As laser and electro-optics technologies continue to evolve from the laboratory into so-called "real-world" applications, industry conferences have devised various ways to address this ongoing transition. This year’s 16th annual Conference on Lasers and Electro-Optics (CLEO ‘96, Anaheim, CA, June 2-7) is no exception. In addition to the typical research-oriented fare, the Optical Society of America (Washington, DC) is launching a new Lasers and Electro-Optic Applications Program (LEAP) aimed at product design and development engineers. Sessions address such issues as systems design and intellectual-property licensing. Of the 14 other topical areas at CLEO, some also highlight applications; printing, data storage, and display technologies, for example, are prominent in the program.
Held concurrently with CLEO ‘96, the Quantum Electronics and Laser Science Conference (QELS ‘96) encompasses six other topical areas covering spectroscopy, photochemistry and photobiology, coherent light sources, nonlinear optical phenomena, quantum optics, and optical interactions with condensed matter and ultrafast phenomena. Together the two conferences include 1064 papers, of which 65 CLEO papers are invited, as are 47 of the QELS papers.
In joint CLEO/QELS ‘96 plenary sessions, M. George Craford of Hewlett-Packard`s Optoelectronic Divison (San Jose, CA) addresses the future of light-emitting diodes, while the impact on optics of the explosive growth of the Internet’s World Wide Web is discussed by Robert Lucky of Bellcore (Red Bank, NJ). And in his talk entitled "Ultrafast Epiphany," Wayne Knox of Bell Laboratories (Holmdel, NJ) points out the potential real-world benefits of ultrafast technology.
The conferences and accompanying exhibit cover a wide variety of ultrafast research. In the Ultrafast Optics and Optoelectronics area, for example, two consecutive Monday sessions address femtosecond lasers, and on Friday two other sessions address ultrafast fiber lasers in the Fiber Lasers, Amplifiers, Devices and Sensors category. In paper CWI1, production of ~10-fs terawatt and petawatt pulses is described by C. Barty and colleagues from the University of California, San Diego (La Jolla, CA) working with ENSTA-LOA (Palaiseau, France) and Japan`s Atomic Energy Research Institute (Ibaraki, Japan). They present results from a three-stage chirped-pulse-amplification (CPA) system designed to produce ~15-fs, >1-J pulses. To date, the first two stages have produced 18-fs, 4-TW pulses at a repetition rate of 50 Hz and an average power greater than 4 W (see related article on p. 93).
An all-fiber CPA system described by A. Galvanauskas and coworkers of IMRA America Inc. (Ann Arbor, MI) and the Optoelectronics Research Centre (Southampton, England) produces high-average-power (up to 1 W) 310-fs pulses. Both the oscillator and power amplifier are based on erbium/ytterbium fiber that is cladding pumped with broad-area diode lasers (paper CFD4).
The generation of ultrashort x-ray pulses is of interest for studying the structural dynamics of materials because the x-rays interact with core electrons. In paper JTuF1, A. Chin and colleagues from Berkeley National Laboratory (Berkeley, CA) and the University of California (Berkeley, CA) report on use of Thompson scattering of femtosecond IR pulses off relativistic electron bunches to produce such pulses. The group was successful in producing ~300-fs, 0.5-Å x-ray pulses and expects such pulses to be very useful for studying ultrafast atomic motion in solids and molecules.
Ultrashort laser pulses, in combination with time-gated photorefractive holography, may yield a potential real-time diagnostic imaging tool for medical applications, according to the Femtosecond Physics Group at Imperial College (London, England). In paper CTuK3, the group describes 3-D depth-resolved imaging through turbid media; the researchers have obtained real-time images with sub-100-µm spatial resolution (see "Enhanced imaging through tissue").
Development of new sources continues at an ever-increasing rate as new materials, pumping techniques, and frequency-doubling configurations emerge that address specific problems in the design or function of solid-state lasers. The application of fiber-coupled transverse pumping by diode lasers to Nd:YAG lasers produces multimode output powers of several hundred watts with optical efficiencies of 45%, according to an invited paper by D. Golla and colleagues at the Laser Zentrum Hannover (Hannover, Germany; paper CWH3). The group says that improved resonators enable CW single-mode (TEM00) output power from this system of more than 75 W.
Compact and efficient tunable sources in the UV have potential in many applications such as atmospheric remote sensing. Candidates for these sources are cerium-doped LiSAF or LiCAF, optically pumped at 266 nm. In an invited paper, A. B. Petersen of Spectra-Physics Lasers (Mountain View, CA), together with researchers from Lawrence Livermore National Laboratory (Livermore, CA) and Lightning Optical Corp. (Tarpon Springs, CA), reports on operation of a Ce:LiCAF laser at 20 kHz when pumped by the fourth-harmonic output of a diode-pumped, Q-switched Nd:YVO4 laser. They obtained a maximum average power of 170 mW, with tuning over the 290-314-nm range (paper CTuJ1).
A high-power CW singly resononant optical parametric oscillator (OPO) based on periodically poled lithium niobate is described in paper CThA3 by W. Bosenberg and colleagues of Lightwave Electronics Corp. (Mountain View, CA) and Stanford University (Stanford, CA). The OPO had a 4.5-W threshold with an idler output tuning range of 3.1-4.0 µm and signal output tuning range of 1.45-1.62 µm; peak output of 1.2 W was obtained at 3.3 µm (see Laser Focus World, April 1996, p. 22).
Highlighting recent developments and heightened interest, the largest number of CLEO sessions are devoted to semiconductor lasers. Vertical-cavity surface-emitting lasers (VCSELs) continue to advance in performance-setting benchmarks for semiconductor lasers overall.
Examples include wall-plug efficiency greater than 50% and threshold currents less than 10 µA, which have been obtained from VCSELs containing aluminum oxide formed from AlGaAs. Such progress over the past two years in device performance, fabrication, and structure of selectively oxidized confined VCSELs is reviewed in an invited paper, JTuH1, given by K. Choquette and fellow researchers from the Photonics Research Department of Sandia National Laboratories (Albuquerque, NM). Noted will be the enhanced optical and electrical properties attained using various oxidized VCSEL structures, such as oxide distributed-Bragg reflectors, which can relax tolerances and overall epilayer thickness, leading to easier device fabrication.
Vertically coupled quantum dots (VECODs) are advantageous for laser applications because of enhanced optical-confinement factors and reduced radiative lifetimes. In invited paper JThC1Z, I. Alferov and others from the A. F. Joffe Physical-Technical Institute of Academy of Science (St. Petersburg, Russia), along with D. Bimberg from the Technische Universitat Berlin (Berlin, Germany), report on evidence of room-temperature injection lasing via the ground state in InAs/GaAs VECODs. The device dot-lattice geometry is such that it permits tunneling between electron and hole states in the InAs islands, which are separated by GaAs layers. For such vertically coupled structures, the threshold-current density, at all temperatures, is lower than for a laser with only one sheet of quantum dots.
Display and storage
Nine CLEO sessions will cover recent work in optoelectronics and optics for printing, storage, and display applications. In keeping with CLEO`s location near the "Magic Kingdom," Eric Haseltine of Walt Disney Imagineering (Burbank, CA) and David Ansley from Hughes Training (Los Angeles, CA) note that the future looks bright for solid-state lasers to soon have sufficient power to be viable projection lamps for large-screen displays. In their invited paper CML4, they outline the expected benefits of laser illumination systems on image quality (color purity and increased contrast), power consumption, and projection display-system size and operating life. Within three to four years, they project reasonably priced red, green, and blue lasers to replace conventional lamps on a "drop-in" basis or for use as directly scanned modulated raster beams.
Another invited paper (CML2), Cees van Uijen of Phillips Research Laboratories (Eindhoven, Netherlands) talks about the impressive growth in optical storage due to the success of compact-disk (CD) systems leading to widespread CD read-only memory applications. He analyzes the market trends and technical systems that are expected to strengthen optical storage as a cost-effective and robust data-exchange technology. These developments include limited application of magneto-optical recording, solutions to digital-video disk read-only and random-access memory, and the implications of InGaN blue laser development—which is updated in another invited paper (CML1) given it the same session by S. Nakamura of Nichia Chemical Industries (Tokushima, Japan). Van Uijen will also discuss optical-storage requirements that could lead to quartz materials replacing plastic substrates and the alternative route to high storage capacity with optical-tape recording.
Other technical sessions at CLEO will feature medical and biological applications. Here in vivo, noninvasive assessment of developing embryos using optical coherence tomography (OCT) is presented in paper CMJ2 by S. Boppart and colleagues from the MIT Research Laboratory of Electronics (Cambridge, MA) and M. Brezinski of Massachusetts General Hospital (Boston, MA). An analog to ultrasound imaging using light backscattering instead of acoustic echoes, OCT features a fiberoptic Michelson interferometer with a 1300-nm low-coherence superluminescent-diode light source. Other imaging methods may require small, transparent, or fixed specimens or not have sufficient resolution. The authors demonstrate that OCT can identify normal and abnormal morphology, genetic mutations, and repeatedly acquire histology for development assessment, using backscatter from within the living embryo.
The sessions on wave-mixing and photorefractives include the topic of dynamic holography. In paper CTuK1, James Millerd and others from MetroLaser (Irvine, CA) and Sudhir Trivedi from Brimrose (Baltimore, MD) outline resonant holographic interferometry. The method records two simultaneous holograms at two wavelengths, one tuned near a chemical-absorption feature, the other tuned approximately less than 0.1 nm off this. The resulting interferogram allows two-dimensional chemical detection useful for combustion and plasma diagnostics, medical imaging, and flow visualization. The presentation evaluates ZnTe:V:Mn at near-infrared wavelengths because such semiconductors can have photorefractive response times of picoseconds, offering potential for high-speed data acquisition.
Finally, other papers will include diverse topics such as a 1-pm spectrally narrowed compact excimer laser for microlithography (paper CThG4 by A. Tada and fellow researchers at the NEC Opto-electronics Research Laboratories, Kanagawa, Japan); development and overview of fiberoptic-cable networks through the upcoming wavelength-division-multiplexing generation (paper CMA1 by N. Bergano of Lucent Technologies, Holmdel, NJ); the challenge of achieving space-based laser altimeter technology for generating three-dimensional images (paper CTuF5 from J. Bufton of the Goddard Space Flight Center, Greenbelt, MD); and the dynamics of pulsed laser ablation and deposition for thin-film growth (paper CWE1 by David Geohegan of Oak Ridge National Laboratory, Oak Ridge, TN, and Alexander Puretzky from the Institute of Spectroscopy, Troitsk, Russia).
Enhanced imaging through tissue
Optical imaging through biological tissue could be a real-time diagnostic tool for applications such as skin-cancer detection and monitoring. The time-gated photorefractive holography method developed at Imperial College (London, England) utilizes the loss of coherence of light scattered through a thin tissue layer. In the schematic, the scattering cell serves as the tissue analog. The reconstructed colorized photo image is of a test object consisting of concentric aluminum cylinders 1 to 5 mm in diameter and displaced by 100 µm.
The researchers, using picosecond pulses at 750 nm to write holograms in rhodium-doped barium titanate, obtained depth-resolved images, through eight mean-free-path thicknesses of scattering medium, to 2 mm depth and 30-µm transverse spatial resolution. Achievable depth and spatial resolution are traded off, depending on the properties of the holographic-recording medium used.—Rick DeMeis