Optics & Photonics News - December 1990

Optics & Photonics News Magazine
December 1990 Issue

Feature Articles

Femtosecond Technology in the '80s

The technology for producing femtosecond optical pulses was first realized in 1981 and improved to produce pulse durations as short as 6 fs by the end of the decade. In addition to the rapid reduction in pulse duration and close approach to the fundamental limits on optical pulse duration, the pulses were produced in highly stable pulse trains at repetition rates as large as 10 kHz. The direct observation of a broad range of previously inaccessible time dependent phenomena made possible by this progress has stimulated research in a number of fields. This work culminates some 25 years of progress that has resulted in reducing the duration of optical pulses by a factor of more than 100,000.

by R. L. Fork

Optical Neural Networks

The optical implementation of artificial neural networks is a subject that combines optics and neural networks. The notion that links the two fields is connectionism. In optical computers, photons are used instead of electrons as the carriers of information. The advantage of doing this derives from the fact that photons do not directly interact with one another. This makes it easier to establish a communication network connecting a large number of processing elements. Therefore, the design of optical computers is naturally guided toward architectures that require many connections.

by Demitri Psaltis and Yong Quio

Optical Interconnections in the '80s

In the late 1970s and early 1980s, the spectacular contributions of fiber optics to long distance communication were motivating the examination of the use of the optics for solving interconnection problems at the highest levels of the interconnect hierarchy, namely at the processor-to-processor level. As early as 1979, Cathey and Smith had proposed a parallel free-space optical data bus for use within a computer. In 1981, Tajima, Okada, and Tamura proposed the development of free-space optical buses for a multiprocessor machine and, in 1983, these same authors reported development of an experimental freespace optical data bus running with a 100 MHz clock speed. Further advances on this project were described in following years; the computer in which this bus is used in often referred to as Dialog.H.

by Joseph W. Goodman

Tunable Lasers: A Solidifying Trend

The 1980's trends in discovery and development of active media for tunable lasers were to move away from the liquid toward the solid state. The dominance of dye lasers as practical tunable sources, based on their widespread development in the 1970s, had been diminished at the end of the '80s by the emergence of several tunable solid state lasers. In this article, we briefly describe the discoveries and development efforts that led to the present mix of available tunable systems. We include dyes, paramagnetic ions, and color centers in our treatment of media.

by Peter F. Moulton

Quantum Optics in the '80s

Spectacular experimental progress in quantum optics was made in the 1980s based on a solid footing of theoretical work in the previous two decades. In the past 10 years, a variety of physical systems have been shown to generate nonclassical states of light whose noise properties deviate by as much as an order of magnitude from the more common states of light such as laser and thermal radiation.

by R. E. Slusher

Phase Conjugation with Photorefractive Materials

At the beginning of the '80s, it was known that one could generate a phase-conjugate wave in just about anything by four-wave mixing (put in three waves and you'll produce a fourth). These phase-conjugate waves would always travel back from where they came, and with their wavefronts reversed, as though they were propagating backwards in time. Four-wave mixing was seen as a kind of "real-time" holography, and it was fashionable to talk about generating "time-reversed" or "phase-conjugate" light beams. However, these wave-mixing experiments required either a powerful laser or material with a strong optial resonance at the laser's frequency.

by Jack Feinberg

Nonlinear Guided Wave Phenomena

The last decade has witnessed the emergence of nonlinear optics in waveguides, both fiber and integrated optics. Although there have been many noteworthy developments, here we concentrate on three specific areas that have caught and sustained interest because of their fascinating physics and/or applications potential.

by George I. Stegeman and Roger Stolen

Ultra-low Threshold Lasers

The increasing use of semiconductor lasers (SCLs) as optoelectronic components has led to recent efforts to substantially lower the threshold current of these lasers. In addition, future computer interconnect circuitry (board-to-board, chip-to-chip) envisages the use of very large numbers of such lasers, on-chip, so that a major reduction of their power requirement is of prime importance.

by Amnon Yariv

Revolution in X-ray Optics

In the past decade, there has been a technological revolution in our ability to generate, control, manipulate, focus, and detect x-rays. The past few years have been witness to significant advances in the development of normal incidence x-ray mirrors and beam splitters, diffraction limited x-ray microscopy, x-ray holography, x-ray waveguides, and CCD x-ray detector arrays. Using these new capabilities, workers in the field are taking the first steps toward the development of sophisticated soft x-ray optical systems, including soft x-ray interferometers, high-intensity x-ray lasers, and projection optics for x-ray lithography.

by N.M. Ceglio

Laser Cooling of Neutral Atoms

The decade of the '80s was the time when many of the dreams of the '70s in laser cooling and trapping of atoms became reality. It was also a time when accidental discovery led to totally new areas of research and deeper understanding of the basic ideas of laser cooling. Indeed, the "technology" of the manipulation of atoms has exceeded the expectations of the first proposals during the 1970's.

by Steven Chu

Optical Bistability

Strictly speaking, a system is said to exhibit optical bistability if it has two steady-state output intensities for the same value of the input intensity over some range of input values. During the '80s, optical bistability was often interpreted more broadly to include all of the steady-state and transient characteristics of nonlinear optical systems that exhibit bistability under some operating conditions. Thus, the topical meetings1-5 on optical bistability and the only single-author text6 on the subject addressed many aspects of the physics of bistable systems, including instabilities (transient phenomena with constant input) and optical switching (controlling light with light).

by Hyatt M. Gibbs

Optical Mapping of Cortical Activity

Neuroscientists have long wanted to "see" events inside the brain. The main objective of their research, after all, is to deduce relevant response properties in various parts of the brain and explore their organizations. The ability to "see" these organizations directly— to visualize regions activated by specific stimuli—has an obvious advantage. This is especially apparent in the mapping of neocortex, where many important organizations occur tangentially in a plane parallel to the surface. Striate cortex (a.k.a., primary visual cortex, area 17, or V1) offers a useful example. Located in the occipital lobe, along with other visual areas, this is the first area (in primates, at least) to receive input directly from the lateral geniculate nucleus (LGN). To a first approximation, this input is mapped retinotopically; neighboring parts of the visual field are represented in neighboring parts of cortex. But there are at least two additional properties—orientation selectivity and ocular dominance—that are mapped laterally as well.

by Gary G. Blasdel

Enhanced Backscattering

When an electromagnetic wave interacts with a dense random medium, the amount of energy scattered in the retroreflection direction (namely, in that of back-scattering), is remarkably larger, on the average, than in any other direction. Examples of optical phenomena resulting from this effect have been known for a long time: the optical glory formed by the bright colored rings surrounding the shadow of an illuminated object over a region of mist, (e.g., the shadow of an airplane over the clouds), is due to a large retroreflectance of light by Mie scattering from the water droplets. Similarly, the heiligenschein consists of a bright region of backscattered light about the shadow that an illuminated object projects over dew-covered grass. These phenomena belong to a wider class of so-called opposition effects; also described in astronomical observations, and among which higher reflectivities of the moon when it is full have been noticed since 1922. More recently, enhanced backscattering of light as it propagates in dense media has been reported.

by M. Neito-Vesperinas

Persistent Infrared Spectral Hole Burning

In 1974, persistent spectral holes were first produced in the inhomogeneous electronic transitions in solids. To this day, persistent spectral features caused by both photochemical and non-photochemical processes are found in ever increasing numbers. In the 1980s, low frequency analogues of these processes were discovered and explored. Persistent IR spectral holes can be generated in the vibrational degrees of freedom of molecular defects in crystals and glasses even though no electronic excitation is involved, i.e., a non-photochemical process. The brief overview of these findings presented below demonstrates that such persistent IR spectral holes do provide a new high resolution technique with which to explore the dynamics of vibrational defects in solids, as well as the properties of the solids themselves.

by A.J. Sievers

Ignoring international optical standards would be costly

Last month, Harvey Pollicove and Robert Novak authored an Engineering Column entitled "Optical Standards—A Costly Change?" (OPN, November 1990, page 33). Their implication was that the U.S. might be better served by keeping the current ANSI Y14.18 optical standard, as opposed to adopting an international standard, ISO 10110. We would argue that it will be more costly to ignore the new international optical standard because it appears likely that it will be adopted unanimously by all the major optics producing nations.

by Robert E. Parks

Science signed, Sealed and Delivered

In an age of fax machines, electronic mail, and video conferences, some 2,500 scientists and over 15,000 fourth through ninth graders have decided to use an older form of communication—the letter. They're part of Science-by-Mail, a penpal program run by the Boston Museum of Science. Each academic year since 1986, students and scientists across the country and overseas have been writing to each other, exchanging notes on everything from pets to solutions for science challenges developed by the Science Museum staff. The challenges, which have ranged from making a square bubble to discovering the wonders of static electricity, are aimed at increasing students' critical thinking skills and their interest in science.

by Susan M. Reiss

Haidinger's brush

It is commonly appreciated that light in the sky is partially polarized and that some insects (e g., honey bees) use the polarization for cues in navigation. It is less well appreciated that our eyes can also detect polarized light and that we can distinguish the azimuth of linearly polarized light under rather common circumstances. The accompanying visual effect is called Haidinger's brush.

by Susan Houde-Walter