This PDF file contains the editorial “Something Special” for OE Vol. 38 Issue 04

The optical wedge engenders a distortion aberration in a collimated beam in general. Presented is a set of distortion formulae and of third-order distortion formulae in the component form of TAx and TAy for optical wedges. The main dependence of the distortion as a function of the apex angle, of the incident angle of the optical axis, and of the view field of the optical wedge is established. The slope formula of a curved line, which is the image of a straight line of an optical wedge, is developed. They are suited for the large incident angle of the optical axis and the small apex angle. The analysis and calculation indicate that the image of a square for an optical wedge is in the shape of a church bell with a slightly convex or flat side rather than with a concave side.

The effect of low power, photorefractive phase-conjugate feedback on the spectrum of a semiconductor laser is considered. Relaxation oscillations are observed at 4.2 GHz on either side of the fundamental lasing mode. Unstable external cavity mode features appear underneath the envelope of the relaxation oscillations. A further increase in feedback results in dramatic linewidth broadening. The experimental results are found to be similar to the effects observed with conventional reflectors and they closely agree with recent theoretical predictions.

Vector quantization (VQ) and block truncation coding (BTC) are two popular schemes for image compression. VQ has been widely used for its simplicity and low bit rate; on the other hand, BTC has the advantages of requiring little computational cost and providing high image quality. To put together the advantages of VQ and BTC, three hybrid methods based on VQ and BTC are proposed in this paper. According to our experimental results, it is shown that the first two methods of ours produce good image quality while keeping low bit rate. Meanwhile, the third method produces higher image quality while keeping acceptable bit rate. Also, these methods require little computational cost. In other words, our methods can be applied to diverse occasions according to their specific requirements.

We show an unsupervised parallel approach called the annealed Hopfield neural network (AHNN) with a new cooling schedule for vector quantization in image compression. The main purpose is to combine the characteristics of neural networks and annealing strategy so that online learning and hardware implementation for vector quantization are feasible. The idea is to cast a clustering problem as a minimization problem where the criterion for the optimum vector quantization is chosen as the minimization of the average distortion between training vectors and codevectors. Although the simulated annealing method can yield the global minimum, it is very time consuming with asymptotical iterations. In addition, to resolve the optimal problem using Hopfield or simulated annealing neural networks, the designer must determine the weighting factors to combine the penalty terms. The quality of the final result is very sensitive to these weighting factors, and feasible values for them are difficult to find. Using the AHNN for vector quantization eliminates the need for finding weighting factors in the energy function, which is formulated and based on a basic concept of the ‘‘within-class scatter matrix’’ principle. In addition, the rate of convergence is much faster than that of simulated annealing. The experimental results show that better and more promising solutions can be obtained using the AHNN than the LBG in image vector quantization. The convergence rate with various cooling schedules is discussed.

Extracting a multivariate nonlinear physical model from a set of satellite images is considered as a multivariate nonlinear regression problem. Multiple local solutions often prevent gradient type algorithms from obtaining global optimal solutions. A method of solving this problem is presented based on the simultaneous perturbation stochastic approximation (SPSA) algorithm. The method is applied to a problem of estimating the distribution of energetic ion populations in the magnetosphere from global images of the magnetosphere. The approach uses multiple objective functions: single image errors and the summation of square image errors. The algorithm is demonstrated on simulated energetic- neutral atom (ENA) images. Within a reasonable number of function evaluations, the process converges and reconstructs the images with a mean square error less than or equal to 0.1% of the original image. Also, the SPSA method is compared with results obtained from simulated annealing (SAN) in a single objective function setting. In the comparison study, SPSA has a 3:1 advantage over SAN in both accuracy and efficiency measures.

Various approaches to edge detection for color images, including techniques extended from monochrome edge detection as well as vector space approaches, are examined. In particular, edge detection techniques based on vector order statistic operators and difference vector operators are studied in detail. Numerous edge detectors are obtained as special cases of these two classes of operators. The effect of distance measures on the performance of different color edge detectors is studied by employing distance measures other than the Euclidean norm. Variations are introduced to both the vector order statistic operators and the difference vector operators to improve noise performance. They both demonstrate the ability to attenuate noise with added algorithm complexity. Among them, the difference vector operator with adaptive filtering shows the most promising results. Other vector directional filtering techniques are also introduced and utilized for color edge detection. Both quantitative and subjective tests are performed in evaluating the performance of the edge detectors, and a detailed comparison is presented.

A novel image restoration method using the modified evolution strategy (MES) is presented. Three main improvements consist of coordinate descending mutation, a ‘‘survival of the fittest’’ selection rule, and hybrid evolution strategies. Theoretical analysis and experimental results demonstrate the efficiency of the proposed method.

A method is presented for the process of elimination of the influence of ellipticity of interfering waves in moire interferometry, using a combination of an analytical solution and direct measurements of ellipticity parameters of each interfering wave. The method is advantageous in that it makes it possible to access and overcome the unfavorable influence of ellipticity. The phase difference of interfering waves is described. A model case is used to show the influence of wave ellipticity and the fluctuation of light source output on the mean irradiation intensity of the interference field, visibility of fringes, and the error of the phase function determination.

Two different types of micromirrors are integrated with a fiber laser to modulate the cavity Q-factor. Both systems operate at frequencies up to 60 kHz and generate a pulse peak power 100 times higher than the continuous emission. We simulate the emitted pulses and find a good agreement with the measured value for the period of relaxation oscillations. The simulations also show the necessity of a shorter rise time of the Q-factor modulation to achieve one single giant and narrow Q-switched pulse.

The main temporal and energetic characteristics of an ultrabroadband nanosecond Ti3+:sapphire laser are measured. The laser resonator employs a prism pair and a diaphragm creating a "spatially dispersive'' cavity, and then is allowed to maintain the lasing conditions simultaneously for different spectral components in different areas of the active element. Second-harmonic radiation from a Q-switched YAG:Nd3+ laser with a 532-nm oscillating wavelength and up to 210 mJ of pulse energy was utilized as a pumping source. The ultrabroadband laser was transversely pumped and operated simultaneously in 685- to 955-nm region with a minimum pulse duration of 25 ns, pulse energy of up to 17.8 mJ, a pulse repetition rate of 10 Hz, and 8.5% real pumping to lasing efficiency. Oscillation build-up time was 19.8 ns at a 775-nm wavelength and was varied for different spectral components by only 7 ns in the 700- to 912-nm region.

We present a technique to improve the performance of an annular field system that has a uniform aberration residual and an element at the stop or one of the pupils. The technique entails to departing from axial symmetry by introducing a nonaxially symmetric asphericity. Depending on the angular field extent an improvement in root mean square (rms) of 30% or more can be achieved.

A simple method to measure the aspherical surface of test elements by immersing them in a liquid crystal container is presented. The theory of the method is given and the experimental results of two lenses, one made of a plane surface and a spherical surface and the other with a spherical surface and an aspherical surface, are demonstrated.

The classical cross-correlator detector is extended to a fuzzy detector used in a digital bandpass M-ary communication system. The fuzzy detector uses the combined fuzzy product to aggregate two fuzzy sets to detect the presence of one signal, buried in a noisy received waveform, out of M possible signals. The error performance of the fuzzy detector is demonstrated via simulation and is compared to the crosscorrelator using M-ary phase shift keying modulation. The error performance of the fuzzy detector is shown to be better than that of the classical correlator detector, in particular, when the noise level is high. The prospect of using the fuzzy detector in an optical communication system is also demonstrated.

A new thinning algorithm for binary images is presented. The algorithm is based on the analysis of crossing segments that are constructed by an almost perpendicular approach to the direction of the line. In the appropriate area, similar crossing segments or similar boundaries as well as special connecting points are located. The experimental results carried out on handwritten characters show the efficacy of the algorithm with respect to other methods.

The problem of image noise estimation for improved noise robustness and discrimination capabilities of optical correlation filters is discussed. Colored noise is often used in the literature as an approximation to the true noise spectral density in the input image of a correlator. This conjecture is verified on different kinds of input images, i.e., their power spectral densities are fitted to a colored noise model. The quality of the resulting approximation is discussed. We then show that incorporating this noise estimation into optimal trade-off filters can significantly improve both the discrimination capabilities and the SNR of the resulting adaptive correlation filter above that of the classical filters for which the noise parameters are not estimated. Although its performance is in general found to be markedly inferior to that of true nonlinear filtering techniques that are optimal for adaptive image correlation, the proposed adaptive method is attractive in terms of computation time. Possible optical implementations of the proposed method are also discussed.

We investigate the effects of high input power on the performance of optical bistable symmetric self-electro-optic effect devices (SSEEDs) using extremely shallow quantum wells (ESQWs). In this study we consider four ESQW SEEDs: antireflection (AR)-coated ESQW S-SEEDs (E-SEEDs), back-to-back AR-coated ESQW S-SEEDs (BBESEEDs), asymmetric Fabry-Pe´ rot (AFP)-ESQW S-SEEDs (AE-SEEDs), and back-to-back AFP-ESQW S-SEEDs (BBAE-SEEDs). As the input power increases, device performances such as on/off contrast ratio and on/off reflectivity difference are seriously degraded due to the ohmic heating and exciton saturation. But switching speed of the device increases up to a certain value and then begins to decrease. With reasonable optimization of the input power, we simulate and analyze the system bit rate of a cascading optical interconnection system using the various ESQW S-SEEDs changing the number of quantum wells for the external bias of 0 V and 25 V. The BBE-SEED with 48 quantum wells shows the maximum bit rate of 100 Mbytes/s at 0-V bias, while the E-SEED with 60 QWs shows the maximum bit rate of 109 Mbytes/s at 25-V bias. In AFP cavity structures, the AE-SEED with 18 QWs and the BBAE-SEED with 28 QWs show maximum bit rate of 311 and 156 Mbytes/s, respectively, at 0-V bias.

Three adaptive multichannel L-filters based on marginal data ordering are proposed. They rely on well-known algorithms for the iterative minimization of the mean square error (MSE), namely, the least mean squares (LMS), the normalized LMS (NlMs), and the LMS- Newton (LMSN) algorithms. We treat both the unconstrained minimization of the MSE and the minimization of the MSE when structural constraints are imposed on the filter coefficients. The performance of the proposed adaptive multichannel L-filters is compared to that of other multivariate nonlinear filters in color image filtering. Adaptive multichannel linear filters and adaptive single-channel L-filters are considered as well. Performance comparisons are made in both RGB and U*V*W* color spaces. The proposed adaptive multichannel L-filters outperform the other candidates in noise suppression for color images corrupted by mixed impulsive and additive white contaminated Gaussian noise.

There is great interest in using commercially available twisted-nematic liquid crystal displays as programmable spatial light modulators. Proper use of such displays, particularly as phase-only filters for optical correlators or as diffractive optical elements, requires knowledge of the physical parameters of the device including the twist angle, the birefringence, and the orientation of the director axes. A simple experimental technique was published for evaluation of these parameters. When we used this technique, however, we found a number of different sets of values that satisfied the experimental data. We demonstrate that additional measurements at different wavelengths can resolve these ambiguities and provide valuable information about the wavelength dependence ofthe birefringence.

A new optically addressed spatial light modulator using a GaAs single crystal plate is developed for incoherent-to-coherent optical image converter. The device has the same structure as the Pockels readout optical modulator. The basic characteristics of the device are measured as half-wave voltage, 10.5 kV; sensitivity, 0.65 ?J/cm2 at visible light; and resolution, 5.6 line pairs/mm. The high-frame-rate (over 500 Hz) operation and high contrast ratio (over 700:1) are experimentally confirmed. The Fourier transforms of alphabetic characters are successfully performed and an application to joint transform correlator is demonstrated.

A solution to the problem of automated detection of targets with unknown spectral properties in multispectral imagery is presented that makes use of three background characterization and suppression algorithms in series. The first, parametric Bayesian clustering, is used to accurately characterize individual elements of the background scene. The second, background suppression filtering, eliminates those dimensions of multispectral space containing the majority of background energy. Finally, a multidimensional extension of the well-known Linde- Buzo-Gray (LBG) clustering algorithm is used to characterize what remains of the background and extract any anomalous target signatures. The results of this process are compared to spectral decorrelation (RX) filtering alone, LBG clustering alone, and RX filtering in combination with background suppression filtering. The process presented is shown to be significantly superior to each of these algorithm combinations.