV proton hplanting into fused silica glas as a function of the depth frorn the sarnple surface, calculated by TRIM.52 Fig.4.3. TE mode optical near-field output at 0.6328 pm wavelength of a planar waveguide in fused silica fabricated by 3 MeV proton implantation ...... 53

(vii) Fig.4.4. (a)(b)TE mode near-field output and surface plot; (c)(d) TM mode near-field output and surface plot, for the plaaar waveguide in fused silica fabricated by proton implantation...... -...... - -.-. -. . - ...... 54 Fig.4.5. TE mode near-field output and the ccrresponding surface intensity plot for the planar waveguide in fused silica, at a 1.55 prn wavelenD@ ...... 54 Fig.4.6, Optical intensity profile of scattering light on the surface of a planar waveguide and the corresponding curve of the logarithm of optical intensity, 10xlo,olol(x), versus depth... .-...... ,...-...... -55 Fig.4.7. (a)TE mode near-field opticai output of the planar waveguide fabricated by 3 MeV proton implantation at 0.6328 p.m wavelength, (b)(c) the corresponding intensity surface plot and line plot, and (d) the curve of the last term in equation (3.10)

versus the depth...... -...... - ...... , ...... -. -. - - -.. -. - 56 Fig.4.8. View of TE mode optical nez-field output of channel waveguide in fused silica fabricated by 3 MeV proton implantation, evaluated at 0.6328 pm wavelength...... 57 Fig.4.9. TE modes (TEo, TEl, TE7, m, TE5) optical near-field outputs at 0.6328 p He-Ne laser wavelen,ath of the channel waveguide in fused silica...... 58 Fig.4.10. TM modes (TMo, TMi, TM2, TM3)optical near field outputs at 0.6328 p He-Ne laser wavelergoth of the channel waveguide in fused silica-...... ,...... 59 Fig.4.11. TE mode opticai near-field outputs of channel waveguide fabricated in fused silica by 3 MeV proton implantation, evaluated at 1.55 pm wavelen,ath...... 60 Fig.4.12. Optical intensity profile of the scattered light on the surface of a channel waveguide and the comesponding curve of decreasing intensity, 10xlogioI(x), versus depth...... -...... - ...... 6 1 Fig.4.13. TE mode near-field output of the waveguide at 0.6328 pn, the corresponding vertical intensity plot, and the reconstructed refractive index profile...... 62 Fig. 4.14. Planar waveguide with wavelengths (a) k= 0.6328 pand (b)1.55 m...... -63 Fig. 4.15 Channel waveguides with wavelengths (a), h= 0.6328 p,and (b) 1.55 pm ...... -63 Chapter 5 Fig.S.l. Transmission loss spectrum of multimode ZrF4-based fiber, in which the broken line is the sprectmm of OH-free silica fiber ...... 65 Fig.5-2 Refractive index and chromatic dispersion of ZBLAN glass versus wavelength...... 66 Fig.5.3. Normalized ion range calculated by TRIM. The peaks correspond successively to the helium ion energies of 2 MeV (red) and 4 MeV (black), respectively...... 67 Fig5-4 TE mode near-field output in 2BLAN:a) CCD picture and b) surface intensity plot; TM mode near-field output : c) CCD picture and d) surface intensity plot...... 68 Fig.5.5. TE mode near-field outputs with vertical illumination to show the surface: a) and b), the correspondîng surface intensity plot :c), and d) the diagram of waveguide structure with negative refractive index induced by 4 MeV Hef implantation...... 69 Fig.5.6. (a) Optical intensity of scattering light, (b) the corresponding curve of optical intensity decreasing versus propagating distance, and (C) the curve without the first 0.1 cm ...... 70 Fig.5.7. CCD picture of theTE mode near-field output at 0.6328 p:a), the corresponding intensity plot : b), and the diagram of double waveguide structure: c), induced by 2 MeV and 4 MeV ~e+implantation...... 72 Fig.5.8. TE mode optical near field outputs for the sample with 2 MeV and MeV double Hef implantation at wavelengths of (a) 0.98 p,(b) 1.06 pn. (c) 1.3 p, and (d) 1.50 p...... 73 Fig.5.9. The curves of square-root intensity, and the reconstructed refractive index profile, Based on the data in digital image in Fig. 5.7(a), versus the depth from the sample surface...... -...*...... 74 Fig.5.10 Sketch of assumed waveguide structure with 13 layers and refractive index Change of - 1x~o-~, simulating the index change induced by a double He+ implantation into ZBLAN...... 75 Fig.S.11 BPM-CAD simulation results of top layer at wavelength of (a) 0.6328 pm, and @) 0.98 p,assuming An = -1~10"...... 76 Fig.5.12 BPM-CAD simulation results with the total width of 8.8 pm, and wavelengths of (a) 0.6328 p,(b) 0.78 pm, (c) 1.06 pm and (d) 1.3 p...... 77 Fig.5. 13 Two ways that the two-layer output (a) reduces to single layer output: (b) two-mode changes to single-mode, and (c) the top waveguide of the . - ongmal two is cut-off...... 78 Fi@ .14 BPM-CAD simulattion results with (a) combined waveguide with index change of .3x10'~. and @) top layer waveguide with index change of 4x10.~...... 79

LIST OF TABLES Chapter 2 Table 2.1 . TRIM calculation results for ion ranges for proton. helium. and germanium implantation into fused silica. a .quartz and Lm03 ...... 27-28 Table 2.2. TRIM calculation results for proton implantation into fused silica ...... 28 Table 2.3.Typical defect models in silica glass and their absorption bands ...... 31 Table 2.4. Sumrnary of some parameters and properties of induced by ion implantation .... 34 Chapter 4 Table 4.1. Linear fitting for planar waveguide ...... 55 Table 4.2. Linear fitting for channel waveguide...... 61 Chapter 5 Table 5.1. Typicai refractive index values of ZBLAN at several wavelenboths ...... 66 Table 5.2. Longitudinal range and straggle corresponding to 4 MeV and 2 MeV helium ion implantation into ZBLAN. as calcuiated by TRIM ...... 68 Table 5.3. Linear Regression for Data in Fig.5.6 ...... 71 Table 5.4. Layer thickness and corresponding refractive index for an assumed waveguide structure ...... 76 Introduction 1

Communication through opticai fibers has been changing this world tremendously. It has the impact that the inventions of the automobile and the airplane once did. Especially over the past few years, there has been a vast expansion of worldwide telecomrnunications and data transmission networks. Everyday, many people are now logging-on to the Intemet and the World Wide Web 0.Communications through networks is so popular that it has become a part of our daily Life. In many localities, fiber to the home and integrated service digital networks (ISDN), have become realities.

Humans have been utilizing optical communication throughout the long history of civilization, but we have been limited to free space transmission until the rnid-1980s. One of the world's earliest optical telecommunication systems was build up in ancient China about three thousand years ago. "Smoking & Fire Towers", penodically Iocated dong the Great Wall, worked as signal trammitter and repeaters to send messages by smoke signals from the emergent frontier, several hundred kilometers away, to the militaq base, and then to the capital by horse-running systems. The signal rate by direct visual methods was quite low. For hundreds of years, the sipaling Iamps and flags were used in military communications and navigation with a little higher signal rate based on some protocols, while still at a visual distance. Since Aiexander Graham Bell's invention of the telephone in 1876, communications experienced meteoric or revolutionary developments. Also, it was Bell himself who invented one of the earliest Light-wave communication devices in 1880, just four years after the invention of the telephone [Il. But incoherent light and its free-space transmission through the atmosphere made the photo-phone impractical for long-distance communications. The advents of lasers in 1960 and low-loss opticd fiber in 1970's eliminated ail these barriers. Introduction 2

The phenomenon of total intemal reflection, which is the basis of guided-wave optics, and of optical fiber communication, was fust dernonstrated by John Tyndall. In late 1969, the concept of integrated optics (10s) was coined by S.E. Müler to emphasize the similarities between planar opticai circuit technology and the well known integrated electronics technology, while in IO wires and radio links are replaced by optical waveguides. It is the technology of integrating various optical devices and components for the generation, focusing cornbining, isolation, polarïzation, coupling, switching, modulation and detection of light, all on a single substrate (chip). Optical waveguides provide the basic constitution of an integrated optic circuit (IOC).

An optical waveguide is a light conduit, consisting of a guiding region like a slab, a strip or a cylinder of dielectric material surrounded by other dielectric materials of lower refractive index. Usually, a waveguide has a three-dimensional structure, as in the case of generd channel waveguides. A p1ana.r waveguide is a structure that is invariant dong one dimension perpendicular to the propagating direction. An optical fiber is a special channel waveguide with syrnrnetric cyiinder core and cladding. Optical fiber waveguide now plays the number one role in optical communication. When we Say optical waveguides, we usualiy mean planar or channe1 waveguides, but not optical fiber. The index profie rnay be thought of as forming an optical 'potential well' with 'energy level' solution at each mode. The Iowest mode travels fastest, almost parallei to the z axis and higher order modes have more zig-zag path lengths and are lirnited by the criùcal angle into the surrounding medium. There are bigger problems of dispersion and loss associated with hiph-order modes. The two most important parameters for a waveguide are the mode field profile and optical Ioss. The former is mainly dependent on the size and refractive index profile of the guide.

Optical fiber communication has advantages of enormous bandwidth, small size and weight, electrical isolation, signal security and system reliability. The cost is continuously decreasing and the capacity is dramaticaliy increasing. Optical communication systems fust deployed in long haul applications over multi-mode fiber at bit-rates of 45 Mbfsec in 1980, rapidly advanced to single-mode system at -500 Mb/sec rates in the mid-1980s, and to Introduction 3 gigabit/sec systems in the late 1980s. Based on the DWDM technique, the signal rate ha reached terabidsec recently. Up to now, the charnel number has been 64 per fiber in practical use, and will soon be more than 100 channels/fiber. It is expected that traffic WUgrow by a factor of 12 from 1998 to 2002[2], mady in Internet and other data communication, while voice communication growth will be very Iimited. The rapidly expending volume of information trfic in modem telecommunication technology requires advanced materials, components and devices to meet the more demanding performance of fiber optic communication system. During the 1980s and the early 1990s, much work had been done, and a lot of excellent techndogy had been dernonstrated while IO was having very little impact in deployed commercial optical communication systems, Integrated optics seemed to be in danger of remaininp exactly a technoiogy of the future, as it was initialiy proposed. Fortunately, di that has drarnatically changed. Red applications now critically depend upon the functionaiity of integated optic components and subsystems, for examples, AWG and LiNbOs modulator and fdter. As stated by Rod Alferness, "For many years, integrated optics was driven by technology and limited by applications; today, applications are driving the field and the growth potential is limited by only the performance and cost of integrated optics components"[3]. Now, in many research laboratories around the world, research groups work on projects related to waveguides for developing inexpensive, efficient and reliable integrated optical components, such as splitters, couplers, switches and multiplexers /demutiplexers.

There are severai methods which may be employed for fabrication of giass optical waveguides, such as thin film deposition, ion exchange process, W-light or x-ray exposure, energetic ion irradiation [4], and more recently, sol-gel [5]. For serniconductor materials, waveguides are usuaily fabncated by MOCVD or MBE thin fdm deposition foiiowed by Iithography and etching. Using ion implantation technique, waveguides have been fabricated in a wide range of materials [6].In Our experiments, fused silica and ZBLAN glasses were used as substrates for waveguide fabrication by proton and helium ion implantation. Readily commercially available fused silica ha a high transrnittance over a wide spectrum and meets the requirements for coupling to the silica optical fiber and fiber-based components. Heavy metal fluoride glass of ZBLAN has low phonon energy and low optical loss in mid-infiared Introduction 4

waveband, It is an attractive material for rnid-infrared optical fiber and laser hosts. A major advantage of ion irnplanted waveguides is that, by a suitable choice of ion energy, a barrier may be positioned at any point beneath the surface. Another advantage is that ion implantation increases the photosensitivity of opticaI materiais. Fluoride-based glasses were initiaily not thought to be photosensitive. In the past few years, photosensitivity of doped fluoride glasses was studied. It was reported that UV induced index change in Ce-doped ZBLALi is about -4~10~[7]. However, photosensitivity in non-doped ZBLAN glass has still not been found [SI. In chapter 5, it will be presented that helium ion implantation into ZT3LAN can induce an index change of about 104. However the study of the photosensitivity of the irnplanted material was beyond the scope of this work.

In this thesis, 1 present Our research on the fabrication of waveguides in fûsed silica and ZBLAN glasses by MeV proton and helium ion implantation, and their optical property characterïzations. It is organized as follow:

In Chapter 1, the structure and theoretical basis of optical waveguides, waveguide fabrication and optical characterization methods are reviewed. Some appiications of waveguide components and devices are also presented-

Then in Chapter 2, the physical basis of ion implantation processes and effects, as well as optical waveguide fabrication by ion implantation are briefly reviewed. Reported experimental results of refractive-index change induced by ion implantation are summarïzed. The simulation tool of TRM code is briefly introduced. Approximate mode solution of implantation induced gradient-index planar waveguide was derived.

Chapter 3 presents the experimentai procedures of waveguide fabrication by MeV proton and helium ion implantation. A discussion about waveguide simulation tool of BPM-CAD is introduced. The waveguide opticai characterization setup and processes is dso described. Refractive-index reconstruction by near-field propagation method foilows. Introduction 5

In Chapter 4, we present the redization of planar and channel waveguides in fused silica by 3 MeV proton implantation with a beam current intensity of about 0.2 pWcm2, and implantation doses of 5x10" ions/cm2 and 1016 ions/cmZ,respectively. We show that, in both TE and TM excitations, the planar waveguide is single mode, and the width of the channel waveguide leads to mukirnode operation. We perform optical characterization and evaluate the loss and the index change. We fmd the results to be in accordance to the literature.

in Chapter 5, we present the first experimental report, based on our knowledge, about the realization of planar waveguides in ZBLAN giass by 4 MeV and by 2 and 4 MeV double helium ion implantation. Previous research on ion implantation effects in heavy metal fluonde glasses focussed mainly on the surface darnage and optical transmittance [9], [IO]. The characterizations of their optical properties, such as optical loss measurement, guided mode profile examination and refractive index profde reconstruction, were perfomed. We show that, the planar waveguides are single mode in the cases of both TE and TM excitations. The results of ion longitudinal range and reconstructed index profile were compared with the results from TRIM calculation and BPM-CAD simulation. Introduction 6

Refiences : [l] A. G. Bell, "Selenium and the Photophone", The Electrician, pp. 214,220,221, 1880. [2] M. Mahan, "Maturing DWDM market expected to soar with increasing capacity demands", Lightwave, pp. 114, lune, 1999. [3] Rod C. memess, "Xntegrated optics: Technology and systern application converge", Optics & Photonics News, p 15, September 1997. [4] S. 1. Najafi, Introduction to GZass Integrated Optics. Artech House, 1992, pp. 114- 127. [5] P. Coudray, J. Chisham, S. 1. Najafi, "Ultraviolet light imprinted sol-gel silica glass low- loss waveguides for use at 1.55 pm". Op?. Eng., vol. 36(4), pp. 1234-1240, April, 1997. [6] P. J. Chandler, L. Zhang and P. D. Townsend, "Optical waveguides formed by ion implantation", Solid State Phen. Vol. 27, pp. 129- 162, 1992. [7] H. Poignant, S-Boj, E. Delevaque, M. Monerie, T. Taunay, P. Niay, P. Bemage and W.X. Xie, "Effkiency and thermal behaviour of cerium-doped fluorozirconate glass fibre Bragg grating", Electronics Letters, vol. 40, no. 16, pp 1339- 1341, Aug. 1994. [8] G.M. Williams, T.E. Tsai, C.I. Merzbacher, and E.J. Friebele, "Photosensitivity of rare- earth-doped ZBLAN fluoride gla~ses"~J. Lightwnve Tech., Vol. 15, No.8, pp 1357, 1997. [9] G. Battaglin, R. Bertoncello, A. Boscolo-Boscoletto, F. Caccavale, I. Lucas, P. Mazzoldi, C. Pledel, P. Polato, "Ion implantation effects in heavy metal fluoride glasses," J. Non- Cryst. Solids, vol. 120, pp. 256-261, 1990 1101 Y. Dai, 1. Yamaguchi, K. Takahashi and M. Iwaki, "Effect of ion implantation and post- treatments on optical transmission of fluorozirconate ghss," Jpn. J. Appl. Phys, vol. 32, Pt.1, NO. 9A, pp. 4026-4032, 1993. Cha~ter1. Optical waveouide: Theo. fabrication and application 7

Optical waveguide: Theory, fabrication and application in optical communication

1.1 Theory of optical waveguides 1.1.1. Electromagnetic mode theory for optical propagation In order to develop a mode1 for light propagation in an optical waveguide, electro- magnetic wave theory must be considered. The bais for the study of electromagnetic wave propagation is provided by Maxwell's equations. For a dielectric medium, these vector + + relationships may be wntten in tems of the electric field E, magnetic field H, electric flux + + density O and magnetic flux density B as :

The four vector fields have relations as foiiows: Chapter 1. Optical waveeuide: Theow. fabrication and application 8

where E is the dielectric perrnittivity and p is the magnetic permeability of the medium. + * Substituting for D and B in equations (1.1) and (l.2), these equations become

Taking the curi of equations ( 1.7 ) and ( 1.8 ) givs

and using the divergence conditions of eqs, (1 -3) and (1-4) with the vector identity

and taking V-Y=O, we obtain the wave equations:

V-1 E-) = p&-a2 d't and + a2G V'H=~&-. d't In equations ( 1.1 1) and (l.l2), V2 is the Laplacian operator, and

where p, and E, are the relative permeability and permittivity for the dielectric medium and poand E, are the permeability and permittivity of free space. In waveguides, electncal and magnetic fields are hnctions of the spatial coordinates, r, and time, t. Equations (1.9) and ( 1.10) can be rewritten as Chapter 1, Optical waveouide: Theorv, fabrication and application 9

For monochromatic waves, the solutions of (1.13) and ( 1.14) have the form

where o is the angular frequency. Substituting (1.15) and (1.16) into (1-13) and (1.14) we obtain

where k =o/c is the free space propagation constant. For convenience, we assume that the wave propagates in z direction. A mode of a waveguide is defined as a field solution of the form

p being the mode propagation constant, then (1.17) and (1.18) become

In some special cases, Le., TE or TM polarization wave, travelling in planar syrnmetrid asymrnetric waveguide, channel waveguide, cylindrical waveguide --- opticai fiber, or polarization maintained optical fiber, the wave equations above can be simplified by symrnetry. Optical waves propagating in waveguides cm be described by these wave equations. However, unlike electrical current that flows through a metal strip according to Ohm's law, optical waves travel in the waveguide in distinct optical modes. A mode, in this sense, is a spatial distribution of optical energy in one or more dimensions. This wiIl be briefly described in the folIowing sections.

1.1.2 Planar waveguide Planar waveguide has a fundamental geometry. The iight guiding region with higher refractive index nl, is asçumed to extend to infinity in the (y, z) directions. And the light confining regions, with lower indices of refraction ni and n3, are assumed to extend to infinity Chapter 1. O tical waveouide: Theory fabrication and application 10

Fig. 1.1. Basic step-index three-layer planar waveguide, in which nz > ni and ns.

in the (tx, y, z) and (-x, y, z) directions, respectively. In this case, the light is confined in one dimension only, which we choose to be in the x-direction. The refractive index n(x) of a planar waveguide and the corresponding modal fields are functions of only this coordinate. By setting alay = 0, infinite extend of mode, one can sirnpliQ the modal differential equations [ 1.11. Equations ( 1.2 1) and ( 1.22) become

Substituting (1.15) and (1.16) into (1-7) and (la$),and by setting a/ay = O, we obtain

A planar waveguide supports transverse electnc (TE) modes with zero longitudinal electric field (Ez = 0) and transverse magnetic modes (TM) with zero longitudinal magnetic field (Hz = O). In the foiiowing, we derive the wave equations governing the two mode types. For TE modes, we have Hy =Er = Ez=O, ( 1.27a ) Chapter 1. Optical wavepuide: Theory. fabrication and application 11

Hx = -(P/ctp))E, Hz = (Jt/ap)ÔE,JÔx, with the Ey component obeying the wave equation a'~,/axZ t [gn2-pz jEy =O For TM-modes, E,, = Hz = H, = O, we have E, = (,O./ae)H,.+ Ez = -(~'/~€)ÜH~/Ôx, with the N, component obeying the wavc equation a%,. ldx' +[kin' =O-

1.1.2.1 The basic three-layer planar waveguide Consider the basic three-layer step-index waveguides structure show in Fig. 1.1. For TE mode, The transverse function Eyhas the generai form

where A, B, C, D, q, h, and p are al1 constants that can be detemiined by matching the boundary conditions, which requires the continuity of Ey and Hz= (i/wp.)aE,,/ax. Since the permeability p and frequency o are assumed to be constant, the condition Hz = (i/q)aE,,/ax translates into a requirernent that aE@x be continuous. The constmts A, B, C, and D can thus be determined by making E, and aE#x continuous at the boundary between region 1 and Region 2 (x=O), and Eycontinuous at x = t. The procedure provides three equations with four unknowns, so that the solution for Ey cm be expressed in terms of a single constant Co Co exp(-qx) ------(OSx4=) ~,[cos(hx)+~~/h)sin(h)]------(-t I x 5 0) ( 1.33 ) Co[cos(ht) + (q 1 h) sin(ht)]e~~[~(x+ t)] - - - -(-+XI-t) Chapter 1. Optical waveguide: Theorv. fabrication and application 12

To determine q, h, and p, we need to substitute (1.33) into (1.27), using the resulting expression of E,(x,z,t) for each of the three regions, the following relations are obtained:

Note in (1.34) that q, h and p are dl given in terms O f the single unknown p, which is the propagation constant in the z direction. By making aEy/ax continuous at x = -t, as required, a condition on p is derived. Taking dEJax from (1.33) and rnakùig it continuous at x = -t yields the condition -h sin(ht) - h(q / h) cos(ht) = p[cos(ht) + (q / h)sin(ht)] ( 1.35) or, after simplification,

tan(ht) = P+4 h(1- pq / h') * The transcendental equation (1.36), in conjunction with (1.34), cm be solved either graphically, by plotting right and left hand sides as a function of /3 and noting the intersection points, or numerically on a computer. Regardless of the method of solution, the result is a set of discrete allowed values of B, corresponding to the allowed modes. For each Pm, the corresponding values of q,, hm and p, can be deterrnined from (1.34). The one remaining unknown constant Co in (1.33) is arbitrary[l.2], depends on the power carried by the mode, and cm be normalized. For the case of TM modes, the development follows the one that was performed for the TE case, except that the non-zero components are H,, Ex,and Ez rather than Ey, H, and Hz. The resulting Eeld components are

H, (x,z, r) = N,(x) exp[i(m - Pz)], i JH, p E, (x,z,t) =--=- H,.(x)exp[i(m - &)]y & GE-

The transverse magnetic component H,(x) is given by Cha~ter1. Optical wavequide: Theory. fabrication and application 13

where h, q and p are again defined by ( 1-34),and where

- rg 4' 4' =74- (1 -39) ni- When boundary conditions are matched in a manner that is analogous to the TE case, it is found that only the allowed values of P for TM modes are those satisfying

where

The constant C' in (1.38) again is arbitrary, and can be normalized.

1.1.2.2 The graded-index planar waveguide Several fabrication processes, in particular ion exchange, UV irradiation and ion implantation, can lead to dielectric waveguide layers with gaded index profiles where the refractive index n(x) varies gadually over the cross-section of the guide. The TE modes in the waveguide with index- profile of n(x), are govemed by the wave equation (1.27) for the E, component d2~y/dx'= [B' - n(x)'k2]~, (1.42 ) If the gradient index n(x) is small enough, the exact mode solutions are avaiiable for some index profiles, for examples. parabolic profile, " lkosh2" profile and exponential profile. In the case of ion implantation, the longitudinal range of the implanted ion is a Gaussian distribution. so it is reasonable to assume that the refractive index profile in the ion stopping range is dso in the Gaussian forrn n(x)= no + h(x) (1-43) Chapter 1. O tical wavepide: Theory. fabrication and application 14 with

The approximate solution wiii be discussed in Chapter 2 (section 2.6)-

1.1.3 Cytindrical waveguide --- optical fiber With radial syrnmetry. an opticai fiber is a cylindrïcal dielectric waveguide made of low- loss materials such as silica glass. It has a core in which the iight is guided, embedded in an outer cladding of siightly lower refractive index. Two factors are most important for opticai fiber: opticai loss, and guided mode profile. As a result of recent technologicai advances in fabrication, the opticai loss is as low as - 0.16 dB/km [1.3]. For the propagating mode field, the cylindrical waveguide is bounded in two dimensions, two integers, l and m, are necessary in order to specify the modes. In cylindricai coordinates, the wave equation (1.17) can be wntten in the scalar form

d2v+--+-- 1dv 1 &+(k2n;-p2)g=0 dr' r dr r' de' where y~ is the field E or H, nl is the refractive index of the fiber core, r and (.I are cylindncai coordinates. The solutions are separable, having the form cosl@ -exp(m -pz) sin l#

The solution given by (1.46) into (1.45) results in a differential equation of the form:

For a step index fiber with a constant refractive index core, Eq.(1.47) is a Bessel differential equation. In the core region the solutions are Bessel functions denoted by JI. The exact solution of this equation involves much aigebra and yields a complex result; and the presentation of this mathematics is beyond the scope of this thesis. A very useful quantity, normalized frequency V, is defined as: V = (2rdh)(d/2)(nr2- ni') '" . (1-48) Where A is the wavelength, d the core diameter, nr and ni the refiactive indexes of core and cladding materiais, respectively. The condition for forming a single- mode waveguide is Cha~ter1. Optical waveeuide: Theory, fabrication and aplication 15

O 5 V < 2,405 ( 1 -49) If X = 1.5 prn, ni = 1.5, and assuming An = 10-~ and An = IO", it is required for single mode condition: d I 17.118 pm and d 5 5.414 pm, respectively. A fiber with typical volues of core diameter of 8 jun, nco, =1.458, cladding diameter of 125 pn and ncladding =1.454, is dehed as a standard fiber.

1.1.4 Rectangular --- channel waveguide The basic rectangular waveguide structure consists of a piding region of index n~ surrounded on ail sides by a confining medium of lesser index. It is not necessary for the index of the surrounding media to be the same in al1 regions. A number of different materials al1 with indices less than ni, may be used to surround the guide. However in tfiat case, the mode in the waveguide will not be exactly symmetric. The exact solution of the wave equation for this general case is cornplicated, and has not been obtained yet [1.2].

Fig. 1.2. Cross-section view of a rectangular dielectric waveguide bounded by regions with lo wer refractive-index.

Marcatilli [ 1.21 [ 1.4) has derived an approximate solution to the rectangular channel wave- guide problem by analyzing the structure shown in Fig. 1.2. The key assumption in his analysis is that the modes are well guided, so that the field decays exponentially in regions 2, 3, 4, and 5, with most of the power being confiied to region 1. The field magnitudes in the shaded corner regions are small enough to be neglected. Hence. Maxwell's equations can be Chapter 1. Optical waveeuide: Theow. fabrication and a~plication 16

solved by assuming relatively simple sinusoidal and exponential field distributions, and by matching boundary conditions only along the four sides of region 1. The wavepide is found to support a discrete number of guided modes that can be groupe into two families of E, and E,. The field components in the five regions shown in Fig. 1.2 (designated by v = 1, 2, 3, 4, 5) have the fom: H,, = exp(-&z + im)M, cos(k,y +a)cos(k,y +P) = exp(-ik,z + ia)M, cos(k, y + a)exp(-ik,, y) H,, = exp(-ik,z + iw)M, cos(k,y +P) exp(-ik,,x) 4, = exp(--2 + iccnt) 4 cos(k,y + a)exp(ik,,y) H, = exp(-ik,z + ia)M5 cos(k,.y +P) exp(-ik,,x)

where Mv is an amplitude constant, o the angular fiequency, 6 the permittivity of free space.

(a) Ey(or Hx)

Fig. 1.3. Sketch of a typical EYoomode, (a) transverse mode, and (b) lateral mode. Chapter 1. Optical wave.mide: Theory, fabrication and application 17

Matching the boundary conditions, the approximate solutions can be reached. The EYil (fundamental) mode is sketched in Fig. 1.3. MarcatiUi's analysis of the rectangular three-dimensional waveguide is very useful in designing such a structure, even though it features an approximate solution to Maxwell's equations. In chapter 4, the mode-profile of a channel waveguide in fused silica fabncated by proton implantation will be analyzed by this approximate method.

1.2 Waveguide fabrication There are several methods which may be employed for fabrication of optical waveguides. These include thin film deposition, dopant atom substitution, epitaxial growth, camer- concentration-reduction, photosensitivity-based irradiation. The different methods are aii airnd at forming a core with a higher index and surrounding it with a lower index substratekover in the case of planar waveguide, or cladding in the case of an optical fiber. Among these methods, thin film deposition and doping-atom-substitution based ion exchange are more popular, mainly used in glass waveguide fabrication, i-e. silica on silicon, or silica on silica; and in most cases, the technique of photolithography is involved. Optical fiber is fabricated by MOCVD followed by fiber drawing. Serniconductor waveguides are usually fabricated by MBE (molecular beam epitaxial) or carrier-concentration-reduction methods. In recent years a new wet method named "sol-gel" was reported[1.5]. Also much attention was focused on relatively low cost polymer waveguide, in order to meet the challenge of the rapidly expending volume of information trafic, and the more demanding cost/performance requirements of fiber optic communication system. In our experiment, we use the method of ion implantation, which is based on both the dopant atom substitution and ion irradiation induced damage. We will discuss this topic in details in chapter 2.

1.3 Waveguide optical characterization Planar and channet waveguide are the most basic elements in integated optical circuit. An accurate knowledge of the optical characteristics of the waveguides is necessary for device Chapter 1. Optical wavemiide: Theorv. fabrication and application 18 design and specification. Foilowing the waveguide opticai characterizations of mode profile, cut-off wavelength, refractive-index profîle, and propagation loss. are briefly summarized.

1.3.1 Mode-profde characterization An optical mode is a spatial distribution of optical energy in one or more dimensions, which corresponds to one of the discrete solutions in an optical wave equation for a given waveguide. In the case of basic three-layer step-index plan= waveguide, for example, the wave equations are reduced to transcendentai equations, eq.(1.36) for TE light incident and eq.(1.40) for TM light incident. The result is a set of discrete solutions of dowed propagation constants p, corresponding to the aüowed modes. The number of guided modes depends on the structure and parameters of the waveguides, such as the syrnmetry of the structure, the guiding region thickness. the index difference between the guide region and the surrounding media, An, and aiso on the operating wavelength. When a given mode is not supported due to some parameter changes, this mode is said to be cut-off. In the case of basic three-layer step-index planar waveguide and TE light incident, the cut- off conditions are given by [1.2], for syrnrnetric guide structure,

and for an asymmelc structure in wkch nz > n3 >> nr,

with ms= (2m+l), m=0, 1,2, 3, ... (1.58) where tg is the guide region thickness, and the incident light wavelength. It can be seen that a symmetric waveguide has at least one fundamental mode (m=O), while it is not the case for an asyrnmetric waveguide. .In chapter 5, when we evaluate the double-waveguide structure in Zl3LAN glass fabricated by 2 MeV and 4 MeV double helium ion implantation, this topic will be discussed in more detail. In the case of optical fiber, the guided mode condition depends on the value of normalized frequency V defined as eq. (1.48), for example the condition O S V c 2.405 corresponding to Chapter 1. Optical waveouide: Theory, fabrication and ap~lication 19 a single mode. Ln the case of an asymmetric channel waveguide, the cut-off condition is usually very complex. In chapter 4, a special condition in asymmetric channel waveguide wilI be briefly discussed.

1.3.2 Index-profile determination From the theory derived above, we see that the refractive-index profde is a very important parameter for a waveguide, and it must be carefully designed before a waveguide is fabricated. The design does not usually pose problems. However du~gwaveguide fabrication, it is extremely difficult to ensure that the pattern exacdy matches the design. It is necessary to characterize the index-profile of the fabncated waveguide. It is aiso helpful to improve the waveguide fabrication controI process. Following are some fiequently used methods for index profile detennination: ( 1) Reflectivity (2) Surface plasmon resonance (3) Incoherent light transmission (4) Pnsm coupling (5)Surface topography with chernical etching --- grating coupling (6)Inversion of the scalar wave equation or propagation-mode near-field method. Determining the index-profile of a waveguide by some of these rnethods may result in damaging the samples, for example, chernical etching. It may dso change the onginal state of the sarnple, for exarnpIe, when coupling light into and out the waveguide through a prism, pressure should be applied, some kind of sofi-material will be damaged; also materials with higher refractive index are not suitable. The method of propagation-mode near field inversion index-profile reconstruction is a non-contact method which is suitable to measure the index- profile in a weakly guiding structure, supporting only the fundamentai mode. In Our experiment, this method was used to determine the ion implantation induced index-profile. The details about this method wiil be described in Chapter 3.

1.3.3 Optical loss rneasurement Optical propagation loss is aiso a very important characteristic of waveguide quality. In any case, the output opticai energy for a waveguide is less than the input energy. The loss cm Chapter 1. Optical waveguide: Theorv. fabrication and ap~lication 20 be caused by absorption, scattering, radiation, nonlinear effects, bends, and so forth. We use the attenuation coefficient, a, to describe the loss in a waveguide; this is defmed by

where Po and Pi are the output and input optical powers, L the waveguide lena&. The techniques for rneasuring the attenuation involve the measurement of transrnitted or scattering iight as a function of propagation distance. The methods used mostly are ( 1) Prism coupling

(3) Scattered light measurement (4) Fabry-Perot interferorneter. In Our experiment, we use scattered light method to measure the optical loss of waveguide. In chapter 3 section 3.3, it will be described in detail.

1.4 Application of waveguides in optical communication Optical amplifiers, especially EDFA, because of their abfity to simultaneously ample multiple wavelength channels, make wavelength-division-rnultiplexing OM) a cost- effective way to upgrade the transmission capacity of existing fiber communication system. ConsequentIy the applications of WDM are driving the field of integrated optics. htegrated optics covers a wide variety of device types, made using a wide array of materials, most cornrnonly LiNb03, silica on silicon, ion exchange in glass, III-V semiconductors, and polymers. Following we briefly present some of the most important waveguide devices based upon three kinds of materials that have been widely used in optical fiber communication.

(A) Silica waveguides --- Arrayed Waveguide Grating (AWG) for DWDM Silica-based integrated-optics has been used to demonstrate a wide variety of passive circuits (splittedcornbiner) and some active (thermo-optic) components, currently its most important component is the arrayed waveguide grating router(AWG or WGR). Multiplexer/Dernultiplexer are key components for construction of dense WDM systems. Several kinds of multiplexer have been developed for practical use. These include inter- Cha~ter1. Optical wavemiide: Theo~.fabrication and application 21

ference-filter-type, fiber-grating-type, and planar-lightwave-circuit (PLC)-type multiplexer/ demultiplexers. The fmt two types include other components such as lens and circulators or couplers, respectively. Therefore, when such multiplexers have a large number of channels, i.e, for DWDM, they require many components and so become highly complex devices.

Fig. 1.4. Structure of an Amy Waveguide Grating (AWG), which cm provide wavelength division multiplexing/demultiplexing or routing.

For PLC-type multiplexers with many channels, compactness is achieved by using PLC fabrication technology which incorporates a photo-lithographie technique. AWG, which is a kind of PLC-type multiplexer, is one of the most attractive components for WDM systems, because of the great flexibility with which its filter response can be designed. Its elegant, highly functional waveguide circuit combines a passive NxN splitter, an array of waveguides with carefully designed path lenad differences, and an NxN combiner[6], as shown in Fig. 1.4. By appropriate design of the path length differences in the waveguide array, the effective dispersion of the array is controlled so that when light from the array waveguide is mixed by the second combiner, the wavelength components are spatially separated and emerge sequentially out the waveguide output ports. Reports have aiready been published on a 128 channels AWG, a flat passband AWG, an unequal channel spacing AWG, and a variable bandwidth AWG. Recently, the commercial AWG demultiplexers with 32 channels, have a range of modules providing a cost-effective method for multiplexing and demultiplexing wavelengths, are issued by PIRI(Photonics Integrate Research Incorporation) and Norte1 Opto-Electronics. The main features are: 32 channels available (40 channels are aIso Chapter 1. Optical wavepide: Theory, fabrication and a~~lication 22 available), 100 GHz or 200 GHz channel spacing, compatibility with ITU grid specifications, fuily hermetic packaging, compact package with interna1 cooler.

(B) LiNb03 --- amplitude or phase moduiator, switch array and filter The simple formation of waveguides in LiNb03 by titaniurn doping was demonstrated by AT&T Beii Labs and gave rise to the establishment of low-loss waveguides in a material with relatively high electro- and acousto-optic materid coefficients [ 1-71. The range of LW03IO devices is already very large, and more applications constantly present themselves. Devices of low complexity include discrete optical switches, amplitude and phase modulators, and fiber optical avoscope sensor elements. More advanced devices are programmable wavelength fikers and WDM transmitters. Now the LiNb03 devices have been demonstrated with amplitude modulation bandwidths of up to 40 GHz and integation levels of 150 switch elements per chip with -40 dB cross-tdk. LiNb03 based optic acousto- optic tunable filter (AOTF) is one of the fast tunable filters over a wide range in the 1.5 prn region. Current AOTF devices have the features of a wavelength range from 0.6 to 2 p, tuning range up to 200 nm at 1550 nm, narrow and variable bandwidth from 1.5 to 10 nm at 1550 nm.

( C ) DFB Waveguide-based semiconductor WDM lasers A significantly more elegant solution that using selective area epitaxy to create active/ passive IOCs on inP was demonstrated in the early 1990s. Selective area epitaxy is a significant break-through in the potential of InP integrated optic circuit, especially very well suited to fabricare IOCs that include a laser and rnodulator. Low-chirp extemal modulation is essential to overcome dispersion Limitation. Using integation to both stable single frequency FDB (distributed feed-back) laser and low-chirp moduiator on the sarne chip offers the required functionality in a single package --- a redization of the originai IO version. Fig.l. 6 shows an integrated laser/modulator realized using selective area growth. The moduIator is of the electro-absorptive type, Le., a voltage-controlled attenuator, based upon the quantum- confined Stark effect (QCSE). Sirnilar to the classical Franz-Keldysh effect in buk material, application of an applied field in the multiple quantum well modulator shifts the bandgap to Chapter 1. Optical waveguide: Theory. fabrication and a~olication 23

higher values to reduce its transparency. The QCES is very strong, allowing hi& extinction over short moduiator lenaes (-150 p)with ody a few volts. In order to be cost effective, it is necessary to fabricate rnulti-wavelength laser transmitters by monolithic integration on one chip to reduce the cost and increase the efficiency of packaging and coupling to the fiber communication system. As early as in 1993, an OIC of this monolithic integration type, strained LnGaAsP MQW device of frequency-division multiplexed ten-channel tunable FDB laser array with space range of 10 GHz and charnel linewidth less than 2.3 MHz, had been fabricated by Sato et ai [ 1.83.

€A Modulator Section 1 p-lnGaAs/f nP Cap

- n-lnP Substrate MOCVD Grown \ MQW-SCH InGaAsP Grating

Fig 1-5. An integrated laser/modulator realized using selective area growth.

Work on IOCs application has continued over the years, with more recent work being directed toward 1-3 pmand 1.5 pmwavelen,a;ths for optical-fiber communication system. As we stated earlier in the introduction part, for many years integrated optics was driven by technology and limited by applications; today, applications are driving the field and the growth potential is limited by only the performance and cost of integrated optics components. Chapter 1. Ootical waveouide: Theoq. fabrication and application 24

References [l.11 T. Tamir, Integrated Optics. 2ndedition, Springer-Verlag, p33, 1979. [1.2] R.G. Hunsperger, Integrated Optics - Theory and technology, Springer, p3843, 1995. [1.3] B.E.A. Saleh, M.C. Teich. Fundamentals of Photonics. John Wiley & Sons, ùic, 199 1, p273. 11-41A.A.J. Marcatilli: Bell Syst. Tech. J. 48,2071(1969). [ 1-51 P. Coudray, J. Chisharn, M.P. Andrews, S-1. Najafi, 4cUltravioletlight imprinted sol-gel silica glas low-loss waveguides for use at 1.55 p".Opt. Eng. 36(4) 1234-1240(April 1997). [1.6] C. Dragone, C.A- Edwards, and R.C. Kistler, "htegrated optics NxN multiplexer on silicon". IEEE Photonics Technology Letters. Vol. 3, No. 10, (October 1991). [1.7] R-V. Schmidt and LP. Karninow, "Metal diffused optical waveguides in lithium niobate," Appl. Phys. Lett. 25(8), 458-460(1974). [1.8] K. Sato, S. Sakine, Y. Kondo, M. Yamamoto: IEEE J. QE-29, 1805(1993). Chapter 2. Optical waveguide fabrication bv ion implantation 25

Optical waveguide fabrication by ion implantation

Ion implantation is one of the methods for inducing permanent refractive-index change, and hence forrning optical waveguides. As early as in 1968, Schineller et al first reported that ion implanted waveguides were produced by proton implantation into fused silica[2.1]. Since then, much work have been dom in this field, and using this technique, waveguides have been fabricated in a wide range of materids[2.2][2.3]. A major advantage of ion irnplanted waveguides is that by a suitable choice of ion energy, a barrier rnay be positioned at any point beneath the surface. It also follows that two barriers rnay be constmcted at different depths. This process may be used to improve the opticai isolation of a single guide by burying it away from the surface, or aitematively, two or more guides rnay be created in proximity. Such devices rnay be used simply to carry one guide beneath another (eg. to act as an interconnector) or else to forrn guides which are opticdiy coupled in the vertical direction. In combination with lateral proximity by means of masking, a three dimensional matrix of guides might thus be produced, In this chapter, we review some notions on waveguide fabrication by ion implantation: basic processes and effects of ion implantation, and induced photosensitivity. At the end of this chapter, permanent index changes in various materials by different ion implantation conditions are summarized. Chapter 2. O~ticalwaveguide fabrication by ion imolantation 26

2.1 Implantor and the basic process of ion implantation An ion implanter is an ion-accelerator. It usually consists of six parts: ion source, accelerating system. andyzer, bearn control system, sample charnber, and vacuum system. The desired ions are generated in the ion source, then accelerated through on accelerating system. Usually an ion source generates many kinds of ions simultaneously, so an anaiyzer is needed to select the desired ions by a strong magnetic field. In order to have a homogenous ion implantation on the substrate, a scannhg device is usiiaily needed. The process must be carried out in vacuum, and controiled fiom a distance. There are two accelerators in the Accekrator Laboratory of the Physics Department at Université Laval: one is the Van de Graaff accelerator with the maximum energy of 7.5 MeV; the other, a Kevatron with the energy between 20 keV to 150 keV. When an energetic ion is implanted into a continuous matrix (target material), it wiIl lose its energy by ionkation, recoiling, exciting phonons, and causing point defects, then it wl stop in the material at a certain depth. Before an implanted ion stops. it causes a senes of nuclear collisions, called a nuclear collision cascade. This process also produces a heatmg effect to the implanted substrate. The path of an implanted ion is in a zig-zag pattern; and the nuclear CO llision cascading region is in a pear shape, as shown in Fig. 2.1.

lmplanted ion O Atom of substrate material

Fig. 2.1. Pear-shape nuclear collision cascade process of ion implantation into solid material. Chapter 2, Optical waveguide fabrication bv ion implantation 27

The depth of penetration of the irnplanted ions depends on their mass, and energy, as well as on the substrate material, temperature and orientation (in the case of a crystalline substrate). The statisticai ion distribution (range) for low doses is almost Gaussian in shape and quite narrow, showing practically no point defects tail extending into the surface region. For high doses, some effects such as sputtering, inner-stress increment and chemicaI component change, must be considered, thereby the range deviates Gaussian with the increase of the tüse, typicaily a half-Gaussian.

2.2 Transport and Range of Ions in Matter (TRIM) By using the Monte Car10 simulation package of TRIM (Transport and Range of Ions in Matter) [2.5], in which incident ions are followed individually through their colfisions in the materiai, all the important information about the physicai effects can be caiculated. The ranges of ion species such as proton, helium, silicon and germanium, implanting into the substrates of fused silica (density, D = 2.202), a-quartz (D=2.648) and LiNb03 (D=4.629), were calculated with TRIM92. Some results are shown in Table 2.1. Range is the peak position of ion distribution in substrate. Ion travels in matter of a zig-zag path, and the straggle describes the radiai deviation of incident ions in the substrate. In the travel path, the incident ions will also ionize the atoms of host matenial-- ionization, excite elastic waves --- phonons, and induce vacancies. In some cases, incident ions coliide with host atoms, then transport thern in the opposite direction. The details of TRIM calculation results, such as range, straggle, ionization, phonons, vacancy and recoil, for 2 MeV proton implantation into fused silica (2.202 @cm3) are shown in Table 2.2.

Table 2-1. TRIM calculation results for the ranges with ion species of proton, helium, and germanium implanted into fûsed silica, a - quartz and LiNb03-

Proton Longitudinal Range ( pm ) Ion Energy Fused Silica cx - quart^ LiNbOs 100 ( kev ) 1.O3 0.866 0.647 1.0 (MeV) 15.3 12.7 9.8 1 2.0 45 -2 37.6 28.8 3 .O 87.8 73 .O 55.4 Chapter 2. Optical waveguide fabrication by ion imvIantation 28

Ion Energy 1 Fused S@ca a - Qum LiNb03 100 (keV ) 1 0.853 0.715 0.48 1.0 (MeV) 3.71 3.10 2.38 1 2.0 (MeV) 7.19 5.98 4.62 3.0(MeV) 1 11.5 9.62 7.41 I

1 Ion Energy ( MeV ) 1 Fused Silica OC - Quartz LiN'bO3

Table 2.2. TRIM calculation results for proton implantation into fused silica (2.202 g/cm3) Ion Energy: 1.O MeV Range Straggle Longitudinal 15.3 pm 4651 A Lateral Proj. 4776 A 6727 A Radial 7621 A 6606 A Vat-/Ion 22.0 Energy Loss (%) Ions Recoils Ionization 99.83 0.03 Vacancies 0.00 0.00 Phonons 0.04 O. 10

Fig.2.2. TRIM caiculation results of 2 MeV proton implantation into fused silica : (a) Longitudinal Range, (b) Phonon distribution. Chapter 2. Optical waveuide fabrication bv ion implantation 29

(Continued from Fig.2.2)

-1

Fip.2.2. TRIM cdculation results of 2 MeV proton implantation into fused sirica : (c) Ionization, and (d) vacancies.

The curves of longitudinal range, phonon, ionization, and vacancies via depth for proton implanting into fused silica are shown in Fig. 2.2- It cmbe seen that the range increases with ion energy, decreases with the atomic number of the ions and the density of the substrate.

2.3 Permanent refractive-index change and photosensitivity 2.3.1 Ion implantation induced permanent refractive-index change The requirernent for the existence of a waveguide in an optical material is that a region of relatively high refractive index should be surrounded by regions of lower index. The effects of various types of radiation on optical materials, for examples, UV-light exposure, x-ray irradiation, and ion implantation, have been extensively studied. Permanent refractive index changes induced by ion implantation have been found in a wide range of materials. The mechanisms can be aise from substitutional dopant atoms, lattice disorder, defect center trapping, or in a combined manner, which is highly dependent on the type of materials being irradiated 12-61. Alternatively, the modification of the refractive-index can be assigned to two basic damage mechanisms: a) the extensive nuclear damage that occurs at the end of range of the ions and b) the ionization that occurs between the irradiated surface and the end of range. Ion implantation can induce dopant atoms which substitutionally replace some of the host stoms in the substrate, bringing about a permanent refractive-index change. Since the new ions have different polarizabilities, sizes, and rnobilities, the refractive index changes in the Chapter 2. Optical wave.mide fabrication bv ion inarilantation 30 doped regions. In the case of crystal materïals, the implanted ions also cause lattice defects and disorder. Basicaüy there are three separate contnbutions to the refractive-index change: from the change of ionic poIarïzability, from the volume change of the glass, and from the stress effect[2.7]. In the section 2.4, implantation parameters via opticai propew changes WU be bnefly discussed; and in section 2.5, some experimental results of refractive-index modification in several optical matenais, induced by ion implantation, wiil be summarized.

2.3.2 Ion implantation into glass and photosensitivity As early as in 1968, Schineller et al frrst reported that ion implanted waveguides produced by proton implantation into fused silica[2.1]. The cornmon effect of implantation of ions into glasses is the formation of point defects, eg, E' and B2-center (O-vacancy types), density/refractive index changes due to volume difatation, surface stresses and hardness changes. As a consequence, it ha. been shown that collisional effects dominate in fused silica. These defects can be partly removed by annealing. Table 2.3 shows some typical defect models in silica glass and their absorption bandsC2.81. In 1978, K.O. Hiil et ai. Fust reported that the refractive index of germanium-doped silica optical materials cm be modified permanently by exposure to UV-lightC2-91; and the terni photosensitivity was introduced. The typical index change values are of the order of 104 -10- in this case. An X-ray irradiation can also induce permanent index change in the UV end. Ion implantation in fused silica can lead to the formation of strong UV absorption bands and to an increase in refractive index of - 10'' near tlie surface[2.4]. Another very important phenornenon, "photobleaching", is that W irradiation at a waveIen,gh close to one of the induced absorption bands, Light from a KrF excimer laser at 248 nm or from an ArF excimer laser at 193 nm, for example, can bleach the absorptions and reduce the refractive-index by 10" [2.10][2.11][2.12]. The first observation of photosensitivity in bulk silica doped by germanium ion implantation was reported by Albert et al [2.10]. Ge-doped silica is one kind of photosensitive glass material. Its photosensitivity is attributed to the extrinsic centers associated with Ge impurities, those are absent in fused silica. Ion implantation can photosensitised fused silica by producing color centers in the UV in large numbers, either Chapter 2. Optical waveguide fabrication by ion implantation 31 because of the actual chemical presence of the dopant ions and more generaiiy because of the radiation damage caused during their implantation. Even in the case of having no Ge ion implantation, the chemical presence of the dopant Ge cm greatiy improve the photo- sensitivity. because ion collisions make much of the impurities of presented Ge resulting in color centers [2.11]. Recently Essid et al reported that 5 MeV Si ion implanted Ge-doped silica increases the refractive-index up to 2% for a dose of 1014 ions/cm2. Three strong absorption bands were found in the regions of 5, 6.5 and 7 eV due to B2, GEC (Ge electron center), GE', and GODC (Ge oxygen deficient center), respectively. Photo bleaching of the absorption bands by both the ArF and KrF excimer lasers leads to a negative refractive-index variation of the order of -105, and the details of the process are different for each laser -- the ArF being the most efficient[2.12]. A thorough understanding of this physical photosensitive phenornenon is stiU lacking [2.11].

E' center 5.8 eV (21 3 nm)

Oxygen hole center(0HC) 2.0 eV (630 nm) or non-bridging OHC

Peroxy radical or superoxide radical -$-@-@ 7.6 eV (1 63 nm)

Peroxy linkage Chapter 2. Optical wave.ouide fabrication bv ion hphntation 32

2.4 Implantation parameters and optical property changes 2.4.1 Substrates Opticai rnaterials cm be grouped into two categories: crystdline materials and amorphous matenais. Typicaily, pure arnorphous materials have lower density than the corresponding crystalline materials. Ion implantation into some amorphous dielectric materials like silica leads to an increase of its refractive-index at the end of the ion range. However in some cases, other effects may dominate the index change. For examples. He+ implantation into fused siiica increases the index, while Hef implantation into soda-lime glass, phosphate, fi uoroaluminate and silicate glasses, decreases the refractive-index [22] [2.13], In the case of crystdline materials, ion implantation generaily produces a decrease in the refractive-index change. which is mainly induced by ion amorphization. Usuaily, multiple energy implantations are required to define a high-index waveguide region by creating low- index boundaries. a-quartz is the crystalline state of silicon dioxide with the density of 2.648

0 During ion implantation. quartz is gndudy converted into arnorphous siiica, producing an index change typicaUy of An - -10-~.Under low temperature implantation of nitrogen ions, index change as high as -5 % was obtained [2.2].

2.4.2 Ion energy and species The distance between the implantation range and the irradiated surface increases with ion energy and decreases with the atomic number of the ion, that is, heavy ion implantation haç a shailow range while light one has a deep range. This can be seen from the TRIM. calculation results shown in Fig. 2.2 and Table 2.1. The refractive index modification mechanism is dso highiy dependent on the ion species. For inert gas ion implantation, it changes the refractive index mainly because of the radiation damage, whiie implantation of Ge, Si, N into silica are acting through the radiation damage, the density change and their chernical presence. In soda lime glass, light ion implantation causes the sodium and calcium ions to move inwards from the surface region ( the index changes reported were a moderate An -5% [2.2]), but for heavier ions the sodium appears to be lost outwards from the surface. Cha~ter2. Optical waveguide fabrÏcation by ion implantation 33

2.4.3 Ion beam density, temperature and implantation dose In the process of ion implantation, the energetic ions continuously are forced enterring the target matenal, and deposit al1 the energy into the substrate. Most of the energy is converted into thermal energy, which heats the substrate. The higher the accelerating energy and the beam density used, the greater the heating effect will be, The substrate temperature induces changes in the optical property through modification of densiq, bonding energy, atomic difussion and migration, saturation of dopant, and lattice parameters in the case of crystaiiine materials. In many papes, it was indicated that the implantation was camed out at room temperature(RT). In these cases, the real temperature on the sarnples were unknown. Even when cooling the target-holder, the samples can be melted by high energy beams. Yunfei Zhao, et al, had derived a semi-empiricd equation to estimate the real temperature on a thin film target during ion beam bombarding[2.14]. During ion implantation, the growth curve of refractive index change in the nuclear darnage region is a pronounced S-shape due to strong initial collaborative mechanisrns giving a 'threshold' effect, foilowed by eventual saturation. For low doses the barrier is almost Gaussian in shape and quite narrow. This barrier broadens asymrnetricaily with further increasing dose. Up to very high doses the surface region shows practicaily no index change.

2.4.4 Mask In the case of non-planar waveguide fabrication by ion implantation, a mask must be used. Such masks must have relatively high aspect ratios to si,dficantly block the ions, and even partial penetration of ions through a mask can lead to higher losses and other problems with fabricated waveguides [2.15]. The typical order of magnitude of the mask dimensions are tens micronmeters in width and several millimeters in length.

2.5. Surnmary of some experimental results of index changes in several optical materials induced by ion implantation Table 2.4 shows experimental results of index changes in several optical materials induced by ion implantation, with different implantation parameters and post-treatment conditions Chapter 2. Optical waveguide fabrication by ion implantation 34

[2.2][2.3][2.13]. It cm be seen that the typical refractive-index change induced by ion implantation is in the order of

Table 2.4. Summary of some parameters and properties induced by ion implantation. Subst. Index Index change Loss dBfcm h=0.633 p.rn An- %

a-quartz

Fused siiica

2.1738 (n,) 2.2797 (n,)

Pyrex &B-SiO2 Fluoroaluminate glas Phosphate glas Silicate glass

The method of ion implantation presents several disadvantages. For example, to fabricate a channel waveguide of a size of about 2-10 Pm, a high energy accelerator with energies of MeV is needed, except in polymer materials. Especially for heavy-ion implantation, energies above 5 MeV is usually required. Thus this increases the cost seriously, and limits its use. The problem rnay be partly overcome by pre-embossing the substrate in the guide-forming region, so that it can decrease the requirement of higher accelerating energyC2.161. Also the multi-energy implantation can be replaced by using a high current hot ion source with somewhat wider energy dispersion (no analyzer is then equiped), by this way it cm greatly increase the implanting efficiency (for amorphous optical materiais). Cha~ter2. Optical waveguide fabrication bv ion imlantation 35

2.6 Index-profile and mode of the graded-index planar waveguide In the case of positive change of refiactive-index induced by ion implantation, the planar and channel waveguide fabrication processes are shown in Fig.2.3. The waveguiding regions are the same as the doped and damaged regions in the substrate. In the case of the negative change, the planar waveguide fabrication process is the sarne as for the positive change, but the waveguiding region is between the surface and the dopecüdamaged region, as shown in Fig.2.4 (a). To fabricate a channel waveguide in the case of negative index change, ion implantation must be carried out at least two times with two energies: the fust tirne to define a planar waveguide, then the second tirne to define the channel by using a he mask, as shown in Fig-2.4.

-- m 1 Dielectrical Material1 Dielectricai Mate rial

Fig.2.3. (a) Planar and (b) channel waveguide fabrication processes in the case of positive refi-active-index change induced by ion implantation.

Fig.2.4. Planar(a) and channel (a and b)waveguide fabrication processes in the case of negative rehctive-index change induced by ion implantation. Chapter 2. Optical wavemiide fabrication by ion implantation 36

As stated in the previous sections of this chapter, ion implantation leads to dielectric waveguide layers with graded-index promes where the refractive index n(x) varies graduaily over the cross-section of the guide. For low doses the bamier is almost Gaussian in shape and quite narrow. This barrier broadens asymmetricaily with further increase in the dose. For high doses, some effects such as sputtering, inner-stress increment and chernical component change, must be considered. Thereby the range deviates from Gaussian with the increase of the dose, typicaiiy a half-Gaussian. Up to very high doses, the surface region shows practically no index change. It is extremely diffxcult to determine the exact refractive index profüe formed by ion implantation, An(x,y,z). In the condition of limited implantation dose, it is reasonable to assume that the refractive index profile in the ion stopping range is also in Gaussian form

n(x)= n, +An, exp where xo is the standard deviation (xo' is the variance), and we have chosen the ion stopping center as the zero coordinate in x. The values of the mean and the variance depend on ion species, ion energy, and the properties of the target material. The TE modes in the waveguide with an index profile of n(x), is governed by the wave equation (1-27) for the Ey component dZ~,/dx'= [p2 - n(~)~k']~~ (1.27 ) The exact solutions are not available for waveguides with this type of index profile. While the approximate solutions of these modes obtained using a paraboiic profile can be adapted to serve as approximations if the gradient of n(x) is small enough. Usually the index change profde is not sharp, in other word, it is broad; that means that

I/xa « 1. The exponentiai term in eq42.1) cmbe developped as :

Negiecting the terrns higher than order 2, eq.(2.2) is written Chapter 2. Optical waveouide fabrication by ion implantation 37 or n(x)=nC(1-x2/2xf) with n,=no+An,md

The index profile of this paraboiic form corresponds to the potential well of the harmonic osciiiator, and the solutions of the wave equation are

E, = H,. (Ar / W)exp(-x2 / w21, where w' = (&hmc), which indicates the degree of confùiement of the fundamental mode. The Hv are the Hennite polynominals defined by

For the lowest orders we have

H,(x) = Sx, (2.9)

The parabolic profile corresponds to a closed well and predicts an infinite set of discrete modes. But as the order v increases, the energy of a mode is spread out filrther from the guide mis, and eventudy the distances are so large that a parabolic profile cari no longer be regarded as a good approximation for the actual guide profile[3.17]. In the case of smaii gradient index of n(x), if we assume that only the fundamental mode exists, we have

E,, = exp(-x' / w'). (2.1 1) In Chapter 4, we will see that in the case of planar waveguide fabricated by proton implantation in fused silica, only a fundarnental mode exists. Equation (2.1 1) gives a good approximate solution of the mode in the center guided region. For high implantation dose, higher order terms in equation (2.2) should be accounted for. And for very high dose, there wiil be a saturation of the index change, and the Gaussian approximation could no longer be used. The solution descnbed in this section will no longer be appropriate. Chapter 2. Optical waveouide fabrication by ion impiantation 38

References [2.1] E. R. SchineiIer, R. P Ham, D. W. Wilmot, J. Opt. Soc. Amer., pp1171,58, 1968. C2.21 P. J. Chandler, L. Zhang and P. D. Townsend, "Optical waveguides formed by ion implantation," Solid State Phen., pp. 129-162, Vol- 27, 1992. [2.3] P. D. Townsend, "Optical effects of ion implantation". Rep. Prog. Phys. p501, a,1987. [2.4] J. Albert, B. Mdo, K. O. Hill, D. C,Johnson, J. L. Brebner, R. Leonelli, "Refractive- index changes in fused silica produced by heavy-ion implantation followed by photo- bleaching" Opt. Lett., pp. 1652-1654, ~01.17,No. 23, 1992. [2.5] J. F. Ziegler, J. P. Biersack, U. Litîmark, The Stopping and Range of Ions in Solidr. Pergarnon, New York, 1985. 12-61R.G. Hunsperger, Integrated optics, lheory and Technology. Springer, pp.55, 1995. [2.7] S. 1. Najafi, Introduction tu Glczss Integrated Optics. Artech House, pp. 88, 1992. [2.8] Sophie LaRochelle, "'Ongin and applications photosensitivity in Germanium-doped silica optical fiber". Ph.D. thesis, The University of Anzona, p63, 1992. [2.9] K.O. Hill, Y. Fuji, D.C. Johnson and B.S. Kawasaki, Appl. Phys. Lett., 647,32, 1978. [2.10] 1. Albert, K.O. Hill, B. Malo, D.C. Hohnson, J.L. Brebner, Y. B. Trudeau, and G. Kajrys. Appl. Phys. Lett. 60(2), 13 Januq, 1992. [2.11] M. Verhaegen, L.B. Allard, J.L. Brebner, M. Essid, S. Roorda, J. Albert, Nucl. Instr-and Meth. In Phys. Res. B 106,438-441, 1995. [2.12] M. Essid, J.L. Brebner, J. Albert, K. Awazu, Nucl. Instr-and Meth. In Phys. Res.0 B 141,pp.438-441,1995. [2.13] P. D. Townsend, P. J. Chandler, and L. Zhang, Optical Effects of ion implantation, Cambridge University Press, pp. 205-235, 1994. [2.14] Yun-Fei Zhao, Shu-Zi Sun, and Shijin Pang. "Study of the temperature of thin film under ion beam bombxding" . SPIE 1519, P411-414, 199 1. [2.15] S .J. Field, D.C. Hanna, A.C. Large, D.P.S hepher, A.C. Tropper, P.J. Chandler, P.D. Townsend, and L. Zhang. Electron. Lett., pp2375-2376, 1991. C2.161 D.M. Ruck, S. Brunner, K. Tinschert, W.F.X. Frank, NIM, B 106,447-45 1, 1995. 12-17]T. Tamir, Integrated Optics. 2"* edition, Springer-Verlag, pp54-55, 1979. Chapter 3. The experiment description 39 .

Experimental procedure

In this chapter, the experimental procedures of waveguide fabrication by MeV proton and helium ion implantation, waveguide simulation tool of BPM-CAD, the waveguide opticai characterization setup and methods, are described. Refractive-index reconstruction by near- field propagation method is also discussed.

3.1 Glass sarnple preparation Two kinds of glass materials were used in Our experiment: fused silica and bulk i313LAN. Their properties will be described in chapter 4 and 5, respectively. The glasses were fustly cut into suitable size by using the cut-saw of h Logîtech Mode1 15 (Materials Technologists & Engineers). Then the sarnples were polished wiîh the polishing machine of Buehler Ecomet El Polisher/Grinder. For each sarnple, three surfaces were polished: the main surface for ion implantation, and two side surfaces for coupling the beam in and out of the wavegude formed by the implantation. In order to obtain perfect sharp edges between the three surfaces, the main surface was fxst polished with a large round holder. Then the two sides were polished with a polishing holder where the sample is placed in a sandwich configuration. In the case of ZBLAN glas side polishing, because it is very soft, a special kind of plastic tacky sheet was used for protecting the main surface from being damaged. The qualities of the polished sarnple surfaces were exarnined by using an opticai microscope with a rnagnif~cationof 400 times; and in some case, the surface height profiles were measured with a Dektak UA height-profilorneter. Chapter 3. The experiment description 40

3.2 TRIM. Calculation In order to induce optical bamers of suitable dimensions by ion implantation, and hence to optimize the waveguide design, the parameters such as range and straggle of proton and helium ion implanted into the substrates of hsed silica and ZBLAN, were calculated with TRIM92 at frrst. The details of the results will be described in the sections of 4.2 and 5.2, separately. The optical confinement of a waveguide is a function of waveguide structure and the refiactive-index profile. For fabricating waveguides in dimensions of several microns, implantation energies of MeV are required. There are two ion implantors available at Université Laval: Kevatron(-120 keV) and Van de Graaff(-7.5 MeV). The TRTM calculation results suggest the use of the Vmde Graaff accelerator in our experiments.

Al.EfiNATEUR

CE COLONNE

, f '-,- SONOE DE MESURE WMPE AUX:UAIRE &WES DE SfA61USATION DU CHAI-1P (FINM)

Fig.3.1 .The diagram of the Van de Graaff accelerator at Université Laval. Chapter 3. The experiment description 41

3.3 Ion Implantation (A) Implanter In Our experirnent, a high-energy Van de Graaff accelerator in the Accelerator Laboratory of the Physics Department at Université Laval, was used. It has maximum acceleration energy of 7.5 MeV. The diagram of the accelerator main parts: ion source, accelerating system and analyzer, is shown in Fig.3.1. The beam that cornes fiom the analyzer passes through tlne beam control system and then reaches the sample chamber. The total control system is Located in another room, at a distance from the accelerator, (B) Sampte holder with sine scanning device and mask The p~lishedsarnples should be carefuliy fmed on a sample holder which protects the three polished surfaces of the sample being damaged, also conducting the heat and the charged io-ns away from the sample. In order to get hornogenous ion irradiation, we designed and fabricated a mechanical sine scanner, and fixed the sample-holder on it. In the case of channel waveguide fabncating, a mask with an adjustable slit was fixed on the sample-holder which is irri front of the glass sarnple. (C) Ion implantation dose The implantation dose is a parameter that should be rnentioned in di the ion implantation experiments. It can be determined by the following equation

where Q = ixt is the total electric charge, i the beam current in ampere(A), t the implantation time in seconds (s), and S the implanted area in square centimeters (cm'). The ratio of i/S is usually referred to as beam density, and it is a key parmeter to thermal-sensitive implantation experiments. However it is easy to understand that an approxirnate estimation of implantation dose is enough, and trying to calculate exact implantation dose is impossible.

(D) Ion species, energy and beam density Two kinds of light ions, proton and helium, were used in Our implantation experiments. Proton implantation with 4 MeV was carried out to fabncate plana. and channel waveguides in fused silica glass. Heiium ion implantation with 4 MeV and 2 MeV was carried out to Chapter 3. The experiment description 42 fabricate planar waveguides in ZBLAN ffuoride glass. Helium ion implantation should not induce chernical interaction between the implanted inert gas ions and the atoms of host materiais. Therefore the induced refractive-index change should mainiy result fiom the radiation damage. As stated earlier, the ion range increases with ion energy and decreases with the atomic number of the ion, So light proton implantation has a deeper range while helium ion has a shailower range. For the Van de Graaff accelerator at Université Laval, the beams with energy between 1.5 MeV and 4 MeV are more stable, and a stronger current density could be reached, hence more efficiency. Moreover, for helium implantion with the energies in this range into ZBLAN glass, the waveguide has an optirnized structure. However in the case of MeV helium ion implantion into ZBLAN glass, in order to avoid darnaging the glass, ion beam density and implantation time must be controlled during the implantation experiments, so that the samples did not be overheated under the high eliergetic ion bearn bombarding. Dunng our implantation experiments, a mechanical sine scanner was used to decrease the bearn density (by means of increasing irradiation area), and the ion beam was periodically stopped by the bearn-viewing screen Iocated just in front of the chamber. More details about proton and helium ion implantation into fused silica and ZBLAN substrates wili be discussed in Chapter 4 and Chapter 5.

3-4 Wavepuide characterizations The optical properties of implanted samples such as propagating guided modes, optical losses, cutoff wavelengths, optical intensity distribution of the modes, were characterized. The optical loss evaluation and the near-field mode-profde measurement for refractive index reconstruction are mainly carried out at the wavelength of 0.6328 pm from a He-Ne laser. The cut-off wavelength of the ZBLAN sarnples were evaluated by detecting guided light at wavelengths of 0.78 p,0.80 pm, 0.85 pm, 0.98 jun, 1.06p1, 1.3 p and 1.5 p.

3.4.1 Mode-profile characterization and the experimental setup The experimentai setup to observe the near field optical mode profdes is shown in Fig.3.2. It consists of a laser source, microscope Ienses, an adjustable sample holder, a CCD camera, a Chapter 3. The experiment description 43

Vidicon camera, a video monitor, a cariera controller with an A/D converter or a frame @ber, and a computer.

Potarizer

Laser source Cornputer

Fig.3.2 Schematic of experimental setup for coupling laser into the waveguides and rneasuring the mode profdes and opticai loss.

The Light out-coupled from the end-face of the waveguide was imaged onto a camera, see Fig. 3.2. The image on camera was recorded by a frame grabber, and then converted into a digital image. In our experiment, two kinds of carneras were used: a vidicon carnera for infrared (IR) Iight, and a Cohu CCD camera (model 48 12-7000) for visible wavelengths. The CCD carnera is controlied by a Spiricon Laser Bearn Analyzer (mode1 LBA-100A), with an Automatic AV linear converter. The recorded CCD output in beam-analyzer fdes was then transferred to an ASCII code file by a Labview program. Finally the data analysis of recorded CCD results was carried out by using the computer software Origin 5.0. A Panasonic Color Video monitor model CT-1384Y was used to view in-situ the near-field output. Color images of near-field outputs were recorded by a Panasonic CTV-CCD, and were converted the images to the digital light intensity profdes by a Matrox Rainbow Runner frame grabber; linear/surface plots were then carried out by Scion Image.

3.4.2 Cut-off wavelength evaluation The cut-off wavelenghs weze evaiuated mainly for the ZBLAN planar waveguides fabricated by helium ion implantation. The experimental setup is the sarne as that for near- Chapter 3. The experiment description 44

field profile measurement. In this setup, the laser sources are coupld to the waveguide following the sene S ource+Lens+Fiber+Fiber+Lens+Waveguide or, in the case that the source has a pigtaii output Source with pigtaii Fiber+Fiber+Lens+Waveguide The two fibers are connected by FCPC connects. When a different wavelength source is needed, the only thing that needs to be done is to connect the two fibers, and leave the remaining parts unchanged. By this way, we cm distinguish the guided-mode disappearance due to waveguide-cutoff from changes in the coupling condition. In Our experiment, the waveguides fabricated in fused silica by proton implantation was characterized by detecting the guided light at only two waveIene&s: 0.6328 pm fkorn a He- Ne laser and 1.5 p from Photonetics wavelength tunable laser diode source (mode1 TUNICS-BT). As wiU be detailed in chapter 4, the almost-symmetric waveguides were single mode at both these wavelen,oths. The cut-off wavelength of the fundamental mode of the asymmetric waveguides in ZBLAN was also evaluated by injecting wavelengths of 0.78 pm to 0.85 pn from Ti:Sapphire laser, 0.98 jun and 1.3 prn wavelengths from semiconductor lasers, and 1.06 p form YAG laser.

3.4.3 Optical loss measurement by scattered-light method The attenuation or propagation loss for optical waveguides has proved to be one of the most important factors in bringing about their applications. It is a function of physical- chernical composition of the waveguide material and the waveguide structure. The optical transmitted power in a waveguide can be expressed in the absolute value

PO = Pi(exp(-~L)1 (3 -2) where Pi and Po are the input and output optical power of the waveguide, respectively, L the length of the waveguide, and y the iinear loss coefficient. In optical communication, the attenuation is usually expressed in units of decibels per unit length defined as following: G~*L= 10*10,0~~(Pi/P~)- (3.3) Chapter 3. The exoeriment desdotion 45

where &B is the attenuation coefncient in decibels per unit length, usually dBlcm for opticai waveguides. The optical los for aii the waveguide samples fabricated by ion implantation in our experiments were measured by a scattered-light method [3.1]. This method aliows to determine the propagation losses of a waveguide non-destnictively and to observe light propagation in devices such as directional couplers, Y-branches, modulators, switches, and defkctors. The principle of the measurement is simple. In an optical waveguide. a guided mode continuously loses a small part of its power by Rayleigh scattering. This scattered power is proportional to the total guided power. Thus the scattered light decreases exponentially along the waveguide. The light streak from the optical waveguide is detected by the CCD camera, and then plotted via the propagation distance. By fitting the measured scattered power to a decreasing exponential function, the attenuation coefkient is obtained. The accuracy depends on the sensitivity of the detection system. The loss measurement experimental setup in our experiment is almost the same as that for near-field profile measurement. There are two ditfcrences, (1) The additional two cyluidrical lenses to expaud the incident beam and duce its divergence, and (2), for grabbing the scattered light, the CCD camera is moved above the sample, as shown in Figure 3.2. The digital CCD recorded results of scattered iight were treated by using the cornputer software 0rigi.n 5.0, for example, by plotting the power-depth curve, fitting the curve to an exponential function, and fïnally cdculating the optical loss of the waveguide. The excitation of radiation modes, coupling losses and end-refkctions also affect the scattered light near the two ends of the waveguide. The data nom these regions are unusable to do loss calculation.

Fig.3.3. Beam waist-- He-Ne Laser couphg into chamel waveguide in hed silica. Chapter 3. The experiment description 46

3 -5 Refractive index reconstruction by near-field propagation method

The refiactive-index profile An(x,y,z), is one of the most important properties of an optical waveguide. In the case of ion-implantation induced wa~e~pides,the index profde rneasurement of a waveguide is very difficult because of the narrow guiding area and low refractive-index difference. In fact, it is tme for al1 optical waveguides. Many approximate methods were developed, such as inverse WKB, and propagation-mode near-field method- The first two method requires muiti-mode effective refractive-index measurement in order to have a high precision, based on prism or reflection techniques [3.2]. The refractive-index profüe reconstruction of optical waveguide from the near-field measurement is a weil established technique 13.31-13-63. It requires the conditions of (i) that the waveguide is weakly guiding; (ii) only the fundamental mode is excited, It has very lirnited success in step-index or sharply discontinuous profile reconstruction. This technique has been successfully applied to waveguides induced in fused silica by ion implantation [3 -53. While in the case of single ion implantation induced negative graded index change, it can not be applied. For the three planar waveguides in ZBLAN formed in Our experiment, ody the lower one formed by double He+ implantation meets the conditions above, and its refractive- index profile is suitable to be reconstructed by the propagation-mode near-field measurement. The other two contain sharp refractive index discontinuities at the surface. Theoretical expression for the relation between the refractive-index change and the near- field intensity I(x) cm be found in references[3.3]-C3.51. In the following section, we review the expressions derived for the implantation-induced three-dimensional optical waveguide and two-dimensional planar waveguide. In chapter 5, we will derive the expression for the relation between I(x) and An(x) for the case of planar waveguides formed by two negative refractive-index barriers.

(A) Three-Dimensional Optical waveguide It is assumed that the permeability of the medium is equai to that of a vacuum and the refractive-index difference is very small. Maxwell's equations are changed approximately into the scalar wave equation Chapter 3. The experiment description 47

V:-y~~,y)+{k~n'(x?y)-~~}~~x,y)=o. (3-4) We can easily get the expression of the refractive-index profile from (3.4) as [3.4]

The Fust tenn on the right-hand side of (3.5) is an unknown constant. However, we can uniquely determine the difference distribution of the squared refractive index, because the constant does not affect the difference distribution. We derive the expression of the refractive-index profile using the near-field intensity P(x,y), which is proportional to the square of a transverse field v(x,y). The second-order partial derivatives of v(x,y) with respect to x and y are derived using the optical intensity P(x,y) as follows:

a2w(x,Y) - 1 ay2 - ~P(x,Y)I ("'a;))} Applying (3.6) and (3.7) into (3.5), we get the index profile expressed by

We cm obtain cross sectional of refractive-index profde at rnost three dimensional waveguides by means of (3.5) or (3.8).

(B) Two-Dimensional Optical waveguide For a TE mode planar waveguide, the refractive-index profile can be expressed as C3.51

where Y(x) is the transverse field and k, =- the free-space wave nurnber; n(x) the 4, refractive index, and p the propagation constant. Introducing n(x) = ne + An(x), where ne is the background refractive index of the implanted optical material, and making the substitution for intensity, I(x)= y2(x), yields: Chapter 3. The experirnent description 48

1 d'JrOrO h(x)=---- P' n, 2n& 2 2nBk,?Jm dx'

The first two terms on the right-fiand side of (3.10) and (3.1 1) are unknown constants. However, we cm uniquely determine the refractive index profile from the square root intensity, because the constants do not affect this profile. In the case of positive refractive index change induced by ion implantation, assuming that in the regions away from the index-change center, the index gradually becomes the same as that of the background, n(x)=nB, or h+O, we can introduce a constant

(3.12) Applying equation (3.12) into the equations of (3.10) and (3.1 l), we get the index profile expressed by

they are proportional to the second-order derivatives of square root transverse intensity. A similar expression cm be derived for the case of implantation induced negative index change, which will be described in the section 5.5. The digital CCD recorded data of guided-mode output were also treated by using the cornputer software Origin 5.0, for example, doing square-root and difference calculations, plotting the power-depth curve. In the case of planar waveguide, an averaging is perfonned over the lateral dimension in the data array within a small region. However the plotting- curve-smoothing is forbidden, because it induces artificial data in the final results. Also averaging over a longer region wiU induce pseudo-data into the caiculation which will also affect the final results, and should be avoided. Chapter 3. The experiment description 49

3.6 BPM-CAD design and simulation The simulation package of BPM-CAD (Bearn Propagation Method, Optiwave Corporation, version 3-O), was used before and after wave,ouide fabrication. Based on the TRIM calculation results, we use BPM-CAD to design and optimize the waveguide structure design. In reverse, it was used to choose implantation parameters. After a waveguide had been fabricated, the software was used to simulate and to compare the experïmental results. In any case, the index change induced by ion implantation is in graded shape. Even at the sample surface, ion irradiation makes the index-step less sharp than in its virgin state. In our experiments, al1 the BPM-CAD simulations in the index change regions were canied out by talcing three-step index profile approximation, that is, each step has an index difference of A n/3 with a thickness of FWJ3M/3. In sections 4.5 and 5.5, we will describe BPM-CAD simulation with the three-step approximation in details.

References [3.1] S. 1. Najafi, Introduction to Glass integrated Optics. Artech House, 1992, pp. 114- 127. C3.21 R.G. Hunsperger, Integrnted optics, Theory and Technology. Springer, pp. 37, 1995. [3.3] K. Morishita, "Refractive-index-profile determination of single-mode optical fiber by a propagation-mode near-field scanning technique," J. Lightwave Technol., vol. LT-1, pp. 445-449, Mar- 1983. [3.4] K. Morishita, "Index Profiiing of Three-Dimensional OpticaI Waveguides by the propa- gation - Mode Near-Field Method," J. Lightwave Technol., vol. LT-4, No. 8, pp. 1 120- 1 124, Aue~~st1986. [3 S] M.L. von Bibra and A-Roberts, "Refractive Index Reconstmction of Graded-Index Buned Channel Waveguides from their Mode Intensities," J. Lightwnve Technol-, vol. 15, No- 9, pp. 1695-1699, September 1997. [3.6] L. McMaughan, E.E. Bergmann, "Index Distribution of Optical Waveguides from Their Mode Profile", J. Lighhvave Technul., vol. LT- 1, No. 1, pp. 241-244, March 1983. Chapter 4 Waveguide fabrication in fused silica bv proton implantation 50

Waveguide fabrication in fûsed silica by proton implantation

In our experiments, fused siiica was used as the substrate for waveguide fabrication by proton implantation, mainly because it is a well characterized and readily available material with low loss. It cm meet the requirements for coupling to the silica optical fiber and fiber- based components. It has a high transinittance over a wide spectrum, a low thermal expansion coefficient, and is resistant to scratching and thermal shock. Hence it is widely used in the optical industry. It is apparent that ion implantation has a potential in the field of opticai commercial application, as its appkability extends over a very wide range of materials, and it is able to produce device geornetries which are not possible using other fabrication techniques. As early as in 1968, the first ion implanted waveguides was realized by proton implantation into silica glass [4.1] - The irradiation of silica glass with protons has been shown to produce an increase in the index of refractive (An) with typical values of +1% and +2% for implantation at temperatures of 300 K and 77 K, respectively. A further enhancement in refractive-index to An = 4%has been produced by the use of chernically active ions such as p.Silica was found to suffer compaction under the influence of nuclear damage and with a slight contribution fkom electronic effects. This produced enhanced index optical wells with very low attenuation Chapter 4 Wavenuide fabrication in fused silica bv proton implantation 51 levels. The damage efficiency is quite high, requiring ody -10'~ ions/cm2 to produce a saturation change of An +2% at 77 K or -1 % at 300 K [4.S].

4.1 Fused silica glass The composition of typical fused silica is pure Si02. The density of fused silica is about 2.202 g/crn3. The thermal properties are: a glas temperature of 1273 K. a melt temperature of 1983 K, a heat capacity of 0.746 JfgK, a thermal expansion coeficient of 0.51~10-~/K and a thermal conductivity of 1.38 W/m.K [4.3]. The dispersion formula is as foiiow

within the range of 0.2 1-3 -71 p,where h is the optical wavelength and n the refnctive index. The refractive-index values of fused silica at wavelengths of 0.6328 pm and 1.55 p are 1.4570 18 and 1.4440, respectively.

Experimental .. \ 1,

t v-absorption Waveguidc n ns \ - imperfections

Fig. 4.1 The attenuation spectrum for an ultra-low-loss single-mode fiber.

The attenuation spectrum for an ultra-low-loss single-mode fiber is shown in Fig4.1. Narrow transmission windows exist in the long wavelength region around 1.3 and 1.55 p. The lowest attenuation is 0.2 dB/krn at a wavelength of 1.55 p,which is usually referred to Chapter 4 Waveguide fabrication in fused silica bv proton implantation 52

as the third optical communications window. Between these two windows there is an OH absorption peak, In Our experiment, the glas material that we used is Coming 7940 optical hiseci sika. Its trammittance is certified at 280% betweem 185 nm and 2220 nm for 1 cm thickness.

4.2 Proton implantation into fused sïlica 4.2.1 Proton implantation In both implantation experhents of planar and channel waveguide fabrication, 3 MeV proton implantation was carricd out ushg Van de Graaff high-energy accelerator, with a current density of about 0.2 pA/cm2. In the est implantation expriment, a planar waveguide in fused silica glass was fabncated by 3.0 MeV proton implantation a dose of 5x10~~ionslcm2. In the second experiment, a channel waveguide was fabncated by 3.0 MeV proton implantation with a dose of 1x10'~ions/cm2. In order to defuie a channel waveguide, a mask with adjustable dits was designed and fabricated. In this experiment, we adjusted the width to about 15 p;the mask was fixed in fiont of the glas surface.

I I 1 I I 1 . O 20 40 60 80 100 Depth (micron)

Fig.4.2 Nomalized range, ionization and vacancy of 3 MeV pro ton implanting into fused silica glas as function of the dep th fkom sample sdace, calculated by TRIM. Cha~ter4 Wavemide fabrication in fused silica bv moton implantation 53

4.2.2 TRM calculation By taking the density value of 2.202 glcm3,the longitudind range, straggle and vacancy of 3 MeV proton implantation into fuseci szca glass were calculated by TRIM, yielding values of 87.7 pm, 2.27 pm and 44 vacancieshon, respectively. Fig.4.2 shows the normalizcd ion distriiution, ionkation and implantation induced vacancy via the depth from sarnple surface, corresponding to the 3 MeV proton impIantation into fùsed silica.

4.3. Planar waveguide fabrication 4.3.1 Near-field mode-profile measurement The near-field mode profile of the fabricated planar waveguide was evduated at the He-Ne laser wavelength of 0.6328 p,with the experimcntal setup as shown in Fig. 3.2. A single mode with a width of about 6 jun was found. The waveguide was located at a distance of about 75pbeneath the surface. Fig.4.3 shows the TE mode optical near-field output. A white light ïilumination on the top of the sample was used to reveal the surface for the photograph. The waveguide depth indicates that the ioduced refiactive index change, An, is positive, since the guided region is almost at the end of the ion range, as shown in the diagram of Fig.2.3 (a). The TE and TM mode near-field outputs and their corresponding surface plots are shown in Fig. 4.4. Both of the TE and TM excitation gave single modes.

Fig.4.3, TE mode optical near-field output at 0.6328 pm wavelength of a phar waveguide in ksed silica fabricated by 3 MeV proton implantation. Chapter 4 Wavepide fabrication in fused silica bv oroton implantation 54

( Fig. 4.4 (a)(b) TE mode near-field output and surface pIot; (c)(d) TM mode near-field output and surface plot, for the phar waveguide in fused silica fabricated by pro ton implantation.

Fig.4.5 TE mode optical near field output and the corresponding surface intensity plot for the planar waveguide in fused silica, at a 1.55 p wavelength.

The guided mode was also evaluated at the wavelengths of 1.55 p.The TE mode optical near field output and the corresponding surface intensity plot are shown in Fig.45 It is stiU in single mode, but the output-profile is broader. Based on the cut-off waveIength analysis presented in Chapter 1, in the case of ion implantation inducing a positive index change, the resulting alrnost-symrnetric waveguide tias at least one fiindamental mode. By assuming a cut- off wavelength for the TE0 mode smaller than 0.633 p and with the guided-region thickness approximation of 6 Pm,the rehctive index change is estimated to be less than 1x IO-^. Cha~ter4 Wavemiide fabrication in fused silica bv proton imolantation 55

4.3.2 Optical loss evaluation

The optical loss of the waveguide was measured at the He-Ne laser wavelength of 0.6328 pm, by the scattered light method which has been described in cbapter 3.4.3. The experiment set-up is shown in Fig.3.2. Fig. 4.6 (a) shows the recorded digital optical intensity pronle of scattered light on the sample's surface, and (b) the corresponding curve of logarithm decreasing optical intensity, lO~logloI(x), versus the propagation distance in cm. The propagation distance is equal to the length of the waveguide, that is 9.65 mm. By linearly fitting this curve, the optical loss of this planar waveguide was calculated, giving an optical loss of about 2.5745 dB/cm. Table 4.1 shows the linear regression results.

Eig. 4.6 Optical intensity profile of scattering light on the surface of phar waveguide and the corresponding curve of the logarithm of optical intensity, lO~log&x), versus depth.

Table 4.1, linear fitting for planar waveguide

Linear Regression for DataSB: Y=A+B*X

Parameter Value Error ------A 14.7903 O. 06053 B -2.5746 0.09421 Chapter 4 Waveguide fabrication in fused silica bv proton hlantation 56

4.3 -3 Refkactive-index reconstruction Since the ion-implantation induced waveguide with positive rehctive index change does not contain sharp refractive index discontinuities, the rehctive index pronle can be reconstructed by pro pagation-mode near-field method. Indeed, for the planar waveguides formed in our expcriment meet the required conditions of (i) a weakly guided mode; ci) excitation of only the foundamental mode. We used near-field propagation method to reconsûuct the rehctive-index profile of the planar waveguide fabricated by 3 MeV proton implantation. Fig. 4.7 (a), (b) and (c) show the TE mode near-field opticai output at wavelength of 0.6328 pm, the corresponding intensity surface plot and line plot of this planar opticai waveguide.

4W(W ! , - , . , . , . i O 10 P 50 40 50 Dlstanœ at arbiry referenca (micron)

Fig. 4.7, (a)TE mode near-field optical output of the planar waveguide fabricated by 3 MeV proton implantation at 0.6328 Pm wavelength, (b)(c) the corresponding intensity surface plot and iine plot, and (d) the curve of the last term in equation (3.10) versus the depth. Cha~ter4 Waveguide- fabrication in fused silica bv vroton implantation 57

Fig. 4.7 (d) shows the curve of Iast term in equation (3.10) versus the depth fiom sample surfze- The pcak value of the second term in Fig. 4.7 @) is 2.7x104, and Co = -2.3~10~.We thus have: An(x), = 2.7 x 104 - (-23x 104) = 5x 104

4.4. Channel waveguide fabrication 4.4.1 Near-field mode-profile characterization Channel waveguide in fused silica was fabricated by 3 MeV proton implantation. Fig.4.8 shows view of TE modes optical near-field output of the waveguide evaluated at the 0.6328 pm He-Ne laser wavelength. Multiple guided modes were found with both of the TE and TM excitations, located in a depth about 75pfkom the sample surface. Fig.4.9 shows the near- field outputs of the higher order TE modes and their corresponding mtensity surface plots. Fig.4-1 O shows one, two, three and four horizontal TM mode optical near-field outputs of this waveguide and corresponding intensity surface plots.

The near-field mode-profiIe measurement of this waveguide was &O carried out at wavelength of 1.55 p.The TE mode near-field outputs are shown in Fig. 4.11. The guided mode was alrnost reduced to a single mode.

Fig.4.8 View of TE mode optical near-field output of channel waveguide m fiised silica fabricated by 3 MeV proton implantation, evaluated at 0.6328 pm wavelength. Chaoter 4 Wavermide fabrication in fused silica by proton imlantation 58

Fig.4.9, TE modes (Tl&, TEi, ï&, TE3, TES) optical near-field outputs at 0.6328 pm He-Ne laser wavelength of the charnel waveguide in fused silica. Chapter 4 Wavemiide fabrication in iùsed silica bv got ton implantation 59

Fig-4.1O, TM modes m,TM[, TA&, TM,) optical near field outputs at 0.6328 pm He-Ne laser wavekngth of the channel waveguide in fùsed silica. Cha~ter4 Waveguide fabrication in fused silica bv moton implantation 60

Fig.4.11, TE mode optical near-field outputs of channel waveguide fabricated in fused silica

by 3 MeV proton implantation, evaluated at 1.55 jîm wavelength.

4.4.2 Optical los s evduation The optical loss of the channel waveguide was measured at the He-Ne Laser wavelength of 0.6328 p,by scattered light method. Fig. 4.12 shows the recorded digital optical intensity prome of the scattered Light on the sample's surface and the corresponding curve of the Iogarithm optical intensity, lO~log~~I(x),versus propagating distance. The propagation distance is equal to the length of the waveguide, that is 11.0 mm. By linearly fitting this curve, Cha~ter4 Wavemiide fabrication in füsed silica bv proton klantation 61 the optical loss of the channel waveguide, was calculateci to be about 2.075 dB/cm. Table 4.2 shows the linear regression results,

Fig.4.12, Optical intensity pronle of the scattered light on the surface of a channel waveguide and the comsponding curve of the decnasing opticd intensity, 10xlogl&x), versus depth.

Table 4.2, linear fitting for channel waveguide

Linear Regression for Data2-B : Y=A+B*X Parameter Value Error

4.4.3 Refiactive-index reconstruction Xn this channel waveguide, it contains small step of rehctive-index discontinuities which result fiom the lateral Bnplantedvirgin material margins. In vertical direction, the waveguide does not have such small step rekactive-index discontinuities, and mats the weakly guided condition for the rehctive-index pronle reconshiction by using near-field propagation method. The coupling condition also shouid be carefuily adjustecl so that only the fundamental Cha~ter4 Waveguïde fabrication in fiised silica bv proton blantation 62 propagation mode is excitd Fig. 4.13 shows the TE mode near-field optical output of the waveguide at wavelength of 0.6328 pm, the corresponding vertical intensity plot, and the reconstructed refractive index profile versus the depth. The pealc index change is approxmiately AE~X~O-~.

X AU (no diadim 1O)

6.0015!, . . . . , . , . , . 1 O 5 1O 15 20 25 30 35 Distance at arbitrary referenœ (micron) O FigA. 13, (a)TE mode near-field output of the waveguide at 0.6328 p@)the corresponding vertical intensity plot, and (c)the reconstnicted refractive index profile.

The value above is near the maximum rehctive index change, in other words, saturation index change, in fused silica glass induced by proton implantation at room temperature (300 K). It is in accordance with the typical value reported by Chander, et al. [4.2]. In section 4.3.3, the refiactive index profile reconstruction gave the peak value of 5x lo4 in the case of planar waveguide with dose of 5x10~~ionslcm2. It is Lower than the typical value for this dose, -IO-) [4.2]. The error may be imluced by horizontdy data averaging. Chapter 4 Waveguide fabrication in fused silica by proton implantation 63

4.5 B PM-CAD simulation The refkactive index changes in fused sllica induced by proton implantation were shuhted by using BPM-CAD with threc-step 5-layer index profile approximation. Fig. 4.14 and Fig. 4.15 show the simulation results of planar and channel waveguides with wavelengths of (a), 0.6328 p,and (b) 1.55 p.The simulation results are all single mode.

1 ct -' . O 2

r> ci :1 CI I -r C> O 3t i 1 :l 7 O O .! -/ a/ 4 Lb tdLi1 (/~rrl) ~1c1ti-l (,#111) (a) (b) FigA 14. Planar waveguide with wavelengths (a) h= 0.6328 pm and (b) 1.55 pm.

(a) Cb) Fig.4.15. Channel waveguides with wavelengths (a) h= 0.6328 pand @) 1.55 p.

In the chaxmel waveguide simulations at 0.6328 pn, the waveguide is single mode over a wide range parameters like index change, thickness and widths. In some cases, no modes were found. The experirnental multirnode results were produced by changing the injection conditions of the input beam. These conditions cannot be simulated with BPM-CAD. This simulation software therefore has its iïmits in cornplex situations. Reference t4.11 E. R. Schineller, R. P b,D. W. Wilmot, J. Opt. Soc. Amer., 58, ppll7 l(l968). [4.2] P. J. Chandler, L. Zhang and P. D. Townsend, "Optical waveguides formed by ion impIantation," Solid State Phen., VOL 27, pp. 129- 162, 1992. C4.31 M. Bass, Handbook of Optics. McGRAW-HILL, INC, 1995, vol- IL, 33.55, Tab. 19&23. Chapter 5. Planar waveauide fabrication in ZBLAN by He' implantation 64

Planar waveguide fabrication in ZBLAN by ~e+implantation

Low phonon energy and low optical loss make heavy metd fluoride glasses an attractive material for mid-infrared optical fibers and laser hosts, eg., opticai fiber amplifiers and fiber lasers. The material ha. recently becorne of major interest for telecommunication application[5-11. Standard flurozirconate gIass, ZBLAN (53% ZrF3-20% Bafi4%LaF3-3% AlF3-30% NaF), has a long wavelength multiphonon edge at 8 pm and a phonon energy of 574 cm-'. It offers an advantage over silica, by decreasing the non-radiative ernission probabilities of the rare-earth ions. Waveguide fabrication in fluoride glasses by several methods, for example, anionic exchange process and vapor phase deposition, has been reported 15-21-l5.41. Previous results on ion implantation effects in heavy metai fluoride glasses were reported, conceming mainly the surface darnage and optical trammittance [5.5], [5.6]. In this chapter, we report planar waveguide fabrication in ZBLAN glass by MeV helium ion implantation. The waveguides are characterized by optical loss rneasurement, cutoff wave-length evaluation for propagating guided modes, and the exarnination of optical mode intensity distribution. The refractive-index profile was reconstructed by near-field propagation method. The results of ion longitudinal range and reconstmcted index profile were compared with the results from TRIM calculation and BPM-CAD simulation. This work was published at an internatinal conference [ 5.7 1. Chapter 5. Planar wavemide fabrication in ZBLAN bv He+ implantation 65

As stated in chapter 2, the mechanism by which the rehctive index is modified is highly dependent on the ion species and the type of materials being irradiated. In the case of helium ion implantation, it changes the refiactive index mainly because of the radiation darnage and the effect of stress. For crystalline materials, HeC implantation decreases the refractive index. However for some amorphous materials, Hef implantation increases the index in fused silica and lead germanate glasses, but it decreases the index in silicate, phosphate and fluoro- aluminate giasses[5.8]. In this chapter, we show that heiïum ion implantation into ZBLAN gass also leads a negative index change.

5.1 ZBLAN glass The standard flurozirconate, ZE%LANglass has the typical composition of 56% -44% B&-6%LaF3-4% m3-20% NaE The density is 4.52 &m3. The thermal properties are: a glass temperature of 543 K, a melt temperature of 745 K, a heat capacity of 0.520 J/g.K, a thermal expansion coefficient of 17.5~10~IK and a thermal conductivity of 0.4 W/m.K [5.9]. Fig.5.1 shows the transmission loss spectnun of rnultimode ZrF4-based fiber, in which the broken line is the spectrum of OH-free silica fiber [5.10].

0.1 ' 1 2 3

Wavelength ( p m)

Fig.5.1 Transmission loss spectrum of multimode ZrF4-based fiber, in which the broken line is the spectrum of OH-free silica fiber. Cha~ter5. Planar wavewde fabrication in ZBLAN by Hef hmlantation 66

The dispersion formula with the range of 0.50-4.8 pm is as foliows

where h is the optical wavelength and n the mfiactive index. The cwes of rehctive index and chromatic dispersion of ZBLAN glass are plotted m Fig.5.2. Table 5.1 @ves the rehctive index values at several wavelengths used in our experiments.

Fig.5.2 Refiactive index and chromatic dispersion of ZE3LAN glass versus wavelength.

Table 5.1. Typical refiactive index values of ZBLAN at several wavelengths

-- 1 Wavelength X (pm) 1 Refiactive index n 1

5.2. MeV helium ion implantation into ZBLAN 5.2.1. He+ implantation In the first implantation experiment, 4.0 MeV Het implantation with a current density of about 0.2 pA/cm2 was carried out with a dose of 1016 ions/cm2 onto the ZBLAN glass. In the second experiment, to define two rehctive-index change layers, Hef implantations with Cha~ter5. Planar waveguide fabrication in ZBLAN by Hef klantation 67 energies of 4.0 MeV and 2.0 MeV were pedormed with a current density of about 0.2 pA/cm2 and a dose of 2x10'~ ions/cm2, onto another ZBLAN glass sample. In order to have homogeneous irradiation and to avoià damaging the ZBLAN glass, a sinusoidal mechanical scanning device was used. This also provides a means to disperse the ion charge that can cause deflection of the hcoming ion beam, and to dissipate the heat produced by the high-energy ion beam that can cause sample fiachiring, due to the glass high thermal expansion of 17.5~10~/K and low thermal conductivity of 0.4 W/mK p.91. In one of our implantation experiments, the ZBLAN glass was broken and molted at the surface, as a msult of exposure to the helium ion at an intensity of 2 pNcm22

5.2.2. TRIM calculation By tahg the density value of 4.52 @cm3 [5.11] and the atomic ratio approximation of 75F:13Zr:7Na:SBa, the longitudinal range and straggle corresponding to 4 MeV and 2 MeV helium ion implantation mto Zi3LA.N were calculated by TRIM, having the longitudinal rangdstraggle values of 11 .O p.rd354 nm and 4.96 w245 nm, respectively, as shown in Table 5.2. Fig.5.3 shows the normalized ion distribution as a fimction of the depth fkom the sample surface, corresponding to the helium ion energies of 2 MeV and 4 MeV, respective1y.

-vm -w-v-1-1 O $ i - i 8 1 O 12 14 Distance (m icron)

Fig.5.3. Normalized ion range calculated by TRLM. The peaks correspond successively to the helium ion energies of 2 MeV (red) and 4 MeV (black), respectively. Chapter 5. Planar wavemide fabrication in ZBLAN by HeC implantation 68

Table 5.2. Longitudinal range and straggle comsponding to 4 MeV and 2 MeV helium ion implantation into ZBLAN, calçulated by TRIM 1 Ion Energy (MeV) 1 Longitudinal Range (p)1 Straggle (MI) I

5.3. Planar waveguide fabricated by 4 MeV He+ implantation 5.3.1. Near-field mode-profile characterization The experimental setup used to observe the near field optical mode profiles is show in Fig.3.1. Fig. 5.4 shows the TE and TM mode near-field outputs at wavelength of 0.6328 pm, and their correspondmg surface plot of plauar waveguide in ZBLAN fabricated by 4 MeV He' implantation. Both the TE and TM modes are single mode.

Fig. 5.4 TE mode near-field output in ZBLAN:a) CCD picture and b) surface intemity plot ; TM mode near-field output :c) CCD picture and d) surface intensity plot. Cha~ter5. Planar waveguide fabrication in ZBLAN by Het implantation 69

The TE mode near-field output with vertical illumination and its intensity surface plot for the sample with 4 MeV He' implantation, are shown in Fig.5.5. A planar single guided mode with the thickness of guided region about 8.8 pwas found. located just beneath the surface. This iudicates that the induced rehctive index change, An, is negative, and the guided region is between the surface and the end of the ion range, as shown in the diagram of Fig.5.5 (d).

Fig. 5.5. TE mode near-field outputs with vertical illumination to show the surface : a) and b). the corresponding surface intensity plot : c), and d) tbe diagram of waveguide structure with negative refiactive index induced by 4 MeV He+ implantation. Chapter 5. Planar wavemide fabrication in ZBLAN bv He+ implantation 70

The guided mode was aIso found at the wavelengths of 0.85 p and 0.98 pn, but not found at 1.3 pn and 1.5 p. The waveguide is cut-off at a wavehgth ktween 0.98 pm and 1.3 pr~Based on the cut-off wavelength calculation of a three layer asymmetric waveguides C5.121, the refractive index change was evaluated between -2.6~10~and -4.6~10~.

5.3.2 Optical loss evaluation The optical loss of the waveguide was measured at the laser wavelength of 0.6328 CM, by the scattered light method which has been descn'bed in chapter 3. The experiment set-up is also presented in section 3.4.3. Fig. 5.6 (a) shows the recorded digital optical mtensity profile of scattered Light on the sample surface, and (b) the corresponding curve of logarithmically decreasing optical intensity, l0xlogld(x), versus propagating distance. By hearly fithg thk curve, the optical loss of this planar waveguide was calculated, yielding an optical loss of about 1.77 d8/cm. Table 5.3 shows the linear fitring results.

Fig. 5.6. (a) Optical intensity of scattering light, (b) the corresponding curve of optical intensity decreasing versus propagating distance, and (c) the curve without the fkst 0.1 cm. Chapter 5. Planar waveguide fabrication in ZBLAN bv He+ implantation 71

Table 5.3, Luiear Regression for Data in Fig.5.6------Y=A+B*X Parameter Value Error ------A 18.77457 0.08768 B - 1.77305 O. 16936

At the entrance of the waveguide, radiation modes are excited. Fig. 5.6 (c) shows the logarithm opticai intensity curve without considering the start 0.1 cm. Linear fitting gives an optical loss value of about 1.07 dBfcm. The absorption induced by irradiation in the direction perpendicular to the substrate is also rneasured in transverse illumination, having an opticai loss of about 2.9 %. This is in agreement with the results from Dai, et al, that implantation clearly reduced the transmittance[5.6]. It is believed to be mostly due to surface defects and irradiation damage-

5.4. Planar double waveguide structures induced by 2 MeV and 4 MeV Hef implantation 5.4.1. Near-field mode-profile evaluation The near-field mode profile measurement for refractive-index reconstruction are mainly carried out at the He-Ne laser wavelength of 0.6328 p for the TE mode only. The cut-off wavelengths of two waveguides were evaluated by detecting the guided light at waveIen,bths of 0.78 p,0.80 pm, 0.85 pn, 0.98 pm, 1.06 pm, 1.3 p and 1.5 p. Fig. 5.7 shows the CCD picture of near-field TE mode output at a wavelength of 0.633 p, and the corresponding intensity surface plot for the sarnple with double HeCimplantation of 4 MeV and 2 MeV. Optical output with double planar single guided-mode was found and the top guided layer is also j~stbeneath the surface. The thickness of the total guiding region is about 10 Pm, and the distance between surface and the first optical intensity minimum is about 5.1 p.This indicates that the two waveguide regions are formed by the surface and two negative refractive index changes with double barrier structures at the end of the ion ranges. This further confjms that the induced refractive index change, is negative. The guided Chapter 5. Planar waveguide fabrication in ZBLAN by HeC klantation 72

modes were ako examineci at wavelengths of 0.85 pm, 0.98 pan, 1.06 pm,1.3 pand 1.5 jim Optical output with double pianar guided layers were clearly found up to a wavelength of 0.98 pm, while it suddenly blurred at 1.06 p,at which point the two layers combined into one planar waveguide with the guided thickness about 9 p,as shown u1 Fig. 5.8. Based on the cut-off wavelength analysis, we believe that the top guided layer is cut-off at a wavelength between 0.98 pm and 1.06 um, which gives the rehctive index change evaluation between - 1.005~10~~and -1.18~10'~. The diactive index change required for a waveguide with thickness of 5 pm to be guiding the fundamental mode is 4 hmes the index change required for a thickness of 10 mictons waveguide p.91. In the cut-off condition of the top layer, the structure delimited by the air-glass interface and the implanted barrier at 9 p (4MeV), the lower guide, is stiü operating. The periodic coupling fringes were clearly seen in al1 the images shown in Fig. 5.7 and Fig. 5.8. The bottom waveguide is symmetric, the guided mode was found at all the wavelengths, that is, it has at least a fiindamental mode.

2 4 s a 1 O Norm ilitod dlmtincm pirillil to th. murfacm

Fig. 5.7. CCD picture of theTE mode near-field output at 0.6328 p:a), the correspondhg intensity plot : and b), the diagram of double waveguide structure : c) induced by 2 MeV and 4 MeV He+ implantation. Chapter 5. Planar wavemïde fabrication in ZBLAN bv He+ imulantation 73

Fig. 5.8. TE mode optical near field outputs for the sample with 2 MeV and 4 MeV double He+ implantation at wavelengths of (a) 0.98 pm, @) 1.06 pm, (c) 1.3 p,and (d) 1.50 pm.

5.4.2. Refkactive-index profile reconstruction We use near-field propagation method to reconstruct the refkactive-index profile of optical waveguides fiom the near-field measurement result shown in Fig. 5.4(a). As stated in Chapter 3, this method requires that (i) the structure be weakly guiding; and (ii) ody the fundamental mode be excited. In our experiment, only the burried waveguide of the doubly implantecl sample meets these conditions. The O ther waveguides contain sharp rehctive index discontinuities at the surface. In the case of ion-implanted buried diffuscd waveguide with a negative refractive-mdex change barrier, the theoretical expression for the relation between the refkactive-index change and the near-field intensity I(x) daers slightly fcom that for positive index change. In the following section, we derive the expression for the refiactive-index profile in the case of a plana waveguide formed by two negative rehctive-index barriers. We start fiom eq43.9) Chapter 5. Planar wave.mïde fabrication in ZBLAN by He+ implantation 74

Introducing n(x) = n~ - A@), where n~ is the background refractive index of ZBLAN glass, and making the substitution for intensity, I(x)= 'P2(x), equation (3.9) yields:

In the case of negative refractive index change in ZBLAN induced by double Hef implantation, assuming that in the region between two implantation ranges, n(x)=nB, the value of second-order derivative term in equation ( 5.3 ) is nearly a constant Co,we have

where P2 " = 2nBktJI(X) Applying equation (5.4) into equations (5.2) and (5.31, we get the index profile expressed by

which are proportional to the second-order derivatives of the square-root intensity

4.i.l.i.i.i.i.i-i.,.i., 4.m!.,.,.,.,.,.,.,.,,,.,., -2 O 2 4 6 B 10 12 14 76 18 20 .Z O 2 4 6 8 10 12 14 16 18 20 Distance (micron) Distance (micron)

Fig. 5.9, The curves of square-root intensity, uiid the reconstructed refractive index profde, based on the data in digital image in Fig. 5.7(a), versus the depth from the sample surface. Chaoter 5. Planar waveguide fabrication in ZBLAN bv Hef implantation 75

The curves of square-root intensity ,/r(x),and the reconstructed refractive index profile, are shown in Fig. 5.9, based on the data in CCD picture of Fig. 5.7(a). The peak vaiue of the second tem in Fig. 5.9(b) is 4xlC3, in the center region, Co = lxLO-~. We thus have : A~(X)~*= -10-~-4~10-~ = -5~10-~.

5.5. BPM-CAD simulation and discussion The refractive index changes were estimated by using BPM-CAD with three-step index profiIe approximation, that is, each step has an index difference of Ad3 with a thickness of FWKM/3. Fig. 5.10 shows a sketch of the assumed waveguide structure with 13 layers and refractive index change of -lx105, sirnulating the index change induced by double helium ion implantation into ZBLAN. The data of layer thickness and corresponding refractive index are listed in Table 5.4. Fig. 5.11 shows the results for the top layer with (a) h= 0.6328 pm and (b)

0.98 p.The position << 0.0 » corresponds to the rnidde position of the top guided layer, that is, 0.5 jm~in Fig.5.10. The top layer has a single guided mode at the wavelength of 0.6328 pn and 0.98 Km, and cut-off at 1.06 p. Fig. 5.12 shows the BPM-CAD simulation results of combined waveguide. It has two-guided-mode at 0.6328 p, single-guided mode at wavelengths frorn 0.78 pm up to 1.8 pn, and totally cut-off at 1.9 p.To obtain results from the simulation software, the index of the cladding has to rernain lower than the index of the waveguide core as the depth goes to m. This is one of the iimits from this simulation software.

Refractive index i Surface Substrate

1 0.5 1 1.2 1 1.6 (micron) Fig.S.10 Sketch of assumed waveguide structure with 13 layers and refiactive index change of -1x105, simulating the index change induced by double He+ implantation into ZBLAN. Chapter 5. Planar waveguide fabrication in ZBLAN bv Hef im~lantation 76

Table 5.4. The data of 13 layer thicknesses and corresponding refractive index. Layer Thickness (pu) Refiactive index

f I Cover 5 1

Substrate 1

Opticai Field Field

, .

Xidt h (jtrn) Width (k.~rn) (a) Fig, 5.1 1 BPM-CAD simulation results of top layer at wavelength of (a) 0.6328 0.98 p,assuming An = - 1x10". Chapter 5. Planar waveguide fabrication in ZBLAN by He' implantation 77

OF ticai Ficici Opi Ecai Field

Width (ur;;j Widtl-i j~tm) O (dl Fig.5.12. BPM-CAD simulation results with the total width of 8.8 pm, and wavelengths of (a) 0.6328 pn, (b) 0.78 p,(c) 1.06 p.m and (d)1.3 m.

By the sarne way as above, the guided-modes with index changes lower than -1~10"were simulated by BPM-CAD. At 0.6328 pn wavelength, the combined waveguide is in two modes upon to An - -2.7~10-~.In this case bath fundamental-mode and first-order mode can be excited by adjusting the bearn coupling conditions. On the other hand, two single-mode waveguides can be excited at the sane time, which also gïves a double guided layer output. How can we distinguish the combination of two single-mode waveguides fiom higher-mode of one waveguide? To answer this question, let us consider the diagram shown in Fig. 5.13. In the case (a), we have a two-layer output at a specifïc wavelen,@. Ln the cases of (b) and (c), when we only change the laser source to a longer wavelength and without any modification to Chamer 5. Planar waveguide fabrication in ZBLAN by He+ imlantation 78 couphg conditions, the output reduces to a single-layer one in two ways. If the single-output layer is wider and located between the onginal two layers, as shown in Fig.5.13@), it indicates that the original output corresponds to one waveguide with two modes, and it reduces to one waveguide with a single mode at a longer wavelength. If the single output layer is not a new one, but is of the same dimensions and standing at the lower layer position of the origmal two, as shown in Fig.5.13(c), it indicates that the original two-layer output corresponds to two single mode waveguides, only the lower waveguide is in the single guided mode at thîs longer wavelength and the upper waveguide is cut-off.

Fig.5.13. Two ways that the two-layer output (a) reduces to single layer output: (b) two-mode changes to single-mode, and (c) the top waveguide of the original two is cut-oE

In our experiment, the output at wavelengths up to 0.98 pm stiU has two layers. At the wavelength of 1.06 pm with an YAG laser source, the upper waveguide disappeared and the lower one stayed at its onguial position It indicates that the original output corresponds to two waveguides with single-mode, and the upper waveguide was cutoff at 1.06 p wavelength. In the case of An = -3x10-~,the combiued waveguide has thme modes. And in the case of An = -5~10'~.the top layer itself has two guided modes at wavelength of 0.6328 p. The simulated results were shown in Fig. 5.14. In our experiment, these modes have never been found So it is not possible that the induced index is qua1or lower than -3x 10-~. Bad on the above discussion, we can conclude that the calculated index change corresponding to the cut-Off wavelength in the double implantation, is about - 1.0~105. While we have to indicate that BPM-CAD program was designed for a regular waveguide : a core with higher index surrounded by lower index cover and substrate. It has Iimited applications to Chapter 5. Planar waveguide fabrication in ZBLAN bv Hef irn~lantation 79

Fig.5.14. BPM-CAD simulattion results with (a) combined waveguide with index change of -3xlo5, and (b) top layer waveguide with index change of -sx~o-~.

the waveguide with negative refractive index change. Moreover, al1 the refractive index estimation from the cut-off wavelength in our expeknt are based on three-layer step-index waveguide theory. For implantation induced waveguide with graded index change, what we denved are only "Equivaient index changes" from the "Equivalent cut-off wavelengths". The TRIM simulation results are in accordance with the experimental results from the double irnplanted sarnple. There is a srnall difference in ion range between the TRIM calculation result and that from the experiment with the single 4 MeV Het implantation. This may be due to the measurement errors, and changes of the glas properties during the implantation when not using sample scanner. For exarnple, under the energetic heliurn beam bombardment, a shallow region on the sarnple was heated to a temperature higher than RT (room temperature), creating a high intemal stress due to high thermal expansion [5.8] of the ZBLAN glas substrate. This cm also explain the inverse index change results to the dose rate in our two experiments of the 4 MeV implantations: An around -3.6~lo4 in the first sample with a dose of 1016 ions/cm2, and An about -1~10-'in the second sarnple with a dose of 2x10~~ ions/cm2. For the second sarnple, more than 213 of the beam was stopped by metal plate mask; and the beam was periodically stopped one minute each 5 minutes by silica beam viewer to avoid heating effects that could damage the sample. It again shows that the implantation parameters affect the change of the optical properties, and this is why the ion beam intensity rnust be controlled during the implantation process. Chapter 5. Planar waveouide fabrication in ZBLAN bv He+ im~lantation 80

In many irradiated glasses, ionic motion appears to be responsible for index changes. For the helium-implantation-induced negative index change in 2BLA.N glass, based on the comparative weak bonding energy between fluorine and heavy element, it seems reasonable to suggest that the mechanisrns of radiation-enhanced diffusion and heavy ion segregation are involved.

References [5. LI A. Akelia, E.A. Downing, L. Hesselink, "New fiuoroindate glass composotions," J. Non- Cryst- Solids, vol. 2 13 & 214, pp. 1-5, 1997. [5.2] O. Perrot, L. Guinvarch, D. Benhaddou, P. C. Montgomery, R. Rimet, B. Boulard, C. Jacoboni, J. Non-Cryst. Solids. vol. 184, pp. 267-262, May, 1995. [5.3] E. Josse, J. E. Broquin, G. Fonteneau, R. Rimet, L. Lucas, "Planar and channel wave- guides on fluoride glasses," J. Non-Cryst. Solidr, vol. 213 & 214, pp. 152-157, 1997. [5.4] Y. Gao, O. Perrot, B. Boulard, J. E. Broquin, R. Rimet, C. Jacoboni, "Preparation of PZG fluoride glass channel waveguides," J. Non-Cryst. Solids, vol. 213 & 214, pp. 137- 141, 1997. [5.5] G. Battaglin, R. Bertoncello, A. Boscolo-Boscoletto, F. Caccavale, J. Lucas, P. Mazzoldi, C. Pledel, P. Polato, "Ion implantation effects in heavy metal fluonde glasses," J. Non-Cryst. Solids, vol. 120, pp. 256-26 1, 1990 [5.6] Y. Dai, 1. Yamaguchi, K. Takahashi and M. Iwaki, "Effect of ion implantation and post- treatments on optical transmission of fluorozirconate glass," Jpn. J. Appl. Phys, vol. 32, Pt.1, No. 9A, pp. 4026-4032, 1993. [5.7] Yunfei Zhao, Sophie LaRochelle, et al. IPR 2000, p74, [5.8] P. D. Townsend, P. I. Chandler, and L. Zhang, Optical Effects of ion implantation. Cambridge University Press, 1994, pp. 205-23 5. [5.9] M. Bass, HANDBOOK of OPTICS. McGRAW-HILL, 1995, vol. II, 33.55, Tab. 19&23. [5.1 O] Shoic hi Sudo, Optical Fiber Amplifiers :Materials, Devices, and Applications. Artech House 1997, p346. C5.1 LI M. Bass, BANDBOOK of OPTICS. McGRAW-HILL, INC,1995, vol. II, 33.44, Tab. 9. [S. 121 R.G. Hunsperger, lntegrated optics, Therory and Technology. S pringer, 1995, p. 37-38. [5.13] S. 1. Najafi, Introduction to Glass Integrated Optics. Artech House, 1992, pp. 114-127. Conclusion 81

Conclusion

Waveguide fabrication in iüsed silica glas and ZBLAN fluoride glas by proton and heiium ion implantation with a Van de Graaff accelerator was studied. The experimental procedures, results, and characterization of the optical properties, were presented. Two simulation tools, BPM-CAD and TRIM, were used to simulate and to predict the experimental results. Based on our knowledge, this is the fust experimental study of waveguide fabrication in ZBLAN glas by helium ion implantation.

Both plana. and channel waveguides were fabricated in füsed silica by 3.0 MeV proton implantation with the current intensity about 0.2 pA/cm2. The conesponding longitudinal range and straggling of 3 MeV proton in fused silica substrate were calculated by TRIM, having the values of 87.7 pm and 2.27 pm, respectiveiy. In the case of planar waveguide fabrication, the proton implantation dose was about 5x10'~ions/cm2. In both TE and TM excitation, the waveguide was a single-mode type at wavelengths of 0.6328 jim and 1.55 pm. The waveguide was located at a distance about 75 pm beneath the sarnple surface with the FWHM of the guiding region about 6 W. Using the scattering-light method, the optical loss was measured to be about 2.57 dBfcrn at wavelen,ath of 0.6328 p.The induced refractive index profile in this sarnple was reconstructed by the nez-field mode-propagation method, having the peak value to be about 5x10~.Based on the cut-off analysis of the second-order mode, BPM-CAD estimates the upper bound of 1x10-~on the refractive-index change.

In the case of channel waveguide fabrication, the proton implantation dose was about 1x10'~ions/crn2. Under both TE and TM excitations, the waveguide was multimode at the wavelength of 0.6328 p.But it was a single mode at 1.55 p.m wavelength. The optical loss Conclusion 82

was measured to be about 2-07 dBkm at the wavelength of 0.6328 W. Based on the near- field mode-propagation rnethod, the restructured refractive index profile had a peak value of about 2xl0-~,almost the saturation index change usualiy reported for fused siiica implanted by protons at room temperature- BPM-CAD gave a value, as a rough estimation, of less than 1x10~~.

Planar waveguides in ZBLAN glasses were fabricated by 4 MeV and by 2 and 4 MeV double helium-ion implantation. For the fmt sample, 4.0 MeV helium-ion implantation with a current intensity about 0.2 pA cm-' was carried out with a dose about 1x10'~ions cm". The longitudinal range and stragglinp were calculated by the TRIM code, having the values of 11.0 pm and 204 nm, respectively. Optical properties of the implanted sample was firstly evaluated at He-Ne laser wavelength of 0.6328 p.From the optical near-field examination, this planar waveguide was single mode in both the TE and TM excitations. The optical loss without post-annealing was measured to be 1.8 dB/cm by scattering light rnethod. The thickness of total guided region is about 8.8 p and is located just beneath the surface. Comparing to the TRIM caiculation result of longitudinal range, we can conclude that the guided region is between the surface and the end of implanted ions; and the induced refractive-index change in ZBLAN @ass by helium ion implantation is negative. The single guided mode was found at wavelen,ghs up to 0.98 pm, but not at 1.3 pm. Based on cut-off wavelength analysis, An was evaluated to be around -3.6~10~.By using BPM-CAD simulation, the refractive index change An was estimated to be between -1.15~10~and - 9.8~10~.

The natures of the negative index change induced in ZBLAN glas by helium ion implantation and the longitudinal range of helium ion calculated by the TRIM code, were verified by double helium ion implantation, which was carried out at two energies ( 2 MeV and 4 MeV ) in a second ZBLAN glas sample with a current density of 0.2 Wcm and a dose about 2x10'~ions/cm. Optical near-field output with a double guihg layer was found at wavelengths from 0.6328 p up to 0.98 p.m. The top guide had a fundamental mode which disappeared at 1.06 m. Cut-off analysis gave An evaluation of about -1x105. The Conclusion 83

distance between the surface and the first optical power minimum corresponding to the 2 MeV implanted region was about 5.1 p.The TRIM calculation gives the value of 4.96 p. The top waveguide contains a sharp rehctive index discontinuity at the air-glas interface,

oniy the buried waveguide of the-doubly implanted sample meets the requirements 06 the -- propagation-mode near-field method. The refractive index profile was reconstructed, having the peak value of -5x10-~,and FWHM about 0.3 p.The equivalent index change at half- maximum was -2.5~IO?

The experimental results are in accordance to those from the BPM-CAD simulation. But in dl experiments of silica glas and ZBLAN fluoride glass implanted by proton and helium ions, the experimental values of longitudinal range were less than those fiom the TRIM calculation. This may be due to measurement errors and bem-induced heating during implantation.

There is an inverse relation between the index change results and the implantation dose rate in the two ZBLAN glass sarnples. The An is about -3.6~10~in the fmt sample with a dose of 1x10'~ions/cm2, and An about -1x109 in the second sample with a dose of 2x10'~ ions/cm2. This rnay be due to the temperature-related implantation panmeter changes when a mechanical sample scanner was used in the second sample. It cmbe concluded that the ion beam intensity and temperature control play important roles for the index change of optical materials using implantation.