Data Storage, New Laser Advances Featured at ILS-XII Meeting
Optical and laser scientists from around the world gathered in Rochester, New York, 20-24 October 1996, for the twelfth annual Interdisciplinary Laser Science Conference (ILS-XII). The conference serves as the annual meeting of the APS Topical Group on Laser Science, in conjunction with the Optical Society of America (OSA). First held in Dallas, Texas, in 1985, the ILS series was established to survey the core laser science areas, including lasers and their properties, nonlinear optics and ultrafast phenomena, the physics of laser sources, lasers in physics and chemistry, and other laser applications.
Optical Data Storage
The most direct means of achieving large increases in the capacity of optical disk storage is by using three dimensions rather than the two presently used, according to F.B. McCormick of Call/Recall Inc. in San Diego, California, who maintains that two-photon absorption induced changes in dye-doped plastic media offer an inexpensive means of producing high-density multilayer memories.
Scientists at the University of Dayton in Ohio have discovered that micromirrors measuring about 100 microns are suitable for angle multiplexing in holographic data storage systems. Such devices demonstrate fast response times, due to their small inertia, and can readily be designed to scan over two dimensions in angle while simultaneously executing displacements that induce phase shifts, according to UD's Steven Gustafson, who spoke at a Monday morning session. More practical advantages include superior ruggedness and low cost.
To achieve improved imaging performance, Stephen Kowel (University of Alabama, Huntsville) has fabricated a liquid crystal adaptive lens using a novel conductive ladder meshing technique to minimize the number of control electrodes. The focal distance and transverse image position can be set by microcomputer, thus improving image quality. Other speakers covered such topics as recent advances and future optical head architectures using increased integration; the optical design and analysis of thin-film media structures used to produce CD-recordable discs; and a novel method of using an extended recording reference to reduce cross-talk noise in angle- multiplexed volume holographic data storage.
Thomas Mossberg of the University of Oregon's Center for Optics in Science and Technology described recent advances in spectral holographic optical data storage. The spectral recording dimension enables multi-kilobit storage at single spatial locations and the writing of frequency-dependent gratings keyed to deflect specific temporal data patterns. According to Mossberg, these capabilities lead, respectively, to high capacity, high speed, optical RAM, and content-controlled optical switching devices.
Norbert Hampp of Philipps University in Germany reported that bacteriorhodopsin processed into a polymeric film constitutes an excellent medium for optical and holographic recording. Bacteriorhodopsin - a relative of the visual pigment rhodopsin - is a photochromic retinal protein with a highly efficient primary photoreaction, large spectral shift, and excellent reversibility. The material's photochromic and related photorefractive properties can be modified over a period of several years, making it well-suited for technical applications.
In the area of telecommunications, the advent of the optical amplifier, together with ever-increasing communication demands, has made wavelength-division multiplexing (WDM) an attractive method for transmission capacity upgrade over existing standard fiber, according to R.C. Alferness (Bell Laboratories/Lucent Technologies). "The potential to build reconfigurable transmission networks and possible evolution into the access portion of the network offer further growth for the application of WDM technology," he said, adding that highly functional, mass producible, cost-effective sources will be key to the ubiquitous deployment of WDM systems.
In addition, a great deal of progress has been made in understanding the active region properties of ultrahigh speed diode lasers, including electron and hold dynamics - alternatively known as the transport and capture problem for quantum well lasers - and the effect of active region doping and strain, resulting in improved dynamic performance of these devices.
In the medical sphere, fluorescence spectroscopy of native tissue can provide a noninvasive rapid diagnostic tool, according to A. Katz (City College of New York), who has found that differences in spectra can be used to distinguish malignant from benign tissues. Currently three generations of optical biopsy diagnostic instruments with organ-specific optical probes are under development for in vitro and in vivo tissue diagnosis, along with various algorithms to improve the prediction accuracy.
Laser-aided manufacturing is another important emerging application. For example, in the automotive industry there is great interest in tailored blank welding and option hole cutting, according to David Rosenberg of General Motors R&D Center. In addition, scientists at the Illinois Institute of Technology are developing a new, versatile, automated laser shaping system for the manufacture of ceramic and ceramic composite components.
Advances in Fiber Lasers
On Monday afternoon, D.J. Richardson of England's Southampton University described recent advances in the development of novel fiber components for short pulse generation and manipulation, particularly the development of high power/high pulse energy fiber sources based on novel doped fiber designs. In addition, all-fiber modelocked lasers have found technological applications in recent years, and cladding-pumped erbium fiber oscillators and amplifiers have enabled the construction of high-power ultrafast fiber pulse sources delivering pulses as short as 100 femtoseconds, and average powers in excess of one Watt.
3-D Imaging and Display
Improved dimensionality from displays has required an increase in the amount of information conveyed, including binocular disparities and motion parallax, according to Stephen Benton of MIT's Media Laboratory, who focused on autostereoscopic technologies that do not require viewing aids (such as spectacles) in his Thursday afternoon overview of the field. Other advances reported in the session include the first demonstration of the use of a photorefractive crystal to display a three-dimensional image in space, and the development of a new technique called optical scanning holography, which extracts holographic information by two- dimensional active optical heterodyne scanning.
In addition, scientists at the University of Alabama in Huntsville are studying the partial pixel approach to 3-D displays, which they believe provides a conceptual framework for such displays that are functionally equivalent to holographic stereograms. According to team leader Gregory Nordin, he and his colleagues have implemented both monochromatic and full color displays based on the use of diffractive optical elements and conventional liquid displays.
In addition to the regular technical sessions, four critical reviews were given on exciting new developments in the field of laser science by recognized experts. First instituted in 1995, this year's lectures covered photonic band gaps, semiconductor cavity QED, wave packet dynamics, and the realization of Bose-Einstein condensation in dilute, trapped alkali gases. The conference also featured a plenary lecture by Will Happer of Princeton University on the medical application of lasers and spins to the illumination of lungs, and the Schawlow Prize Address on laser spectroscopy of atomic hydrogen by Theodor Hnsch of the Max Planck Institute for Quantum Optics.
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