This PDF file contains the editorial “May I have your attention, please!” for OE Vol. 42 Issue 01

We propose and demonstrate a novel holographic multiplexing technique, polarization multiplexing, in LiNbO₃ with a 90-deg geometric scheme. The technique provides the possibility for storing the index fringes by two waves with the polarization state orthogonal to each other.

We describe the choice of parameters involved in designing a system in which an unmanned airborne vehicle using a camera relays image data to a nearby ship where a human, possibly with the help of a recognition system, is alerted to the possibility of a collision with a whale or other large marine mammal. A key constraint is that the system be inexpensive and, where possible, realized from off-the-shelf equipment.

The noise of a hybrid soliton pulse source (HSPS) with a linearly chirped fiber Bragg grating is analyzed by couple-mode equations, including spontaneous emission noise when the HSPS is mode-locked. The relative intensity noise (RIN) is calculated using numerical solutions of these equations. It is shown that the noise peak in the RIN spectrum locates at the inverse of the round-trip time in the compound cavity and this peak is a little broad. In this range, a high noise level affects the operation of device, and therefore transform-limited pulses are not obtainable. It is also shown that in the mode-locking range, except approximately at the frequency of the compound cavity, the RIN value is low.

We propose new methodology for designing an almost ideal bandpass filter as a cascade of two piecewise uniform fiber Bragg gratings. The first grating is designed to get the desired bandpass amplitude spectrum. The second is used to flatten the group delay characteristics of the first. The parameters of the gratings are determined by using the simulated-annealing technique and the steepest-descent algorithm. At the expense of a complicated structure, we are able to obtain an almost ideal bandpass filter with a flat group delay.

We report on results of laboratory and field tests of dual-band focal plane arrays (FPAs) in the medium-wave infrared (MWIR) and long-wave infrared (LWIR), produced under the Army Research Laboratory's Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system, and by BAE Systems using quantum-well infrared photodetector (QWIP) technology. The HgCdTe array used the DRS HDVIPTM process to bond two single-color detector structures to a 640×480-pixel single-color readout integrated circuit (ROIC) to produce a dual-band 320×240 pixel array. The MWIR and LWIR pixels are co-located and have a large fill factor. The images from each band may be read out either sequentially (alternating frames) or simultaneously. The alternating-frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using III-V materials grown by molecular-beam epitaxy (MBE). The LWIR section consisted of GaAs quantum wells and AlGaAs barriers, and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel, allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two-color operation), which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The advantages and disadvantages of the two material systems for producing medium-format dual-band FPAs are discussed.

Discrete thresholded neural networks of the Hopfield type have been employed to recover the regularized least-squares (LS) solution from noisy image data. The goal is to attain this solution efficiently by conserving computational and storage requirements as the dimensions of the problem grow large. This paper discusses configurations of these networks that recover the LS solution by partitioning the networks and adopting a switching operation between active and inactive partitions to optimize the objective function. Sequential and parallel update procedures on active partitions offer a means to avoid limit cycling and imposing zero-self-feedback constraints. Examples of image noise reduction and the identification of the LS solution from corrupted image data are presented.

White-light interferometry (WLI) for object topography measurements relies on an accurate rate of change of the optical path difference (OPD) between the object and reference beams. However, the motion of the scanner realizing the OPD change is not perfect, and scanning errors directly impact measurement accuracy. We describe a white-light interferometer that is capable of accounting for these errors and correcting them in real time through the monitoring of the scanner motion. This monitoring is achieved by embedding an additional high-coherence interferometer into the system. Besides allowing for monitoring the scanning progress, this system also provides the means to automatically calibrate the WLI. Significant improvements both in the accuracy and repeatability are demonstrated experimentally. Data analysis is discussed as well.

A phase-measuring Fizeau interference microscopy with a laser diode (LD) source is studied that is based on a phase-shifting method using wavelengths varied stepwise by changing the current in the LD. The effect of the multiple-beam interference fringes on three kinds of phase-extraction algorithms is theoretically and numerically investigated. The phase error due to multiple reflections has a periodicity to one-fourth of the fringe spacing and a four-stepping method has the smallest error among interesting algorithms. The measurement error with a four-stepping method can be minimized by averaging two sets of measurements with an initial phase difference of π/4. This eases the adjustment of an initial phase by a tunable LD. A phase error caused by the power change in the LD is eliminated by normalizing the captured interferograms. Experimental results are presented with a Fizeau interferometer to show the usefulness of such procedures.

Removing speckle noise in electronic speckle pattern interferometry (ESPI) from a single speckle fringe pattern while keeping the fringe features is a difficult problem and remains unsolved. In this paper, the spin filtering proposed by the authors is developed further with curved surface windows to filter out speckle noise from a single speckle fringe pattern. With the large curved surface windows, the speckle noise can be removed nearly completely by the spin filtering. The most important advantage of the method is that no blurring effects occur for speckle fringes and the smooth fringe pattern of a phase field is retrieved and derived. The skeletons of the smoothed fringe pattern are extracted automatically.

We propose a new interferometric approach for optical coherence tomography applications. The device uses the Young's-hole principle to produce the fringes. The low-cost and versatile ion-exchange technology has been chosen to fabricate an integrated optics chip. This new approach has allowed us to eliminate optical delay lines presenting mechanical instabilities. We present fringes using both laser and white light.

A shearing interferometer is used to measure the temperature and temperature profile inside the axisymmetric flame of a Bunsen burner and the two-dimensional flame of a slot burner used in an atomic absorption spectrophotometer. Experimentally obtained results are compared with the results measured with a platinum–platinum-rhodium thermocouple and multilogger. The values are in good agreement. The effect of aspiring water through the nabulizer on the temperature profile of the slot burner is also presented.

We have investigated the utility of a Lau phase interferometer with linear gratings and white light source to measure the temperature and temperature profile of a flame produced by a slot burner used in an atomic absorption spectrophotometer. The temperature values measured using a Lau phase interferometer and a thermocouple and multilogger are in good agreement. The effect of aspirating water through a nebulizer on the temperature profile and details of theoretical analysis and experimental investigations are also presented.

We have developed a measurement method to evaluate deformation of a metallic membrane (thickness 2 μm) in a micro-radio-frequency switch. The method is based on a modified Michelson interferometer incorporated with optoelectronic devices including a He-Ne laser, conventional optics, a CCD sensor, and a photodiode. To detect the deformation of the membrane in the rf switch, a He-Ne laser probe 10 µm in diameter is directed onto the specimen. The laser beam reflected off the membrane is combined with a reference beam. The combined laser beams are regulated to follow a common path. The resulting circular interference fringe pattern is simultaneously recorded by a CCD sensor and a photodiode. The deformation of the membrane is determined from the order of the resulting fringe pattern. As demonstrated by the experimental results, the proposed method is capable of measuring deformation of the rf switch at submicron levels.

A differential absorption lidar system for simultaneous measurement of O₃ and NO₂ in the lower troposphere is developed. The system is based on a pair of Nd:YAG pumped dye lasers each capable of emitting two wavelengths on alternate pulses. Sum frequency generation is used to generate laser pulses of wavelengths 446.8 and 448.1 nm used for NO₂ measurement. O₃ concentrations of approximately 30 to 60 ppb and NO₂ concentrations of approximately 4 to 45 ppb are obtained for a vertical range of 1000 to 1500 m. Changes in O₃ and NO₂ concentration over a time interval of a few hours suggests an anticorrelation between these quantities. The measurement error, estimated from statistical error and instrumental error, is 2.5 ppb for O₃ concentration and 5.5 ppb for NO₂ concentration. In the case of NO₂ measurement, the error due to differential backscatter gradient is found to be a major source of error, but does not fully account for the measurement error.

An optical fiber interferometer is described for measuring the out-of-plane displacement of microelectromechanical structures. The interferometric system has a theoretical measurement dynamic range greater than 108. Experimental results characterizing an electrostatically actuated polysilicon flexure beam are presented. The results are in good agreement with modeling data based on coupled boundary-element and finite-element analysis.

Silicon, shallow junction, Geiger-mode avalanche photodiodes (APDs) can be manufactured with complementary metal-oxide semiconductor (CMOS) compatible processing steps and provide single-photon-counting sensitivity. As we move toward providing integrated detection of increasingly nanoscopic-sized emissions, small-area detectors and arrays that can be easily integrated into marketable systems will be required. Geiger-mode diodes with diameters of 10, 15, and 20 µm are manufactured and the dark counts measured at 10 V above breakdown are 9, 95, and 990, respectively, at room temperature. The simulated and measured optical crosstalk is found to be significantly reduced for detector pixel pitches beyond 300 µm. The activation energy of the dark count with temperature is found to be 0.58 eV, representing an order of magnitude drop in dark count for every 27°C decrease in temperature. The responsivity of the detectors, without antireflection coatings, is found to peak between 550 and 650 nm with a photon detection probability of 43% at 10 V above the breakdown voltage. The low dark counts of the detectors and high photon detection probability highlight the potential these detectors have for fluorescence decay experiments and also in future integrated photonic detection systems.

A far-infrared photodetector based on twenty layers of self-assembled InAs/GaAs quantum dot superlattices is fabricated. Measurements by Fourier-transform infrared spectroscopy at 77 K reveal that the response of the device is in the range of 9 to 16.3 μm, with a peak absorption wavelength near 13.2 μm (756 cm^–1). The electrical characteristic of the device's current-voltage response is 63 μA at –1-V bias voltage when a 1-cd light source is at 15-cm distance.

We describe the development of a high-resolution absolute optical encoder that achieves a resolution of 4,194,304 division/rev (22 bits) at a diameter of 34 mm, based on a new angle detection principle in which the two sets of interpolation data from two differing cycles by multitrack and M-code (maximum length sequence) data are combined. This absolute encoder also achieves an accuracy of 20 arcsec (16 bits) to minimize the sinusoid wave distortion of the signal from the photodetector.

A method of 3-D optical metrology based on a triangulation system using holographic axilens to increase the depth-measuring range without any decrease in the axial or the lateral resolution is presented. The element is designed according to an analytic ray-tracing optimization approach and is recorded as a computer-generated hologram. A sixfold increase in the depth-measuring range is experimentally obtained while nearly diffraction limited light spots are completely maintained.

This study proposes a new method for measuring the 3-D surface profile of an object moving at constant speed. A sinusoidal grating pattern is projected on a moving object. Then, grating patterns that are deformed according to the profile of the object are acquired by three linear array sensors. Since a linear array sensor records light intensity along an original grating line, the resultant image forms a fringe pattern that represents the profile of the object. Using three images obtained by the three linear array sensors, the phase distribution of the fringe pattern is calculated by a phase stepping method without any phase stepping devices. Consequently, the profile of the moving object can be evaluated from the phase of the fringes, since the value of the phase is simply proportional to the height of the object. Experimentation demonstrates that the accuracy is about 4.3% for the wrapped phase range of 2π rad with a speckle pattern present. The proposed method can be used for inspection of moving objects such as products on an assembly line.

We report a noncollinear optical parametric generator (OPG) using periodically poled LiNbO₃ (PPLN) pumped by a Nd:YAG laser. Signal tuning is achieved from 1.58 to 1.81 μm by angular rotation of the PPLN crystal. A signal linewidth of 68 nm was found at 1.76 μm. A 532-nm-pumped narrow-linewidth OPG is employed for its idler to serve as a seed source for the Nd:YAG-pumped broadband OPG. Tuning the seed wavelength only, tuning of the noncollinear OPG is achieved, with its linewidth reduced to 1.7 nm throughout its bandwidth. Such a system can be employed for wavelength division multiplexing, optical coherence tomography, and spectroscopy.

Recently the demand for high-speed and high-density optical recording media using a direct overwrite scheme is very high. Among some of the potential candidates, the AgInSbTe alloy appears to be one of the promising materials that has drawn worldwide attention. It can give direct overwrite capability within a short period of time and is reported to give a well-defined shape with sharp edges, leading to intrinsically lower jitter values, thereby increasing the linear density. Hence, considerable interest has been focused on the study of the crystallization behavior of AgInSbTe alloy. Different crystalline phases are observed due to thermal annealing of AgInSbTe four-element alloy films. Results of the x-ray diffraction analysis of amorphous and crystalline (AgSbTe)_x(In_1–ySb_y)_1–x films with x = 0.2, 0.4 and y = 0.7, deposited by thermal evaporation technique are presented. The difference in crystallization behavior of the crystalline phases formed after 1 h of isothermal annealing at temperature between 200 and 400°C are studied through x-ray diffraction analysis. The experimental results are presented for a composition close to eutectic Sb₆₉Te₃₁ in which some of the Te is replaced by Ag and In. The results indicate that the growth of the crystalline phases depends on the annealing temperature and it is also affected by the change in the composition.

The optical method of reflected caustics is used for the determination of axial force in polymeric members and small components. The mechanical properties of the polymeric member are not required. The method has the advantage of simplicity. It is proposed that the member itself be used to determine the load in a two-force machine and structural elements. The test models are statically loaded in tension. A classification of errors with respect to their sources and influence on the results are discussed. The mechanical and optical parameters are investigated to optimize the design of the experiment. Two-loading apparatus are used to verify the results obtained using the proposed method.

Gain-controlled prelase is used to obtain highly stable pulse energy and width in a laser diode (LD)-pumped acousto-optic (A-O) Q-switched Nd:YAG laser. The gain-controlled prelase method can overcome prelase spiking caused by loss-controlled prelase and keep the Q-value dynamic range unchanged. With the gain-controlled prelase, 45 μJ output energy with ±0.8% instability is achieved at the repetition rate of 50 Hz. The pulse time jitter is 211 ps at the pulse width of 19.48 ns. Gain-controlled prelase has significant advantages of implementation simplicity and reliability. It is applied in an obvious way to other LD-pumped actively Q-switched solid state lasers such as Yb:YAG or Nd:YVO₄ laser systems.

A high-speed 3-D shape measurement technique based on digital fringe projection has been developed and experimented. This technique uses a computer-generated color fringe pattern whose red, green, and blue channels are sinusoidal fringe patterns with a 120-deg phase shift between neighboring channels. When this color fringe pattern is sent to a digital-micromirror-device (DMD) based video projector with no color filter, three gray-scale fringe patterns are repeatedly projected to an object surface in sequence with a cycle time of approximately 10 ms. The three phase-shifted fringe patterns deformed by the object surface are captured by a CCD camera with proper synchronization between the camera and the projector. The 3-D shape of the object surface is reconstructed by using a phase wrapping and unwrapping algorithm and a phase-coordinate conversion algorithm. Experimental results demonstrated the feasibility of this technique for high-speed 3-D shape measurement with a potential measurement speed up to 100 Hz.

A combined thermal and optical simulation study is performed on an all-silicon waveguide integrated thermo-optic Fabry-Perot modulator to assess the impact of the driving signal shape on the device speed performances. The results clearly indicate that the cooling phase after the application of a heat pulse constitutes the true speed-limiting factor of a device, which is otherwise in thermal equilibrium with the environment. Therefore, to enhance the heat exchange in this phase, we analyze the hypothesis of applying a thermal bias and hold the modulator at a higher average temperature with respect to the environment and the substrate heat sink. With this approach, a new driving signal is proposed and predicted to allow a transmission rate of 2.2 Mb/s, representing over a 30-fold bandwidth widening with respect to the standard modulation scheme. We also discuss the fabrication process of this kind of modulator, reporting on its first prototypal realization and preliminary optical characterization.

As one of the disparity estimation algorithms for stereoscopic images, we propose the block-based adaptive field-dependent search (AFDS) algorithm to reduce encoding time and increase image quality at the same time. The AFDS algorithm is a fast disparity estimation algorithm that uses the homogenous direction property of the disparity vector and the high spatial correlation between casual neighboring blocks on the epipolar line. Performance of the disparity estimator is improved using a dynamic search range and decreasing the disparity compensation error that is caused by objects taken by a camera at a short distance. As a result of the simulation, we can reduce the computational load up to 78% but can improve the peak SNR (PSNR) up to only about 0.2 to 2.8 dB, compared with the block-matching algorithm.

One main focus of research in stereo image coding has been disparity estimation, a technique used to reduce the coding rate by taking advantage of the redundancy in a stereo image pair. Significantly less effort has been put into the coding of the residual image. These images display characteristics that are different from that of natural images. We propose a new method for the coding of residual images that takes into account the properties of residual images. Particular attention is paid to the effects of occlusion and the correlation properties of residual images that result from block-based disparity estimation. The embedded, progressive nature of our coder enables one to stop decoding at any time. We demonstrate that it is possible to achieve good results with a computationally simple method.

In inverse synthetic aperture radar (ISAR) imaging, due to the noncooperative motions of maneuvering targets, the Doppler shifts of scatterers are usually time-varying, and the radar return signals are usually chirps. Thus, chirp estimation plays an important role in the performance of ISAR imaging. Li and Stoica (1996) presented an adaptive finite impulse response (FIR) filtering approach to estimate the amplitudes and phases of sinusoidal signals and applied it to the synthetic aperture radar (SAR) imaging with the sinusoidal signal model. We extend the Li and Stoica's algorithm to estimate the chirps and then apply it to the ISAR imaging of maneuvering targets. The extended algorithm is verified by some raw ISAR data.

We propose a novel method for recognizing airplanes in an inverse synthetic aperture radar (ISAR) image. The method utilizes the conspicuous scatterers at fixed positions in a 2-D ISAR image, and a point-scatterer model is used to model the airplane. The geometric relationships of the scatterers can be used to generate a set of cross-ratio vectors and a database for recognition can thus be built. A pair of stable cross-ratios is applied to the database as a search key to reduce the cost of the search. Furthermore, ordered geometric invariants and the circular matching technique are employed to alleviate the computational complexity. The experimental results show that the proposed method is both robust and effective in recognizing airplanes in ISAR images.

This paper presents stochastic models and estimation algorithms for speckled images, with an emphasis on synthetic-aperture-radar images, and where the speckle may not be fully developed. We treat speckle from a novel point of view: as a carrier of useful surface information rather than as contaminating noise. The stochastic models for surface scattering are based on a doubly stochastic marked Poisson point process. For each of these surface-scattering statistical models, we present estimation algorithms to determine the average surface reflectivity and scatterer density within a resolution cell, using intensity measurements of speckled images. We show that the maximum-likelihood estimator is optimal in the sense that the variance of the error is the smallest possible using any conceivable estimate having the same bias with the same data.

We present a new texture analysis method, namely a texture feature coding method (TFCM), for classification of the Brodatz's natural textures. The TFCM is a coding scheme that transforms an image into a feature image, in which each pixel is encoded by TFCM into a texture feature number (TFN) that represents a certain type of local texture. The TFN of each pixel in the feature image is generated based on a 3×3 texture unit as well as the gray-level variations of its eight surrounding pixels. The TFN histogram and TFN cooccurrence matrix are derived to generate many texture features for texture classification. The texture features of a gray-level cooccurrence matrix (GLCM), texture spectrum (TS), and cross-diagonal texture matrix (CDTM) have been used for comparison in discriminating natural texture images in experiments based on minimum distance and Bayesian classifiers. Experimental results reveal that the features of the TFCM are superior to the ones of the other three methods for classification.

The authors propose a true-time-delay system for wideband continuous phased array beamforming using a chirped grating-based true-time-delay beamforming network incorporating a novel multiwavelength tunable fiber ring laser with equally increased or decreased wavelength spacing. The wavelength of each lasing is determined by the fiber Bragg grating in the laser cavity and tuned by compressing or stretching the fiber Bragg grating. For simultaneously achieving N tunable lasings, N fiber Bragg gratings are used. To ensure wavelength tuning with equally increased or decreased wavelength spacing, the N fiber Bragg gratings are mounted on an organic plastic plate with angles between adjacent gratings such that the tuning forces applied to the gratings experience equal increments. Time delays with equally increased or decreased time-delay difference for the equally spaced wavelengths are obtained when the modulated light waves are reflected at different locations of a linearly chirped grating. A four-channel chirped-grating-based true-time-delay system using the proposed light source for continuous beamsteering at microwave frequencies up to 10 GHz has been constructed and demonstrated.

To segment moving objects in head-and-shoulder image sequences, a new morphological image segmentation algorithm is proposed using a hierarchical structure based on a binary split algorithm. The split method is performed using the combination of spatial and temporal similarities to reduce the residual errors between the original and segmented image. Spatial similarity is used to split those regions with the most different intensity values, while temporal similarity, based on the displacement vectors between two images, is used to split those regions with different motions. As a result, the proposed hierarchical segmentation method, based on a binary split algorithm, can increase the subjective image quality with only a small number of regions. Furthermore, a new contour simplification technique composed of three filtering steps is repeatedly applied to the segmented image, thereby eliminating any noisy contours of segmented regions.

Affine transformation is widely used in image processing. Recently, it has been recommended by MPEG-4 for video motion compensation. We present a novel low-power parallel architecture for texture warping using affine transformation (AT). The architecture uses a novel multiplication-free algorithm that employs the algebraic properties of the affine transformation. Low power has been achieved at different levels of the design. At the algorithmic level, replacing multiplication operations with bit shifting saves the power and delay of using a multiplier. At the architecture level, low power is achieved by using parallel computational units. At the circuit level, using low-power cells contributes to the power savings. The proposed architecture is used as a computational kernel in video object coders. It is compatible with MPEG-4 and virtual reality modeling language (VRML) standards. The architecture has been prototyped in 0.6-µm CMOS technology with three layers of metal. The performance of the proposed architecture shows that it can be used in mobile and handheld applications.

We propose a new method for objective measurement of video quality using the wavelet transform. We exploit the fact that the sensitivity of the human visual system varies over spatiotemporal frequencies. First, in order to compute spatial-frequency components, the wavelet transform is applied to each frame of source and processed videos. Then, the difference (squared error) of the wavelet coefficients in each subband is computed and summed, producing a difference vector for each frame. By repeating this procedure for all the frames of a video, we obtain a sequence of difference vectors, and then the average vector is computed. Each component of this average vector represents a difference in a certain spatial frequency. In order to take into account the temporal frequencies, we also propose a modified 3-D wavelet transform. In either case, a single vector represents the difference between the source and the processed video. From this vector, a number is computed as a weighted sum of its elements and is used as an objective score. In order to find the optimal weight vector, an optimization procedure is proposed. Experimental results show that the proposed method compares favorably with the current objective methods for video quality measurement.

Communication between computing systems is recognized as the main limitation to increase the speed of all-electronic systems beyond levels currently achieved in existing supercomputers. Optical interconnects hold great promise in eliminating current communication bottlenecks because of properties that stem from optics inherent parallelism. Wavelength-division multiplexing (WDM) technology, by which multiple optical channels can be simultaneously transmitted at different wavelengths through a single optical transmission medium, is a useful means of making full use of optics parallelism in an application of interconnects for massive parallel processing. We first briefly review the bottlenecks of electrical interconnects in massively parallel processing. Then we discuss the advantages of optical interconnects and present our approach of optoelectronic interconnects in massively parallel processing by WDM technology. We then review the working principles of wavelength division (de) multiplexers [WD(D)M] for optical interconnects in massively parallel processing and address the optical design issues of WD(D)Ms. Finally, we report experimental data of WD(D)Ms for this application. The devices exhibit low insertion loss, high reliability, and low cost.

We propose and demonstrate a bidirectional dense wavelength division multiplexing cable television system that uses large effective area fibers (LEAFs) as transmission fibers and chirped fiber grating (CFG) as a dispersion compensation device to reduce the dispersion and cross-phase modulation (XPM)-induced crosstalk. By comparing the system link with the transmission length of a 50-km single-mode fiber (SMF) without CFG, good performances of carrier-to-noise ratio (CNR) ≥ 50 dB, a composite second order (CSO) ≥ 69 dB, and a composite triple beat (CTB) ≥ 66 dB are obtained in our proposed system over a 50-km LEAF transport.