This PDF file contains the editorial “1998 In Review” for OE Vol. 38 Issue 02

Slant path calculations are shown of the transverse coherence length (r0), the isoplanatic angle (?0), and the Rytov variance (?2R), using a 6-yr data set of refractive index structure parameter (C2N) from 49.25-MHz radar observations at White Sands Missile Range, New Mexico. The calculations are for a spherical wave condition; a wavelength (?) of electromagnetic radiation of 1 ?m; four different elevation angles (3, 10, 30, and 60 deg), two path lengths (50 and 150 km); and a platform, such as an aircraft, at 12.5 km MSL (mean sea level). Over 281,000 radar-derived C2N profiles sampled at 3 min intervals with 150-m height resolution are used for the calculations. The approach, an ''onion skin" model, assumes horizontal stationarity over each entire propagation path and is consistent with Taylor's hypothesis. The results show that refractivity turbulence effects are greatly reduced for the three propagation parameters (r0, ?0, and ?2R) as the elevation angle increases from 3 to 60 deg. A pronounced seasonal effect is seen on the same parameters, which is consistent with climatological variables and gravity wave activity. Interactions with the enhanced turbulence in the vicinity of the tropopause with the range weighting functions of each propagation parameter is evaluated. Results of a two region model relating r0, ?0, and ?2R to wind speed at 5.6 km MSL are shown. This statistical model can be understood in terms of upward propagating gravity waves that are launched by strong winds over complex terrain.

We report a compact flat-top beam shaper fabricated by laser-assisted chemical etching upon an ion-exchanged high-energy- beam-sensitive glass. The beam shaper combines a phase filter element and a focusing lens element into a single diffractive optical element with a quasi-continuous surface relief profile. The fabrication takes advantage of the laser generation of gray-level patterns and one-step etching that are effective especially for such beam shaper fabrication. The demonstrated beam shaper with good flatness and edge features has an energy efficiency of 82%.

The magneto-optical rotation at room temperature was measured for three Ga:S:La:O chalcogenide glasses at several laser lines in the visible. The first sample was a binary system constituted by 70 mol % Ga2S3 and 30 mol % La2O3, whereas in the second and third ones the lanthanum oxide was partially substituted by lanthanum sulfide, keeping the amount of gallium sulfide fixed. A pulsed magnetic field between 50 and 80 kG was used for the Faraday rotation measurements. The Verdet constant for one of the ternary samples was found to be as high as 0.205 min G2-1 cm2-1 at 543 nm, indicating that these chalcogenide glasses are very promising for magneto-optical applications. The data for each sample were fitted using the expected analytical expression for the magneto-optical dispersion. Measurements of the refractive index of the glasses at 632.8 nm are also reported. Data on the magneto-optical properties of two high Verdet constant, heavy-metal oxide diamagnetic glasses are also included for comparison.

One method for influencing the polarization of light is the use of wire-grid polarizers. For the visible region, this type of element can be realized as a metal-stripe grating with periods less than the wavelength. We fabricate metal-stripe gratings with periods down to 190 nm in thin chromium layers of 35-nm thickness using electron-beam lithography and ion-beam etching. A detailed investigation of the influence of grating period and duty cycle on the polarization effect is carried out to verify the conformity of rigorous diffraction theory and experimental results. The comparison between the two indicates good performance. Polarization ratios of the order of 5 with transmission efficiencies of about 60% in TM polarization are obtained. The connection of the polarization effect realizable and the fabrication technology used is discussed.

A simple referencing technique is presented for a fiber optic reflective displacement sensor. Two receiving fibers are arranged side by side to operate in the negative and positive slopes of the output signal. The ratio of the two outputs provides common mode rejection. A theoretical model of the new referencing technique is developed and verified experimentally. The linear operating range of the sensor depends on the numerical aperture of the fibers and the diameter of the second receiving fiber. The minimum detectable displacement of a representative sensor is 1.57 nm/?Hz in the audio frequency range, which agrees well with the theoretical value. The measured dynamic range of the sensor is 111 dB. Alternative sensor structures to reduce the size and to increase linear range and resolution are discussed.

A novel variable-rate vector quantizer (VQ) design algorithm, which is a hybrid approach combining a genetic algorithm with the entropy-constrained VQ (ECVQ) algorithm is presented. The proposed technique outperforms the ECVQ algorithm in the sense that it reaches a nearby global optimum rather than a local one. Simulation results show that, when applied to image coding, the technique achieves higher peak SNR (PSNR) and image quality than techniques using the ECVQ algorithm.

The design, fabrication, and characterization of an optical, noncontact microsystem for the measurement of torque on a drive shaft are described. The heart of the system consists of a glass integrated optical interferometer chip; ball lenses are used for collimation of measurement beams onto reflectors embedded in the shaft. The system requires no contact with and only minimal modification of the drive shaft.

Theoretical considerations and experimental results related to the principle of dual-wavelength absolute distance interferometry (ADI) are presented. It is predicted that the required initial uncertainty of a length to be measured was related to the synthetic wavelength, its stability, and the length to be measured. An absolute distance measurement approach employing a dual-longitudinal-mode He-Ne laser is proposed. In this approach, a heterodyne interferometer with two acoustooptic modulators (AOMs) is used to directly measure the fractional fringe at the synthetic wavelength formed by them. The common optical path configuration in the simplified interferometer eliminates the influences that air turbulence and mechanical vibration have on measurement. Factors that influence measurement accuracy are analyzed theoretically and experimentally. Mode mixing is determined as the main influence on the accuracy of the approach, and the error caused by it can be compensated through data processing if the amount of mode mixing is fixed. Additionally, an experimental result is presented that obtains an accuracy of 140 ?m for 25 m.

A scanning lidar was used at Wallops Island, Virginia, to locate regions where aerosols are generated by breaking waves and to characterize the resultant aerosol plumes. False-color lidar images show views of both the horizontal and the vertical extent of the plumes above the surf zone. A 30-min-average location of aerosol generation is shown as a contour overlay on the lidar images. The strength of the backscatter is separated into three regions: the land side of the surf zone, the surf zone, and the ocean side of the surf zone.

Several equalizers are applied to digital versatile disk-readonly memory (DVD-ROM) systems. As a result of an imperfect writing process, nonlinear distortion may be generated in the replay signal. Neural equalizers, which have strong nonlinear mapping capabilities, are applied to the system to compensate for the nonlinear distortion. We perform computer simulations for two systems: a DVD-ROM system with and without run-length-limited (RLL) modulation code. Nonlinear distortion such as domain bloom is the dominant interference of the channel in a DVD-ROM system with RLL code because this code reduces the linear intersymbol interference (ISI). Thus the experimental results with RLL code show that the signal-to-noise ratio (SNR) gain of equalizers using neural networks over conventional equalizers increases in proportion to the nonlinearity of the DVD-ROM channel. When RLL is not used, however, linear and nonlinear ISI is the dominant interference of a DVD- ROM channel. Therefore equalizers with a decision-feedback section in a DVD-ROM system with RLL code outperform equalizers without a decision-feedback section in eliminating ISI. Consequently among the equalizers in our computer simulations, the neural decision-feedback equalizer (NDFE) showed the best bit error rate (BER) performance.

Morphological shared-weight neural networks previously demonstrated performance superior to that of MACE filters and standard shared-weight neural networks for target detection. Empirical analysis showed that entropy measures of the morphological shared-weight networks were consistently higher than those of the standard shared-weight neural networks. Based on this observation, an entropy maximization term was added to the morphological shared-weight network objective function. In this paper, target detection results are presented for morphological shared-weight networks trained with and without entropy terms.

An optical neural network system is proposed utilizing photochromic materials as an optically addressed spatial light modulator (OASLM) that preserves the analog data stably. By combining this OASLM and the conventional SLM (SLM-OASLM strategy), the system controls massive weights of interconnection, which are represented as the local photoisomerization in the OASLM, quite efficiently in a parallel and active manner. A series of computer simulations are carried out in terms of the self-organization pattern mapping (SOPM) as an example of application to optimize the model parameters and to check the noise stability. Based on the results of simulation and the physical properties of photochromic materials, the practical data processing performance of the system is estimated.

The problem of object recognition in digital images is one of the more important topics for many industrial and military applications of automatic vision systems. In addition to noise effects, the identification of imaged objects is greatly hindered by perspective modification of the object's shape and other optical-geometric effects such as the apparent position and scale (magnification). The application of a new algorithm designed to automatically assess orientation, position, and scale of objects portrayed in digital images is discussed. The algorithm, which utilizes a scene description based on image edges and the related Hough transform, also performs object-background separation. The obtained procedure is tested with excellent results using synthetic and real images of flying objects. The purpose is to exploit new techniques and algorithms for automatic target detection or tracking.

A number of aberration compensation systems are investigated for use with multiple-layer optical data storage. A Burch-type objective lens in conjunction with a Galilean telescope is found to be a compact, simple and effective optical system for spherical aberration compensation.

Optical correlators used in pattern recognition should have their operation optimized according to physically measurable behavior related to the correlator's task. The optimization is best done with explicit consideration of the limited set of complex values available to the filter. We earlier developed a theoretical filter computing strategy, MEDOF (minimum Euclidean distance optimal filter), that optimizes a wide variety of correlation metrics on physically realistic modulators. We use a single reference image and add noise according to a stated model. We show laboratory results for filters optimizing a variety of metrics on a highly coupled SLM. The results show improvements in the metrics by using MEDOF when compared with some other filter computing strategies such as matching phase. Optimizing SNR appears preferable to the other metrics when detecting an object in clutter of known power spectral density.

This paper presents a method of evaluating the influence of future alternative interconnection technologies at system level. A substitution algorithm has been developed that replaces single critical paths in complex CMOS systems by optical interconnects so that the effect of each substitution on overall system performance can be calculated, in order to decide whether this substitution is feasible under such technological constraints as cost, speed, area, and power consumption. In this paper the focus is on determining the maximum clock frequency, as a typical frequency for data exchange, which is at least up to now the upper frequency limit for processing elements. To attain the optimum ratio of electrical to optical interconnections for systems conceived as hybrid (i.e., containing both types of interconnections), the model takes into account the influence of every single substitution on the electrically wired rest of the system. Thus an optimum interconnection technology can be determined even before a system of given complexity is designed.

We describe how an evolution strategy is used to automate the design of luminaire reflectors. In particular, we outline a computer simulation, consisting of a 2-D optical reflector with point light source, which is implemented for this purpose. The reflector shape is modeled using a Bezier curve representation, and photometric distributions are calculated in the near, middle and far fields using a ray-tracing approach. The automation of the design process is achieved through the use of a novel evolution strategy, termed differential evolution. For the effective operation of differential evolution, a merit function specific to luminaire reflector design is presented. Finally, we describe our investigation into the validity of the evolution strategy approach to reflector design. Based on our results, we propose that the technique is not only valid but also feasible.

Photographic media can be exposed by two-photon absorption, rather than the more usual one-photon absorption. This leads to the question of whether the simultaneous absorption of a pair of photons could be accompanied by a twofold spatial-resolution enhancement. We find that ordinary two-photon absorption merely enhances the photographic contrast, or gamma. While this improves the spatial resolution somewhat, it does so at the expense of requiring tighter control over the incident light intensity. Instead, we introduce a new type of exposure arrangement employing a multiplicity of two-photon excitation frequencies, which interfere with one another to produce a stationary image that exhibits a true doubling of the spatial resolution. © 1999 Society of PhotoOptical Instrumentation Engineers. [S0091-3286(99)00402-X]

A direct phase-to-radial distance mapping algorithm for 360 deg profilometry with time delay and integration (TDI) imaging is presented. This method, based on an inherent mapping relationship, is capable of speedy and accurate measurement without the determination of any geometric parameter. The capability of the mapping algorithm is demonstrated by measuring a plane and a shoe.

A four-band (8.2 to 9.2, 10.5 to 11.5, 11.5 to 12.5, and 8 to 14 ?m), prototype, thermal-IR radiometer, model CE 312 [CE 312 is the company model number. In previous papers, the CE 312 was called the CLiMAT (conveyable low-noise IR radiometer for measurements of atmosphere and ground-surface targets)], with a built-in radiance reference is been fabricated by CIMEL Electronique (Paris, France) for use as a field instrument. The instrument is briefly described, laboratory characterization is detailed, and its field measurements are compared with those from three other radiometers. The CE 312's main characteristics are linearity of better than 0.8%, field of view of 9.5 deg; noise-equivalent temperature difference of 0.06 to 0.2 K (depending on the band) for brightness temperatures of 0 to 75°C; SNR greater than 1100 for the broadband and greater than 400 for the other bands for brightness temperatures between 10 and 80°C; and repeatability of the measured radiance smaller than 0.35% after four field campaigns, corresponding to 0.2 K in terms of brightness temperature. Field measurements were conducted over different periods during 1996 at Jornada Experimental Range, New Mexico, Lunar Lake and Railroad Valley, Nevada, and Lake Tahoe, California. The CE 312 compares quite favorably with the other instruments: the brightness temperature at two different sites compared to within 0.3 K with two instruments. These measurements show that the CE 312 thermal-IR radiometer is very stable for ambient temperatures varying between 15 and 60°C and that the availability of several filters in the thermal-IR region can help tremendously to improve the accuracy of the radiance determination.

We present a new approach to implementing the joint transform correlator based on a single ferroelectric spatial light modulator used as a binary phase-only device. The correlator is a 1/f joint transform correlator that uses two passes through the optical system to generate the correlation peaks. The input scene and reference image are encoded with a checkerboard pattern to produce a spectrum with no zero order. The spectrum is then convolutionally binarized and replayed on the same spatial light modulator. Experimental results demonstrate strong correlation peaks, with the zero order in the correlation plane reduced to 6 dB below the correlation peaks. The correlator also demonstrates enhanced differentiation between closely correlated images.

Most spatial light modulators (SLMs) are limited in that they cannot produce arbitrary complex modulations. Because phase and amplitude are usually coupled, it is difficult to computer design appropriate modulation patterns fast enough for the real-time applications for which SLMs are suited. Dramatic computational speedups can be achieved by using encoding algorithms that directly translate desired complex values into values that the modulator can produce. For coherently illuminated SLMs in a Fourier transform arrangement, pseudorandom encoding can be used. Each SLM pixel is programmed in sequence by selecting a single value of pixel modulation from a random distribution having an average that is identical to the desired fully complex modulation. While the method approximates fairly arbitrary complex modulations, there are always some complex values that are outside the encoding range for each SLM coupling characteristic and for each specific pseudorandom algorithm. Using the binary random distribution leads to methods of evaluating and geometrically interpreting the encoding range. Evaluations are presented of achieving fully complex encoding with SLMs that produce less than 2? of phase shift, identifying an infinite set of encoding algorithms that encode the same value, identification of the maximum encoding range, and geometric interpretation of encoding errors.

Design optimization and experimental studies of bandpass optical filters useful in the near-IR region from 1200 to 2000 nm are carried out. It is seen that the use of Si/SiO2 multilayer coating in the form of a multicavity Fabry-Perot (F-P) filter, either alone or in combination with a long-wave-pass (LWP) filter, on an intrinsic silicon (Si) substrate simplifies the filter configuration, obviating the use of the conventional cemented filter configuration. The filter manufacturing studies demonstrate the production of filters with high in-band optical transmittance T>0.90 and excellent rejection T<0.001 from 400 to 2000 nm. The spectral stability of the filters with respect to temperature is found to be sensitive to substrate temperature; the higher the substrate temperature, the higher is the spectral stability. For filters deposited at a substrate temperature of 600 K, the spectral stability is in the range of 2.27 to 1.58x 104 K, over a filter temperature from 298 to 373 K, which is mainly dictated by the refractive index coefficient (RICFT) of Si films.

In cone-beam tomography, relatively small misalignment of the imaging system is geometrically magnified and may cause severe distortion of the reconstructed image. We describe a method for alignment of a cone-beam tomography system built on an x-ray microfocus tube, an image intensifier, and a high-resolution CCD camera. To obtain geometrical parameters of system misalignment, we suggest measuring two 180-deg-opposed cone-beam radiographs of a specially manufactured calibration aperture. An advantage of the aperture over other calibration objects is that we can easily restore its idealized picture by applying a certain threshold to the measured data. The method permits the lateral displacement vector and lateral tilt angle to be accurately found. Unlike other alignment methods, our approach enables virtual system alignment by using mathematical processing of the measured data, rather than moving the parts of the system. The virtually aligned system data are used for 3-D image reconstruction by a standard filtered back- projection algorithm. Experimental results demonstrate considerable improvement of the image quality after applying the alignment method suggested.