The field of all-optical signal processing has matured to the point that prototype devices are now being built. The development of this field, which has potential advantages over electronic processing for certain applications, depends critically on the availability of nonlinear materials that work at relatively high speed and low power. This requirement raises interesting scientificquestions and challenges the existing technology.

Semiconductor microstructures whose dimensions are comparable to atomic dimensions and whose interfaces are atomically smooth exhibit novel optical properties not encountered in the parent compounds. Quantum well structures, which consist of ultrathin semiconductor layers alternately grown one on the other, possess remarkable optical nolinearities and electro-optical properties with potential applications in optoelectronics. In this paper we review our studies of GaAs/AIGaAs quantum well structures (QWS). We briefly discuss the physics of absorption in QWS at room temperature, and we describe the mechanisms of saturation of excitonic absorption and refraction and that of electroabsorption in QWS. Finally, we review the applications of these effects to optical processing and optoelectronic devices.

The minimum power (=5 mW) for room-temperature optical bistability is very similar in bulk and multiple-quantum-well (MQW) etalons. No exciton resonance is obvious in the absorption spectrum of bulk GaAs at 300 K because it blends into the band edge, but it is present, and its area is two-thirds that of the MOW heavy-hole exciton. It is shown that the excitonic nonlinear index change is consistent with the observed bistability.

The first digital all-optical circuits based on intrinsic bistable devices have now been realized. Experimental results with two systems, InSb etalons and nonlinear interference filters containing ZnSe, are presented and discussed. These results indicate that a digital optical processor incorporating devices of this type would appear to be feasible.

This paper discusses the feasibility of all-optical information processing using the nonlinear refractive index at the band gap in direct band semiconductors. Comparison is made of the nonlinearities due to dynamic state filling, also known as the dynamic Burstein-Moss shift, and excitonic nonlinearities. It will be shown that the lowest possible intensity thresholds for nonlinear processes such as optical bistability and degenerate four-wave mixing occur from the dynamic state filling nonlinearity. The wavelengths around 3 µm in mate-rials such as InAs appear to be an excellent choice for parallel processing of a large amount of information.

Liquid crystals as nonlinear optical materials are reviewed. Potential applications of these materials in wave mixing, phase conjugation, and optical bistability are discussed. The extremely large optical nonlinearity and the slow response time of these materials make them unique for studies of some highly nonlinear optical processes and their dynamic characteristics.

We review the conditions of high gain in wave-mixing experiments with photorefractive Bit 2Si020 crystals. These interactions are based on the recording of a moving interference pattern in the volume of the crystal ("drift" recording mode with an externally applied electric field). This technique has recently resulted in large values of the exponential gain coefficient r = 8 to 12 cm-1 for an optimized grating velocity and grating spacing. Applications to image amplification, laser beam steering, amplified phase conjugation, and self-induced optical cavities are presented.

Potential device applications of third-order nonlinear phenomena in guided wave structures are discussed. One of the waveguiding media is assumed to exhibit an intensity-dependent refractive index that results in both an intensity-dependent propagation wavevector and field distribution for the guided waves. These two characteristics lead to many interesting all-optical devices whose operating principles and power levels are outlined here. For example, the intensity-dependent field patterns can be used for thresholding and switching operations in a waveguide context. The intensity-dependent wavevector, used in conjuction with a distributed input or output coupler, can lead to devices such as optical limiters and light-controlled spatial scanners. When gratings are used as distributed feedback elements within a nonlinear waveguide, a whole class of novel devices such as bistable switches and optically tunable optical filters should be feasible.

In this paper, fibers and transmission system designs using nonlinear optical amplification and frequency conversion in single-mode fibers (SMF) are discussed. Optical amplification of signals issued from laser diodes in the first and second optical transmission windows is reported. Raman gains of 40 dB have been obtained for 1.32 um laser diode light at the rate of 11 dB/W in a 2.5 km long SMF. Parametric amplification of laser diode signals at 1.57µm in a 30 m long SMF pumped in the low dispersion region at 1.32 um is presented. Gain values in excess of 30 dB are measured. Up and down frequency conver-sions using stimulated four-photon mixing experiments are also demonstrated.

Because of the large variety of materials and material forms available, organic materials are attractive candidates for ultrafast nonlinear optical signal processing systems based on the intensity-dependent index of refraction, i.e., the change of index of refraction with a change in applied optical intensity. As an example, we consider an organic polymeric system, the polydi-acetylenes. Through molecular engineering (organic synthesis and Langmuir-Blodgett film growth combined with monolayer deposition techniques), the polydiacetylene was prepared in the technologically important format of a planar waveguide on a grating. By measuring the change in the angle that the incident laser radiation couples into the guided-wave mode (via the grating) with the change in intensity of the laser radiation, we determined the intensity-dependent index of refraction to be 10-6 (MW/cm2)-1 in the transparent region of the material. Implications of this reseach for future work related to signal processing applications are discussed.

Two-photon absorption coefficients /32 of ten direct gap semiconductors with band-gap energy Eg varying between 1 .4 and 3.7 eV were measured using 1.06 µm and 0.53 um picosecond pulses. $2 was found to scale as E43, as predicted by theory for the samples measured. Extension of the empirical relationship between $2 and Eg to InSb with Eg = 0.2 eV also provides agree-ment between previously measured values and the predicted 02. In addition, the absolute values of $2 are in excellent agreement (the average difference being <26%) with recent theory, which includes the effects of nonparabolic bands. The nonlinear refraction induced in these materials was monitored and found to agree well with the assumption that the self-refraction originates from the two-photon-generated free carriers. The observed self-defocusing yields an effective nonlinear index as much as two orders of magnitude larger than CS2 for comparable irradiances. This self-defocusing, in conjunction with two-photon absorption, was used to construct a simple, effective optical limiter that has high transmission at low input irradiance and low transmission at high input irradiance. The device is the optical analog of a Zener diode.

Professor Ryszard Pryputniewicz of Worcester Polytechnic Institute has assembled a significant group of papers on the subject of holographic interferometry in celebration of the first twenty years of activity in this field. Several of these papers were received too late for inclusion in this issue but will be published as a group in the next issue of Optical Engineering. Taken together, these papers are an indication of the tremendous progress made during the twenty years of this field's existence.

Described is a technique for optically subtracting the phase terms in holographic interferometry to yield fringe patterns showing the differences between a master object and an object under test. The technique may be applied to the strain fringes produced in holographic nondestructive testing or to contour fringes used to measure shapes of parts. In either case, the fringe spatial frequency is greatly reduced by eliminating unwanted phase terms. The test stress may then be greatly increased, yielding increased detection sensitivity, or the contour interval may be greatly reduced, providing more accurate measurement.

This series of tests demonstrates the feasibility of recording deformation holograms of reflecting subjects that are not mounted on an optical bench and vibration-isolated and that, in fact, can even be submerged in a transparent liquid. This represents an important extension of research into the use of fiber optics in holography. Various considerations for the ultimate development of a practical fiber optic device for making in-situ or "production environment" deformation field measurements by continuous wave holographic interferometry are also discussed.

Wider use of hologram interferometry for quantitative measure-ments has been delayed by the fact that interpolation between the fringe maxima and minima to obtain the optical path difference at a particular point in the field is laborious and inaccurate. A solution to this problem is quasi-hetero-dyne interferometry, which permits rapid and accurate measurements simultaneously at a number of points distributed over the interference pattern. In this technique a television camera is used in conjunction with digital electronics to measure and store the irradiance values at points on a rectangular sampling grid covering the real-time interference fringes. The phase difference between the interfering wavefronts at each point is then calculated from the irradiance values obtained from successive scans of the camera made while the phase of one of the wavefronts is shifted either continuously or in steps. A practical system is described with which values of the optical path difference for 10,000 data points can be obtained with an accuracy of ± A/200 in less than 10 s. The application of quasi-heterodyne hologram interferometry to the measurement of vector displacements and to holographic contouring is discussed.

Optical holography has been successfully utilized for a wide variety of applications in such areas as structural analysis, flow visualization, biomechan-ics, and analysis of particle fields. Pulsed lasers capable of delivering multijoule pulses of holographic quality with pulse widths of the order of 10 ns play an important role in such applications where the phenomena under study are inherently transient or where it is difficult to isolate the experimental apparatus from ambient vibrations. This paper reviews the design, performance character-istics, and selection criteria for the two most popular pulsed lasers used in holography, ruby and frequency-doubled Nd:YAG lasers. In addition, two other lasers, alexandrite and CO2 lasers, which are not commonly used in this area but which offer great potential, are briefly discussed.

The advantage of the rapid development of fiber optics for communication purposes is that now holographic investigations can be made much more flexible if fibers are used. The use of optical fibers in connection with cw lasers has already found interferometric applications within metrology. The possibility of using a pulsed laser with fibers will open important new applications for hologram interferometry within science, industry, and medicine. A fiber coupling technique for pulsed lasers is described, as well as suitable equipment for pulsed or cw laser holographic investigations. Possible applications also are discussed.

A model is described for the formation of diffraction-limited optical images of chrome on glass photomasks using an optical microscope. The effect of the imaging transducer is also considered, and a Gaussian model for a television camera is presented. National photomask calibration standards were examined using bright- and dark-field illumination, and the results are compared with the images predicted by the models. From the results it is concluded that the physical edge and the material properties of the chrome coating cannot be ignored when making precise linewidth measurements on photomasks. A dark-ground measurement technique is described that makes use of the physical edge of the chrome in order to determine linewidths. It is demonstrated how this technique may be used with bright-field illumination in order to measure linewidths on semiconductor wafers precisely. Both the photomask and wafer linewidth measurements may be automated, and a method of adopting the technique for automation is described.

Dither rendition of images is a well-known method to produce pseudogray-scale images using bilevel output devices. Moreover, it reduces the data to 1 bit/pixel. For fixed subpicture (matrix) size there is, however, a contradiction between resolution and the dynamic range of the gray scale. In this paper we show how this conflict may be overcome using variable resolution. For this purpose we introduce a detail-dependent segmentation of the image into variable-size subpictures. We first describe the construction of suitable dither matrices of size 2m X 2m, with m =2, 3, 4. The next section deals with adaptation criteria to obtain the appropriate pseudopixel size. A major aspect of the paper is the fast in-place computer method to break the full picture up into subpictures of appropriate size. This subpicture size depends adaptively on the local "activity" of the picture. The method is demonstrated using 8-bit test pictures and shows good gray-scale rendition and sufficient detail resolution.

Recent advances in high sensitivity moire interferometry are described. In-plane displacement fields are shown for very challenging problem areas: adhesive joints in tension and in bending, flexure of quasi-isotropic composite laminates, and fatigue cracks in aluminum. Reference gratings with 2400 lines/mm (60,960 //in.) were used, providing a sensitivity of 0.417 Am/fringe order (16.4 Ain./fringe order). Excellent fringe contrast was obtained in all cases, even for gradients of 64 fringes/mm (1600 fringes/in.). A four-beam optical system to produce two orthogonal virtual reference gratings was employed. The question of unknown relative displacements in two-body problems was answered by use of a soft bridge between the bodies to provide a continuous path for the fringe count.

A new mode of operation of photoconductive detectors in which a nonlinear current-voltage characteristic is induced by illumination is described. Outputs at harmonic frequencies can be obtained from devices biased with alternating current or at the modulation frequency of a modulated, alternating bias. Several different ways to achieve the nonlinearity are described, one of which uses conventional detector structures. The advantages of this mode of operation are that the large bias-voltage pedestal, normally associated with long-wavelength photoconductive detectors, can be separated from the photo-signal by simple electrical filters and that the problem of flicker noise, in both the detector and the following electronics, is considerably reduced. Photoconductive devices thus become competitive with photodiodes, which may be more difficult to produce with ideal performance and stable characteristics, in detect-ing radiation with very low modulation frequencies. Possible applications for the device may be in slow, parallel-scanned linear arrays; in "staring" arrays; in ultrasensitive detectors employing long integration times; and in position-sensitive, point-source detectors.

A new product for the direct visualization of cardiovascular and peripheral arterial systems has been investigated and developed using a miniature 1.0 mm diameter fiber optic system. This paper presents documentation of the major design parameters that should be considered in angioscope instrument development. The instrument described incorporates a concentric image system/outer shaft design for truer imaging; a working diameter of 1.5 mm, affording visualization of approximately 80% of the coronary arteries; a depth of field experimentally honed to 2 mm to 25 mm; and a system resolution of over 60 l/mm within the typical inspection range.

The measurement of the relevant parameters of single-mode optical fibers (namely, cutoff wavelength, mode field diameter, chromatic dispersion, and refractive index profile) is discussed, with explicit reference both to the work carried out within the Comite Consultatif International Telephonique et Telegraphique (CCITT) and to original techniques developed by the authors. The main research evolution trends are pointed out.

The development of angioscopy has resulted in a variety of approaches and equipment, all with the fundamental purpose of looking inside the cardiovascular system. In this paper, the variations in design of commercially available angioscopes are discussed, along with the differences in methods of introduction into the patient, balloon placement, and steering capabilities. Finally, suggestions are made for the design of future angioscopic devices that would provide more usable units for clinical studies.

Biased Laplacian masks are useful for enhancing the sharpness of edges in images degraded by moderate amounts of blur and noise. Further-more, their implementation on pipelined processors is fast and convenient. However, as we show, they become limited by noise very rapidly. The use of a mask whose bias adapts to local variance measured in 3 X3 windows is explored in this paper. Results demonstrate a significant enhancement of the power of biased Laplacian masks.

A high speed holographic movie camera system has been developed to investigate the dynamic behavior of cavitation bubbles in liquids. As a light source for holography, a high power multiply cavity-dumped argonion laser is used to record very long hologram series with framing rates up to 300 kHz. For separating successively recorded holograms, two spatial multiplexing techniques are applied simultaneously: rotation of the holographic plate or film and acousto-optic beam deflection. With the combination of these two techniques we achieve up to 4000 single holograms in one series.

A technique is developed that permits gas temperatures to be determined in highly pressurized combustion chambers by means of specially designed emission gauges. Two different designs are developed and tested, and sample results of temperature measurements are presented.

Heavy metal fluoride glasses are a recently synthesized class of nonoxide amorphous materials whose molecular structure, morphology, and optical behavior are at variance with the more common and conventional oxide-based glasses. This study will treat two specific classes of fluoride glasses, namely, the fluorozirconates and the fluorohafnates, in which the network formers are ZrF4 and HfF4, respectively, with modifiers being BaF2 and fluorides of rare earths, group III elements, or alkalis. These glasses exhibit high transparency over a frequency range spanning the mid-IR to the near-UV. The primary goal of this study is to focus on the intrinsic Rayleigh scattering of these glasses. The principles of quasi-elastic and inelastic light scattering in glasses will be presented and discussed in great detail. The application of Rayleigh and Brillouin light scattering to various heavy metal fluoride glasses has resulted in valuable information for understanding the possible scattering mechanisms in these glasses. In addition, Brillouin scattering measurements allowed the calculation of the elastic and elasto-optic (Pockels) coefficients of the same fluoride glasses. The full implications that the scattering behavior has on the possible fiber optic waveguide application of heavy metal fluoride glasses will be discussed. The physical significance of the elastic constants and elasto-optic coefficients will also be considered on the basis of existing theoretical models.

The behavior of the Hurter-Driffield curve in the presence of infrared presensitization is determined.

Experiments carried out over diagonal lines of sight through the entire atmosphere support the concept of spatial coherence degradation through forward scattering as described by an aerosol transfer function that strongly affects the wavelength dependence of imaging through the atmosphere. Airborne particulate size and concentration are affected strongly by wind strength and soil moisture. Changes in weather that result in changes in average particulate size of airborne soil-derived particulates also strongly change the wavelength dependence of resolution through the atmosphere as a result of changes in the wavelength dependence of the scattering coefficient. Knowledge of such effects can therefore permit prediction of spectral regions most suitable for imaging through the atmosphere. The roles of aerosols in determining optimum spectral regions for imaging through the atmosphere are quite significant since the turbulence modulation transfer function displays very little spectral dependence.

For several years the hand-held programmable calculator has been known to be an attractive tool for solving some relatively simple problems in optical computation, such as general ray tracing, first-order optics, and optical system analysis. The range of application, however, can be extended far beyond this level and includes the semiautomatic design of unsophisticated optical imaging systems such as the air-spaced doublet and the Cooke triplet, for which examples are presented. In view of the limitations in the number of program steps, it is mandatory to implement such programs with utmost economy of use of these program steps and to utilize the inherent properties of these optical systems in order to obtain convergence in the automatic design improvement part of the programs. While the most appropriate application may be in support-ing the teaching of elementary lens design, these calculator programs can be quite helpful if relatively simple optical systems have to be designed without easy access to a large computer facility and associated software.

The photodeposition effect of selenium hydrosols was investigated for transient photoactivation properties. It was found that volume photoprecipitation (VP) has a faster response time compared to surface photodeposition (SP). SP can be used for permanent recording of plane images. The faster VP process may be used in large volume display applications in near real time.

Long wavelength infrared light-emitting diodes have been fabricated from the ternary Pb-chalcogenide material Pb1-xSn,Se with composition x = 3.2%. As captured in an f/2 aperture optical system, these devices emitted approximately 25 nW of 11 Am power with a bandwidth of 0.2 um between half-power points. The line center shifted from 11.6 um at 16 K to 10.4 Am at 50 K, in agreement with results previously obtained from laser emission data. Such devices are useful as radiometric calibration sources for low background infrared detection systems.

A methodology for computing the effective radiant sterance by numerical integration techniques is presented, and computational accuracy is discussed.

Ii had been struggling with this, my first editorial, for some time, and had just about arrived at a version that suited me. However, before attempting to put the finishing touches on it, I decided to get in the proper frame of mind by taking one of my frequent early morning bicycle rides. That ride was never completed. While cranking along a bike lane just outside Tucson, I was crunched from the rear by-of all things-a pickup truck belonging to a porta-potty company. My beautiful gold bike was completely trashed, and ended up with a fractured left scapula, multiple contusions and abrasions, and a lot of road rash. In addition, I was pretty badly bruised and scraped. I do seem to be mending, though, and hope to be riding again within a couple of months. So much for the war stories.

The incident angle of solar beams, an angle between the solar rays and a line perpendicular to the receiving surface, is the most important measure of the intensity of total sun radiation reaching a terrestrial surface. The equations for calculating the incident angle are cited in this article, and the importance and uses of this knowledge are discussed. A nomograph that allows calculation of the incident angle in a few seconds is included.