DOCSLIB.ORG

Absorption and Wavelength Modulation Spectroscopy of NO2 Using a Tunable, External Cavity Continuous Wave Quantum Cascade Laser

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Absorption and Wavelength Modulation Spectroscopy of NO2 Using a Tunable, External Cavity Continuous Wave Quantum Cascade Laser Absorption and wavelength modulation spectroscopy of NO2 using a tunable, external cavity continuous wave quantum cascade laser Andreas Karpf* and Gottipaty N. Rao Department of Physics, Adelphi University, Garden City, New York 11530, USA *Corresponding author: [email protected] Received 16 September 2008; revised 5 December 2008; accepted 9 December 2008; posted 10 December 2008 (Doc. ID 101685); published 9 January 2009 The absorption spectra and wavelength modulation spectroscopy (WMS) of NO2 using a tunable, external cavity CW quantum cascade laser operating at room temperature in the region of 1625 to 1645 cm−1 are reported. The external cavity quantum cascade laser enabled us to record continuous absorption spectra −1 of low concentrations of NO2 over a broad range (∼16 cm ), demonstrating the potential for simulta- neously recording the complex spectra of multiple species. This capability allows the identification of a particular species of interest with high sensitivity and selectivity. The measured spectra are in excellent agreement with the spectra from the high-resolution transmission molecular absorption database [J. Quant. Spectrosc. Radiat. Transfer 96, 139–204 (2005)]. We also conduct WMS for the first time using an external cavity quantum cascade laser, a technique that enhances the sensitivity of detection. By employing WMS, we could detect low-intensity absorption lines, which are not visible in the simple ab- sorption spectra, and demonstrate a minimum detection limit at the 100 ppb level with a short-path ab- sorption cell. Details of the tunable, external cavity quantum cascade laser system and its performance are discussed. © 2009 Optical Society of America OCIS codes: 000.2170, 010.1120, 120.6200, 280.3420, 300.1030, 300.6340. 1. Introduction damaging of these pollutants and have multiple im- The accurate and precise monitoring of trace gases pacts on the environment and public health. They has applications in a wide range of fields, including play important roles in the formation of photochemi- the detection of environmental pollutants, tracking cal smog, the formation of tropospheric ozone, and of contaminants in closed environmental systems, the formation of acid rain. medical diagnostics, defense, and homeland security. Laser-based techniques for monitoring environ- Real-time trace gas detection is particularly impor- mental pollutants in the air have several advantages tant in environmental science (e.g., in atmospheric over chemical and other techniques, especially their physics/chemistry concerning air quality control, as ability to provide real-time monitoring capabilities well as for complying with Environmental Protection and greater sensitivity. Until recently, the ap- Agency [1] air quality regulations and safety moni- proaches have included (i) observing the visible and near-IR overtones of molecular transitions (which toring). In these applications, the concentrations of are much weaker than the fundamental mid-IR tran- pollutants typically range from several parts per mil- sitions) using room-temperature tunable diode lasers lion to the parts-per-trillion level and thus require [2–5]; (ii) observing the strong mid-IR transitions techniques that are both highly sensitive and selec- using tunable lead–salt diode lasers (which require tive. Nitrogen oxides (NOx) are some of the more cryogenic cooling and are typically multimode) [6] or strong tunable molecular laser systems such as 0003-6935/09/020408-06$15.00/0 CO lasers (which require cryogenic cooling and pro- © 2009 Optical Society of America vide only limited tuning around the molecular lines 408 APPLIED OPTICS / Vol. 48, No. 2 / 10 January 2009 and overtones) [7,8]. The advent of the quantum cas- The output power at 1632:5 cm−1 (where we con- cade laser in the mid-IR region covering the 4–24 μm ducted our WMS) was 8 mW. Broad-range tuning range has provided an attractive source for investi- is achieved entirely by rotating the external cavity gating the spectroscopy of trace gases in the atmo- grating via a stepper motor; no current or tempera- sphere and constructing portable gas sensors [9]. ture tuning was necessary. The system may be set to Using quantum cascade lasers (which can operate one of six preset tuning rates, the slowest of which in a room-temperature environment using thermo- was 3:125 cm−1=s (the tuning rate used to record electric cooling), one can access the strong mid-IR our NO2 absorption spectra). A piezoelectric transdu- fundamental rotational–vibrational transitions of cer (PZT) positioned between the stepper motor and molecular species [10], thus offering improved the grating itself allows one to tune smoothly at any sensitivity over techniques based on detecting the desired rate over a smaller range (1 cm−1) as well as overtones of these lines. By employing an external modulate the laser frequency with a sinusoidal sig- cavity arrangement, a quantum cascade laser offers nal (allowing us to perform WMS). Additionally, its Δν ∼ 0:001 −1 a narrow linewidth ( cm ), highly stable room-temperature operability and compact size and reproducible tunable CW output, and a wide make it a suitable candidate for a portable device continuous tuning range, all of which are essential for in situ measurements. for the study of complex spectra, as is the case in We carried out absorption spectroscopy of NO2 investigating trace gas components in the atmo- using a short-path absorption cell (12:5 cm) filled sphere [11]. with different concentrations of NO2. The schematic The current work focuses on using a quantum cas- of the experimental arrangement is shown in Fig. 1. cade laser operating at room temperature to detect Two rectangular, flat, protected gold mirrors were NO2 with high sensitivity and selectivity. The pro- ’ aligned to reflect the beam back and forth nine times ject s aim is to ultimately develop a portable, self- through the cell, creating a homemade multipass cell contained device that may be used for high-precision, that increased the path length to 1:125 m. The signal in situ atmospheric measurements. To this end, a was detected using a two-stage, TE-cooled, IR photo- short-path absorption cell was used with a multipass voltaic detector (PVI-2TE-8 manufactured by Vigo), configuration, and wavelength modulation techni- which can be operated in a room-temperature ques were employed to enhance the sensitivity. In wavelength modulation spectroscopy (WMS) [12–15], environment. The detectors were optically immersed the laser output is modulated at a frequency (f ), and in a high refractive index, hyperhemispherical lens. Phase-sensitive detection was done using a chopper detection is done at higher harmonics of the fre- 3:9 quency (e.g., 2f ) so that the excess laser noise, which running at kHz and a lock-in amplifier (Stanford varies as 1=f , is reduced, thereby enhancing the Research Systems SR830 DSP) with the time con- 3 signal-to-noise ratio. Specifically, the modulation fre- stant set to ms. quencies are lower than the absorption linewidth. The NO2 mixtures used were prepared by loading the experimental cell with a precalibrated 1% mix of NO2 and 99% N2 (mixed and certified by National 2. Experimental Details Specialty Gases to Æ2% of the specified concentra- The quantum cascade laser source was purchased tion) to a desired pressure. Additional pure N2 was from Daylight Solutions (Model TLS-CW-MHF) and then added to increase the pressure to the desired −1 has a tuning range of 1625 to 1645 cm . The external final value. For example, for the 50 ppm concentra- cavity is of Littrow configuration, which allows a tion used for the absorption spectroscopy, the wide range of mode-hop-free tuning, provides a nar- row linewidth (∼0:001 cm−1) and is thus well suited for spectroscopic measurements. Because of manu- facturing issues with the antireflective coating on the laser chip, the mode-hop-free tuning range did not span the entire 20 cm−1 tuning range of the laser system, but rather it was limited to the region be- tween 1629 and 1645 cm−1. We were able to record −1 a continuous NO2 spectrum over this 16 cm range without mode hops. Between 1625 and 1629 cm−1, the laser tuned over short ranges (of the order of 1 cm−1) before hopping to another mode. The continu- ity and linearity of the laser tuning were monitored using an etalon and the laser controller’s display screen, as well as by comparing recorded spectra with line positions identified in the high-resolution transmission molecular absorption (HITRAN) data- base [16]. The laser power varies as a function of tun- ing, with a minimum output power of 5 mW at Fig. 1. Schematic of the experimental setup employed for absorp- −1 −1 1625 cm and a maximum of 18 mW at 1645 cm . tion and WMS. 10 January 2009 / Vol. 48, No. 2 / APPLIED OPTICS 409 −1 experimental cell was first loaded with 1:3 Æ 1 mbar and 1632:511 cm (note that the ðν1 ν3 ν3Þ– of the precalibrated 1% mix of NO2 before additional ðNKa KcÞ level notation is used). The separation of −1 pure N2 was added to reach a final pressure of the lines within this doublet is 0:003 cm , which 260 Æ 20 mbar. Because of limitations in the accu- is much less than the pressure broadened widths racy of our vacuum/mixing apparatus, the mixed con- of the lines at 120 mbar, meaning that it should show centrations are expected to be accurate to Æ20%. The as a single absorption “line.” We measured the mixing apparatus was tested by generating several HWHM of the resulting unresolved line to be −1 concentrations of NO2 and comparing the recorded ∼0:018 cm and used this value for purposes of de- absorption line intensities with the calculated inten- termining the modulation depth of the sine wave.
Load more

Recommended publications
  • 020-101363-04 LIT MAN USER CP42LH.Book
    User Manual 020-101363-04 CP42LH NOTICES COPYRIGHT AND TRADEMARKS Copyright © 2019 Christie Digital Systems USA Inc. All rights reserved. All brand names and product names are trademarks, registered trademarks or trade names of their respective holders. GENERAL Every effort has been made to ensure accuracy, however in some cases changes in the products or availability could occur which may not be reflected in this document. Christie reserves the right to make changes to specifications at any time without notice. Performance specifications are typical, but may vary depending on conditions beyond Christie's control such as maintenance of the product in proper working conditions. Performance specifications are based on information available at the time of printing. Christie makes no warranty of any kind with regard to this material, including, but not limited to, implied warranties of fitness for a particular purpose. Christie will not be liable for errors contained herein or for incidental or consequential damages in connection with the performance or use of this material. Canadian manufacturing facility is ISO 9001 and 14001 certified. WARRANTY Products are warranted under Christie’s standard limited warranty, the complete details of which are available by contacting your Christie dealer or Christie. In addition to the other limitations that may be specified in Christie’s standard limited warranty and, to the extent relevant or applicable to your product, the warranty does not cover: a. Problems or damage occurring during shipment, in either direction. b. Projector lamps (See Christie’s separate lamp program policy). c. Problems or damage caused by use of a projector lamp beyond the recommended lamp life, or use of a lamp other than a Christie lamp supplied by Christie or an authorized distributor of Christie lamps.
    [Show full text]
  • Pulsed/Cw Nuclear Magnetic Resonance
    PULSED/CW NUCLEAR MAGNETIC RESONANCE “The Second Generation of TeachSpin’s Classic” • Explore NMR for both Hydrogen (at 21 MHz) and Fluorine Nuclei • Magnetic Field Stabilized to 1 part in 2 million • Homogenize Magnetic Field with Electronic Shim Coils • Quadrature Phase-Sensitive Detection with 1° Phase Resolution • Direct Measurement of Spin-Spin and Spin-Lattice Relaxation Times • Carr-Purcell and Meiboom-Gill Pulse Sequences • Observe Chemical Shifts in both Hydrogen and Fluorine Liquids • Compare Pulsed and Continuous Wave NMR Detection • Study Pulsed and CW NMR in Solids • Built-in Lock-In Detection and Magnetic Field Sweeps Instruments Designed For Teaching PULSED/CW NUCLEAR MAGNETIC RESONANCE The 21 MHz Digital Synthesizer produces rf power in both INTRODUCTION pulsed and cw formats. There is sufficient rf power to produce Nuclear Magnetic Resonance has been an important research a π/2 pulse in about 2.5 microseconds. It also produces the tool for physics, chemistry, biology, and medicine since its discov- reference signals (in 1° phase steps) for the quadrature detectors. ery simultaneously by E. Purcell and F. Bloch in 1946. In the The Pulse Programmer digitally creates the pulsed sequences of 1970’s, pulsed NMR became the dominant paradigm for reasons various pulse lengths, number of pulses, time between pulses, and your students will discover using the apparatus described in this repetition times. The Lock-In/Field Sweep provides a wide range brochure, TeachSpin’s second generation of our classic PS1-A&B. of magnetic field sweeps, as well as a lock-in detection system for This new unit was designed in response to requests for additional examining weak cw NMR signals from solids.
    [Show full text]
  • Time-Synchronized Continuous Wave Laser-Induced Fluorescence on An
    Time-synchronized continuous wave laser- induced fluorescence on an oscillatory xenon discharge Cite as: Rev. Sci. Instrum. 83, 113506 (2012); https://doi.org/10.1063/1.4766958 Submitted: 07 August 2012 . Accepted: 26 October 2012 . Published Online: 27 November 2012 N. A. MacDonald, M. A. Cappelli, and W. A. Hargus ARTICLES YOU MAY BE INTERESTED IN Time-resolved laser-induced fluorescence measurement of ion and neutral dynamics in a Hall thruster during ionization oscillations Journal of Applied Physics 118, 233301 (2015); https://doi.org/10.1063/1.4937272 Ion dynamics in an E × B Hall plasma accelerator Applied Physics Letters 106, 044102 (2015); https://doi.org/10.1063/1.4907283 Tutorial: Physics and modeling of Hall thrusters Journal of Applied Physics 121, 011101 (2017); https://doi.org/10.1063/1.4972269 Rev. Sci. Instrum. 83, 113506 (2012); https://doi.org/10.1063/1.4766958 83, 113506 © 2012 American Institute of Physics. REVIEW OF SCIENTIFIC INSTRUMENTS 83, 113506 (2012) Time-synchronized continuous wave laser-induced fluorescence on an oscillatory xenon discharge N. A. MacDonald,1,a) M. A. Cappelli,1 and W. A. Hargus, Jr.2 1Stanford Plasma Physics Laboratory, Stanford University, Stanford, California 94305, USA 2Air Force Research Laboratory, Edwards AFB, California 93524, USA (Received 7 August 2012; accepted 26 October 2012; published online 27 November 2012) A novel approach to time-synchronizing laser-induced fluorescence measurements to an oscillating current in a 60 Hz xenon discharge lamp using a continuous wave laser is presented. A sample-hold circuit is implemented to separate out signals at different phases along a current cycle, and is followed by a lock-in amplifier to pull out the resulting time-synchronized fluorescence trace from the large background signal.
    [Show full text]
  • Laser Measurement in Medical Laser Service
    Laser Measurement in Medical Laser Service By Dan Little, Technical Director, Laser Training Institute, Professional Medical Education Association, Inc. The global medical industry incorporates thousands of lasers into its arsenal of treatment tools. Wavelengths from UV to Far-Infrared are used for everything from Lasik eye surgery to cosmetic skin resurfacing. Visible wavelengths are used in dermatology and ophthalmology to target selective complementary color chromophores. Laser powers and energies are delivered through a wide range of fiber diameters, articulated arms, focusing handpieces, scanners, micromanipulators, and more. With all these variables, medical laser service personnel are faced with multiple measurement obstacles. At the Laser Training Institute (http://www.lasertraining.org), with headquarters in Columbus Ohio, we offer a week-long laser service school to medical service personnel. Four times a year, a new class learns the fundamental concepts of power and energy densities, absorption, optics, and, most of all, how lasers work. With a nice sampling of all the major types of medical lasers, the students learn hands-on calibration, alignment, and multiple service skills. Lasers used in the medical field fall under stricter safety regulations than other laser usages. Meeting ANSI compliances are critical to the continued legal operation of all medical and aesthetic facilities. Laser output powers and energies are to be checked on a semi-annual basis according to FDA Regulations and are supported by ANSI recommendations which state regular scheduled intervals. In our service school we exclusively use Ophir-Spiricon laser measurement Instrumentation. We present a graphically enhanced presentation on measurement technologies and the many, varying, critical parameters that are faced with not only each different type of laser but design differences between manufacturers.
    [Show full text]
  • Dynamics of Passively Coupled Continuous-Wave and Mode
    Dynamics of Passively Coupled Continuous-Wave and Mode-Locked Lasers by Sudarshan Sivaramakrishnan A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Electrical Engineering) in The University of Michigan 2017 Doctoral Committee: Professor Herbert G. Winful, Chair Professor Almantas Galvanauskas Professor Peter D. Miller Professor Kim A. Winick Sudarshan Sivaramakrishnan [email protected] ORCID iD: 0000-0002-6224-2094 © Sudarshan Sivaramakrishnan 2017 DEDICATION To my parents and to my younger brother ii ACKNOWLEDGMENTS My PhD journey has been deeply enriching and enlightening, as well as the most challenging endeavor that I have ever undertaken. As such, I am grateful to all involved for the motivation, support, and engagement that made graduate life as fulfilling and fun of an experience as it was. First and foremost, I would like to thank my research advisor, Professor Herbert Winful, for his dedication, patience, and enthusiasm to help me successfully complete my PhD. It has been a privilege to work with and learn from him across a diverse array of fascinating topics, and I am indebted to him for aiding my growth as a researcher and as an individual. From my first meeting with him (an advising appointment when I was an undergraduate freshman declaring my major in Electrical Engineering) to joining his research group in the summer before graduate school to the present, he has been a persistent source of wisdom, encouragement, and equanimity. His thoughtful guidance has enabled me to explore new scientific problems and steer my own path through the research process, while time and again his friendly smiles and ever-positive outlook replenished my belief in the value of my work whenever I lost my way.
    [Show full text]
  • Micro and Nanoscale 3D Printing Using Optical Pickup Unit from A
    ARTICLE https://doi.org/10.1038/s42005-021-00532-4 OPEN Micro and nanoscale 3D printing using optical pickup unit from a gaming console ✉ Tien-Jen Chang 1 , Lukas Vaut1, Martin Voss1, Oleksii Ilchenko 1, Line Hagner Nielsen1, Anja Boisen 1 & En-Te Hwu1 Conventional photopolymerization-based 3D printing still requires developing a concise and cost-effective method to improve the printing resolution at the nanoscale. Here, we propose the use of a gaming console optical drive pickup unit for 3D photopolymerization. This mass- produced optical pickup unit features a finely adjustable diode laser, allowing us to adjust the printing resolution from tens of micrometres down to hundreds of nanometres without 1234567890():,; requiring oxygen radical scavenging or costly femtosecond lasers. We evaluate the 3D printing performance using a commercial photopolymer under different laser exposure parameters. The proposed printing system achieves a resolution of 385 nm along the lateral direction and XYZ nano-resolution linear stages enable a printing volume of up to 50 × 50 × 25 mm3. Finally, we demonstrate the fabrication of 3D stereoscopic microstructures. The substantially simplified optics proposed here paves the way for affordable high-resolution micro/nanoscale 3D fabrication. 1 The Danish National Research Foundation and Villum Foundation’s Center for Intelligent Drug Delivery and Sensing Using Microcontainers and ✉ Nanomechanics (IDUN), Department of Health Technology, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark. email: [email protected] COMMUNICATIONS PHYSICS | (2021) 4:23 | https://doi.org/10.1038/s42005-021-00532-4 | www.nature.com/commsphys 1 ARTICLE COMMUNICATIONS PHYSICS | https://doi.org/10.1038/s42005-021-00532-4 he 3D printing technology proposed in the 1980s has optical system is based on an expensive 3D printer (Photonics Tpioneered a new generation of manufacturing1–3.
    [Show full text]
  • Lasers, Free-Electron
    1 Lasers, Free-Electron Claudio Pellegrini and Sven Reiche • Q1 University of California, Los Angeles, CA 90095-1547, USA e-mail: [email protected]; [email protected] Abstract Free-electron lasers are radiation sources, based on the coherent emission of synchrotron radiation of relativistic electrons within an undulator or wiggler. The resonant radiation wavelength depends on the electron beam energy and can be tuned over the entire spectrum from micrometer to X-ray radiation. The emission level of free-electron lasers is several orders of magnitude larger than the emission level of spontaneous synchrotron radiation, because the interaction between the electron beam and the radiation field modulates the beam current with the periodicity of the resonant radiation wavelength. The high brightness and the spectral range of this kind of radiation source allows studying physical and chemical processes on a femtosecond scale with angstrom resolution. Keywords free-electron laser; undulator; microbunching; SASE FEL; FEL oscillator; FEL amplifier; FEL parameter; gain length. 1 Introduction 2 2 Physical and Technical Principles 5 2.1 Undulator Spontaneous Radiation 6 2.2 The FEL Amplification 7 2.3 The Small-signal Gain Regime 10 2.4 High-gain Regime and Electron Beam Requirement 11 2.5 Three-dimensional Effects 13 OE038 2 Lasers, Free-Electron 2.6 Longitudinal Effects, Starting from Noise 14 2.7 Storage Ring–based FEL Oscillators 16 3 Present Status 17 3.1 Single-pass Free-electron Lasers 17 3.2 Free-electron Laser Oscillators 18 4 Future Development 20 Glossary 21 References 23 Further Reading 24 1 accelerator and can be used to accelerate Introduction the electron beam to higher energies.
    [Show full text]
  • High-Resolution Doppler-Free Molecular Spectroscopy with a Continuous-Wave Optical Parametric Oscillator
    High-resolution Doppler-free molecular spectroscopy with a continuous-wave optical parametric oscillator E. V. Kovalchuk, D. Dekorsy, A. I. Lvovsky, C. Braxmaier, J. Mlynek, A. Peters and S. Schiller* Fachbereich Physik, Universität Konstanz, M696, D-78457 Konstanz, Germany We present a reliable, narrow linewidth (100 kHz) continuous-wave optical parametric oscillator (OPO) suitable for high- resolution spectroscopy applications. The OPO is based on a periodically-poled lithium-niobate crystal and features a specially designed intracavity etalon which permits its continuous tuning and stable operation at any desired wavelength in a wide operation range. We demonstrate Doppler-free spectroscopy on a rovibrational transition of methane at 3.39 µm. Considerable technological progress in the fabrication of linewidth < 5 kHz and 1 W single-frequency output periodically poled nonlinear crystals, durable optical power at 1064 nm is used as a pump source. The coatings, ultra-stable solid-state pump lasers and OPO frequency can be tuned by up to 40 GHz (9 GHz without design has recently led to a new generation of continuous- mode hops) by changing the laser crystal temperature. An wave optical parametric oscillators (cw OPOs) with single optical isolator prevents backreflections from the OPO frequency output covering a very wide range of the near- cavity. The OPO is based on a periodically poled lithium infrared spectrum (0.8 – 4.0 µm). These OPOs can now niobate (PPLN) crystal of 19 mm length and 0.5 x 50 mm provide substantial amounts (10 – 250 mW) of coherent aperture. The crystal contains 33 gratings with periods radiation with narrow linewidth (< 160 kHz), continuous ranging from 28.98 to 30.90 µm.
    [Show full text]
  • BARE Physician Brochure Vertical
    Laser away unwted hair for good Professional Laser Hair Removal System Maximize profit with an effective & cost-efficient hair removal solution Laser hair removal continues to be the number one requested aesthetic treatment and a staple of any aesthetic business. The ongoing battle against recurring hair regrowth ensures a constant revenue stream when armed with an effective treatment solution. The BARE 808 system offers a cost effective solution with no consumables to help improve your bottom line. The BARE 808 system uses diode laser technology, designed to quickly and efficiently remove unwanted hair on the face and body. This gold-standard technology combined with advanced pulse modes and chiller-tip contact cooling, offers a safe and effective treatment with less pain to a wide range of skin types and patients. Benefits to your business Improve your bottom line Fast treatments Attractive cost and no Continuous wave delivery consumables to reduce treatment times & save chair time Treat a wide range of Easy to use, ergonomic patients handpiece design Safe & effective for a wide User-friendly interface range of patient needs with built-in pre-sets Advtages A Large spot size and ergonomic handpiece design B Advanced pulse technology C Fast treatment speed D Chiller-tip contact cooling E Convenient user interface Advanced technology for the ultimate treatment experience FOR • No consumables OWNER • Reasonable device and maintenance costs, long life-cycle • Ergonomic handpiece design FOR • Easy to use interface with pre-sets USER • Treatment speed selection FOR • Safe and effective PATIENT • Fast treatment time BARE 808 Advantages • Contact cooling for increased comfort An easy and eicient laser hair removal solution A Large spot size and ergonomic handpiece design The BARE 808 system offers a 14mm x 14mm spot size, ideal for treating all areas of the body.
    [Show full text]
  • Continuous Wave Nuclear Magnetic Resonance: Estimation of Spin-System Properties from Steady-State Trajectories
    Continuous wave nuclear magnetic resonance: estimation of spin-system properties from steady-state trajectories James Christopher Korte ORCID: 0000-0001-9152-1319 Ph.D Engineering (351AA) August, 2017 Department of Biomedical Engineering, Melbourne School of Engineering The University of Melbourne Submitted in total fulfilment of the degree of Doctor of Philosophy 2 Abstract Magnetic resonance imaging (MRI) is a powerful imaging modality, widely used in routine clinical practice and as an investigational tool in basic science. The contrast in MRI is related to both the underlying tissue properties, which undergo disease or injury related changes, and to the MRI method and sequence parameters used. It is the latter with which this thesis is concerned: the design and implementation of novel MRI acquisition paradigms and associated reconstruction methods. The majority of MRI methods excite the object of interest with a series of short RF pulses, varying the weaker spatial magnetic field using the gradients, and ensuring the RF transmitter is inactive while acquiring a series of decaying MR signals. This regime linearises the inherently nonlinear behaviour of a magnetic resonance spin-system, allowing the acquired signals to be considered in a spatial frequency space and an image to be reconstructed using the well known Fourier transform. It is our assertion that nonlinear behaviour of the magnetic spin signal will lead to advantageous attributes in future MR methods, just as moving beyond conventional linear spatial gradients to nonlinear encoding fields led to methods for accelerated imaging and variable spatial resolution. Reconstruction of spin-system properties from nonlinear MR signals requires algorithms beyond the Fourier transform.
    [Show full text]
  • Continuous-Wave Versus Pulse Regime in a Passively Mode-Locked Laser with a Fast Saturable Absorber
    234 J. Opt. Soc. Am. B/Vol. 19, No. 2/February 2002 Soto-Crespo et al. Continuous-wave versus pulse regime in a passively mode-locked laser with a fast saturable absorber J. M. Soto-Crespo Instituto de O´ ptica, Consejo Superior de Investigaciones Cie`ntifica` s, Serrano 121, 28006 Madrid, Spain N. Akhmediev Optical Sciences Centre, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, Australian National University, Australian Capital Territory 0200, Australia G. Town School of Electrical and Information Engineering (J03), University of Sydney, New South Wales, 2006, Australia (Received February 27, 2001; revised manuscript received August 15, 2001) The phenomenon of modulation instability of continuous-wave (cw) solutions of the cubic–quintic complex Ginzburg–Landau equation is studied. It is shown that low-amplitude cw solutions are always unstable. For higher-amplitude cw solutions, there are regions of stability and regions where the cw solutions are modu- lationally unstable. It is found that there is an indirect relation between the stability of the soliton solutions and the modulation instability of the higher-amplitude cw solutions. However, there is no one-to-one corre- spondence between the two. We show that the evolution of modulationally unstable cw’s depends on the sys- tem parameters. © 2002 Optical Society of America OCIS codes: 190.5530, 190.5940, 140.3510, 320.5540. 1. INTRODUCTION loss (both linear and nonlinear). Some delicate balances between them give rise to the majority of the effects ob- Passive mode locking allows for the generation of self- served experimentally. On the other hand, we assume shaped ultrashort pulses in a laser system.
    [Show full text]
  • Continuous-Wave Infrared Laser Spectrometer Based on Difference Frequency Generation in Aggas2 for High-Resolution Spectroscopy
    Canarelli et al. Vol. 9, No. 2/February 1992/J. Opt. Soc. Am. B 197 Continuous-wave infrared laser spectrometer based on difference frequency generation in AgGaS2 for high-resolution spectroscopy P. Canarelli, Z. Benko, R. Curl, and F. K. Tittel Department of Electrical and Computer Engineering, Rice Quantum Institute, Rice University, R0. Box 1892, Houston, Texas 77251 Received May 7, 1991; revised manuscript received September 26, 1991 A high-resolution cw spectrometer based on difference frequency generation (DFG) in a 20-mm-long AgGaS2 crystal pumped by two stabilized single-frequency cw dye-Ti:sapphire lasers is described. Experiments per- formed with a Rhodamine 6G and DCM dye laser combination pumped by a 20-W argon laser are reported. In- frared radiation is generated from 7 to 9 Atm with an ultranarrow linewidth (<0.5 MHz) and an output power of 1 W by making use of 90° type I phase matching. The performance of the DFG laser spectrometer is evaluated by using a portion of the v2 band of NH3 near 1177 cm-'. LiIO , Wellegehausen INTRODUCTION linewidth (<10-4 cm'1). Using 3 et al.7 obtained coherent infrared radiation from 2.3 to High-resolution laser spectroscopy in the infrared region 4.6 gm at -0.5 ,uW. A combination of both crystals was requires a tunable coherent cw radiation that is capable of used by Bawendi et al.' for high-resolution spectroscopy precise wavelength resolution, high spectral brightness, between 1.9 and 5.3 ,m. and good frequency and amplitude stability. A wave- The optical and physical properties of the nonlinear- length resolution of better than 1 MHz and a power optical material is critical to generation of cw infrared greater than 1 ,uW are desirable for high sensitivity and radiation by difference frequency mixing.
    [Show full text]