This PDF file contains the editorial “Speed Bumps” for OE Vol. 42 Issue 09

All-optical Boolean exclusive OR operation is demonstrated on a high-speed serial bit stream with a three-optical-input-fiber Sagnac interferometer that employs a semiconductor optical amplifier as the nonlinear element. Full duty cycle bit switching has been achieved at 20 GHz with low switching energies and contrast ratio as high as 10.4 dB.

An all-optical recirculating shift register with inverter that uses entirely semiconductor optical amplifiers (SOAs) is demonstrated operating at 10 GHz. SOAs provide nonlinear switching in a Sagnac interferometer as well as feedback amplification. The device could hold arbitrary pattern profiles recirculating stably for hours. This demonstration also proves the cascadability of the SOA-assisted Sagnac switch up to 10 GHz.

A fast algorithm for discrete cosine transform (DCT)-domain image resizing is presented. To reduce computations, fast Winograd DCTs are applied to a recently reported image resizing scheme that uses DCT low-pass truncated approximation. Our fast algorithm yields significant improvement in computational complexity over the fast algorithm of the reported method.

A new optoelectronic hardware architecture of a neural network processor is proposed. A basic cell in the system is composed of electronic neurons that share the same time-distributed input; the input is introduced by means of an optical broadcast. The system combines the computational strength of electronics and the communication strength of optics in an optimal manner and it is potentially scalable to a very large number of neurons. A description of the system is given and the behavior of a first prototype is shown.

We describe the fabrication of a 2×2 port optical device based on a thin film filter, and also newly propose a 4-channel optical add drop multiplexer (OADM) module using these devices. The optical performance of the proposed OADM module is evaluated theoretically and experimentally, and compared to that of the typical OADM modules using 1×2 port optical devices for 4, 8, 16, and 32 drop channels. Since the 2×2 port optical device accomplishes the function of wavelength multiplexing and demultiplexing simultaneously in the proposed OADM module, the insertion loss of through channels can be improved by 1.2 dB compared with that of a typical OADM module using 1×2 port optical devices. In addition, both the size and price of the module can be reduced by ~40 to 50%.

The Naval Observatory Flagstaff Station 1-m telescope is evaluated for the addition of adaptive optics capabilities to its instrumentation suite. Zernike decomposition of the optical system based on phase diversity measurements shows that the static optical aberrations are small enough that they will not degrade the performance of the deformable optical element. The analysis makes use of annular pupil Zernike polynomial reconstruction of the wavefront to accommodate the large obscuration in this telescope and compares this with the results from using filled circular Zernike polynomials.

We perform a detailed investigation of the light and x-ray response of a newly developed planar avalanche photodiode (APD) of the deep diffused type. This type of photodetector design has the low noise characteristics obtained from the deep diffusion process, but it is built using only standard planar technology. We measure an excess noise factor of 1.8 at a gain of 6, which is similar to that of other commercial APDs. With this type of structure, the expected gain is ~500 at 1600 V. We do not achieve this gain because of an early breakdown at the surface. Future designs will incorporate improvements in the structure to avoid surface breakdown. In spite of the low gain, the level of noise that we find indicates that this structure, reaching full breakdown, has potential for very good performance. At this gain, it is found that the dominant contribution to the broadening of the energy resolution comes from preamplifier voltage noise. The measured capacitance is 20 pF, and the rise time is 9 ns at 1300 V. The dependence of the gain on the density of primary carriers generated is also investigated. When the gain is measured with α particles (these particles generate a large number of primary electrons in an area of a few micrometers), it is lower than the gain measured with light (primary electrons generated by light are spread over a large area). The decrease starts to be seen at a gain of 2. When a large amount of charge (90×10⁶e) is injected via a light pulse into a larger area (0.3 mm diameter), no effect is seen.

We compare the thermal tuning capabilities of two type I gratings written in unloaded and in hydrogen-loaded germanium-doped silica fibers, and a type IIa grating written in unloaded fiber. The hydrogen-loaded grating is annealed after writing, to remove any unreacted hydrogen. We further study the gratings' response stability with temperature, an important property when these devices are used as optical filters in wavelength division multiplexing systems. Experimental results show that the worst option is the type I grating in unloaded fiber, which presents the highest bandwidth and group delay variations with temperature, so that it is the least suitable as a thermotunable optical filter in wavelength-multiplexed systems. For the other two options, the bandwidth and group delay variations are much smaller, the lowest values being obtained with the hydrogen-loaded type I grating, and the best thermal tuning efficiency with the type IIa grating.

In parallel beam computed tomography, measured projections at conjugate views are mathematically identical. This symmetry can be exploited for either reducing the scanning angle or the size of the detector arrays. In single-photon emission computed tomography (SPECT), because the gamma rays in the conjugate views suffer different photon attenuation, measured projections at conjugate views are generally different. Therefore, it had been widely considered that projections covering 360 deg and the whole detector face are required for exact reconstruction of the distributions of gamma-ray emitters. In this work, we show that projections covering 360 deg, but acquired with a half-detector viewing half of the image space, provide the necessary information to reconstruct a unique image in SPECT with nonuniform attenuation.

It is often necessary in optical experiments to produce computer-generated holograms (CGHs) of reasonable quality for the purpose of routing light. Such holograms are often created using a pressing process that is not normally done in-house and is very expensive for small runs. We present a novel technique for the production of such holograms using a standard compact disk (CD) writer, which is fast and very cost effective, along with an analysis of results obtained using this technique.

We present pulse characteristics and evolution processes of a Q-switched ytterbium-doped double-clad (YDDC) fiber laser. At different pump powers and repetition rates, the laser shows different pulse shapes, buildup times, and duration. Due to the self-mode-locking effect in the Q-switching process, the Q-switched pulses split up. These phenomena are analyzed and the control of Q-switched pulses is discussed.

The influence of the cross-sectional shape of the inner cladding on the absorption characteristics of double-clad fiber lasers has been investigated by wave coupling theory. Wave equations are solved taking into account mode coupling effects between eigenmodes. The coupling between eigenmodes arises from the nonuniform absorption in the doped core region and leads to nonexponential absorption of the pump power. A characteristic parameter is proposed to compare the absorption characteristics among various cross-sectional shapes of double-clad fibers. It is found that a triangular cross section has the best absorption characteristics among the cross-sectional shapes examined.

The transmission properties of Hermite-Gaussian optical pulses in fiber are studied. A theoretical solution is obtained by the momentum method, and the result is compared with a numerical simulation. Moreover, short Hermite-Gaussian optical pulse transmission over 1440 km in a dense dispersion-managed link is investigated on a numerical simulation basis. It is demonstrated that the pulse can propagate stably in zero, positive, and negative net dispersion compensation systems. In our simulation, both the second dispersion and the third-order dispersion are compensated. It is found that a Hermite-Gaussian optical pulse can transmit stably with reasonably lower power.

He-Ne laser lightwaves in an optical fiber are modulated when a section of the fiber is subjected directly to audio-frequency-modulating electric fields. The laser light emerging from the output endface is intensity modulated, phase shifted, or modulation frequency multiplied under special conditions. We present the correlation between this observed electro-optic effect of the fiber itself and the theoretical analysis of the phenomena.

A microampere dc current is measured by a fiber optic sensor. The phase change of the guided light induced by ohm's heat over a golden-coated fiber is detected by an interferometer. The thermal conduction of the optical fiber and the relation of the temperature-induced phase change of the guided light are explored. A Michelson interferometer plus a Faraday rotator mirror enable system immunity from the polarization-induced signal fading due to the environmental disturbance, especially by vibration and temperature. A passive phase-generated carrier (PGC) demodulation scheme enables the system to demodulate the interference signal stably by shifting the signal away from the high-noise, low-frequency band through a carrier. The sensitivity of the sensing system is improved by a biased method. The minimum measurable dc current is 3.416×10^–7 A with a dynamic range of 89.33 dB.

A diaphragm-type fiber-optic acoustic sensor is developed. In such a sensor, the sound-induced strain is imposed on the fiber on the diaphragm-type sensing element, and can be measured by the use of a Mach-Zehnder interferometer. The diaphragm structure can provide directional response with high sensitivity. The experimental measurement frequency range is from 100 Hz to 5 kHz. The sensitivity and minimum detectable acoustic pressure of the system are –139 dB re V/μPa and 82 dB re μPa, respectively.

The effect of wavelength of illumination on the trap occupancy of an optically biased GaN metal-semiconductor field effect transistor (MESFET) for its improved electrical characteristics is presented. The model evaluates the I_d to V_d characteristics and transconductance of the device for the cases of light turning both on and off at a reference time t = 0. The photovoltaic effect across the Schottky junction and the depletion width modulation in the active layer are taken into account along with the gate-length modulation and parasitic source and drain resistances. It is shown that the trap-related degradation of the drain characteristics could be overcome when the device is used under illumination with light acting as a dual gate.

We develop a technology for detecting the inner wall (not an "edge portion") of a small hole in machine parts such as a die, a nozzle, and a gauge directly and measuring the inner diameter (usually, the inner diameter is 0.3 to 10.0 mm and the length of hole is 1 to 20 mm). Especially, we discuss theoretically and experimentally the measurement error in a gap larger than 0.3 mm of a quadrilateral hole sample using an optical noncontact measuring machine. By projecting a spot image onto the sample's inside wall and detecting the reflected image with a CCD camera, the position of the inside wall of the sample can be measured. The inner diameter of the sample can be measured from the distance between the positions of each inside wall. Experimental results show that the measurement error of the gap varies with the gap size of the sample. In a comparison with theoretical simulation results, it is verified that the spherical aberrations of the lens in the measuring machine is the cause of this error. With the maximum spherical aberration of the lens at 50 μm, the maximum measurement error of the gap is +1.0 μm for a 0.3×0.3-mm square hole, and is –0.30 to –0.35 μm for a quadrilateral hole with a gap larger than 0.6 mm. These theoretical results are in good agreement with the experimental results. The measurement error is always negative when the spherical aberration is assumed to be 0.

This paper presents a new supervised classification technique for hyperspectral imagery, which consists of two algorithms, referred to as the greedy modular eigenspace (GME) and the positive Boolean function (PBF). The GME makes use of the data correlation matrix to reorder spectral bands from which a group of feature eigenspaces can be generated to reduce dimensionality. It can be implemented as a feature extractor to generate a particular feature eigenspace for each of the material classes present in hyperspectral data. The residual reconstruction errors (RREs) are then calculated by projecting the samples into different individual GME-generated modular eigenspaces. The PBF is a stack filter built by using the binary RRE as classifier parameters for supervised training. It implements the minimum classification error (MCE) as a criterion so as to improve classification performance. Experimental results demonstrate that the proposed GME feature extractor suits the nonlinear PBF-based multiclass classifier well for classification preprocessing. Compared to the conventional principal components analysis (PCA), it not only significantly increases the accuracy of image classification but also dramatically improves the eigendecomposition computational complexity.

When an image is highly compressed, video coding using the discrete cosine transform (DCT) and quantization produces noticeable image degradations, such as blocking artifacts and ringing noise. In order to reduce the degradations, a loop filtering algorithm is proposed, which consists of simple deblocking filtering and a fast decision rule for ringing and blocking conditions. The proposed method utilizes a four-point integer DCT for detecting blocking and ringing conditions. It also adopts an early termination policy during detection of the blocking and ringing conditions in the DCT domain. This policy speeds up computation of the filtering. The proposed method is compared with the loop filtering of the Joint Video Team (JVT) codec. As an experimental result, the computation time of the proposed method is approximately 20% faster than that of the JVT loop filtering, while the PSNR is almost the same.

Morphological image processing has been widely used to process binary and grayscale images. To extend the concept to color images, an ordering of the data is required. The solution is not unique, because color spaces are not totally ordered and the ordering process is not straightforward. In this work, two algorithms for color morphology are proposed: A Mahalanobis-color-distance-based morphological ordering algorithm, and a corrected componentwise morphological ordering algorithm. Both algorithms implement the Mahalanobis color measure to replace the angle-valued pixels by a scalar, and are based on a combination of reduced and conditional ordering of the underlying data.

Many properties of the atmosphere affect the quality of images propagating through it by blurring and reducing their contrast. The atmospheric path involves several limitations, such as scattering, absorption of light, and turbulence, which degrade the image. Recently developed atmospheric filters, which correct for turbulence blur, aerosol blur, and path radiance simultaneously, are implemented here in the digital restoration of Landsat thematic mapper (TM) imagery. The turbulence modulation transfer function (MTF) is calculated from meteorological data or estimated if no meteorological data were measured. Aerosol MTF is consistent with optical depth. The product of the two yields atmospheric MTF, which is implemented in the atmospheric filter. Restoration improves smallness of size of both resolvable detail and contrast. Restorations are quite apparent even under clear weather conditions. A way to evaluate restoration improvement is presented here by the use of quantitative criteria as well as subjective opinions of human observers in perception experiments. Not all the restoration criteria represent improvement in the same tested image under the same restoration conditions. When one criterion suggests an enhancement, there is a chance that another one might represent a lower value for restoration success.

We report a theoretical model for characterization of a double heterojunction (DH) photovoltaic detector for application in the midinfrared (MIR) wavelength region. The physics-based closed-form model developed here has been applied to study the characteristics of an n⁺-InAsSbP–n⁰-InAsSb–p⁺-InAsSbP DH detector grown by liquid-phase epitaxy (LPE) for possible application in the 2- to 5-μm wavelength region. The results obtained from the experimental measurements have been compared and contrasted with those predicted on the basis of the theoretical model. The model can be used to explain the various physical mechanisms that shape the characteristics of the device under room-temperature operation. It can also be used to optimize the performance of the photodetector to suit specific requirements. The study reveals that compositional grading in the active and the cladding regions of a DH grown by LPE is responsible for the reduction of the responsivity and detectivity of the device in the shorter wavelength region.

The effective-index-based matrix method (EIMM), which was earlier used to analyze single-mode continuous waveguides, has been also extended to the analysis of periodically segmented waveguides (PSWs) formed by Ti indiffusion in LiNbO₃. The Ti-concentration profiles and the induced refractive index changes of Ti:LiNbO₃ PSWs have been computed for different waveguide dimensions and fabrication parameters at the design wavelength by using approximate analytical models. The effective index profile of the PSW is obtained by averaging the contributions from different regions along the segments, and EIMM is then applied for computing propagation constants of the PSWs. The cut-off wavelength of PSW has been computed as a function of duty cycle, keeping the period constant. This PSW, when used as one arm of an asymmetric directional coupler (ADC) along with the other arm formed by a continuous waveguide, can be used as a tunable filter.

A theoretical presentation of evanescent coupling is offered, with consideration of the refractive indices, between a side-polished optical fiber and an infinite planar waveguide with a conductor cladding (PWGCC). The PWG is suspended at a constant distance from an unclad fiber core and coated with a perfect conductor on the far side. The behavior of the distributed coupler is examined using a coupled mode model, which takes account of the two dimensions of the waveguide configuration. The coupling and propagation of light were found to depend on both the relation between the refractive indices of the materials and the configuration of the side-polished fiber used in the PWGCC. The spreading of light in the unconfined direction of the PWGCC is described in terms of a simple geometrical interpretation of the synchronization condition that is in agreement with a previous investigation of the problem based on the coupled mode theory. The power of the light propagated in the fiber decreases exponentially along the fiber axis as power is transferred to the PWGCC, where it is carried away. Then a 3-D finite-difference beam propagation method was applied to the fiber-PWG composite; results of the simulation by this means presented.

The spatial carrier phase shifting method (SCPSM) has advantages over both the Fourier transform and phase shifting methods. Unlike the phase shifting method, only a single fringe pattern is needed, and unlike the Fourier transform method, SCPSM is not computationally intensive. However, these advantages are at the cost of increased susceptibility to noise. In this work, a novel bandpass filter is developed and applied for noise suppression in SCPSM. As an angular carrier frequency of π/2 is widely used, the bandpass filter can be simply designed from a known low-pass filter. The developed filter is applied to a simulated fringe pattern to show its benefits.

A method to accurately determine the relative phase between images acquired using phase-shifted moiré interferometry is presented. Phase shifting, combined with the accurate determination of the relative phase shift, provides an improvement of the sensitivity of moiré interferometry from 0.417 μm/fringe to 27.8 nm/fringe. Specifically, a 1-D continuous wavelet transform technique was developed to effectively reduce the transient environmentally induced noise and the static background noise that complicate phase determination. Using this technique, accurate relative phase shifts of the interferogram frames were determined with an error of ≈2%. As a result, very smooth and accurate phase-map reconstruction of the initially noisy interferogram was accomplished.

A compact perception system—originally developed for real-time acquisition of 3-D images for robotics applications—is presented. For distances up to 12 m, the resolution is about 2 cm at a rate of 5 images/s, with a field of view better than ∓20 deg. The setup is based on a specifically designed phase-shift laser range finder coupled with two micromachined silicon mirrors with a size of 4×4 mm. A sensor gives the Cartesian position of the laser spot. Data obtained from scanning are stored in numerical form in a 4096 10-bit words RAM. This design intends to provide a device with as many silicon components as possible, in view of achieving a very large integration in the near future. We describe the current device and analyze experimental results.

Optical computed tomography (OCT) is often used for measuring thermophysical parameters. Unfortunately, viewing access in many tomographic experiments, such as in plasma physics, is extremely limited, which leads to a highly undetermined inversion problem. Four major approaches to this problem are compared: a novel algorithm based on maximum entropy (ME), and three traditional algorithms, namely the simultaneous iterative reconstruction technique (SIRT), algebraic reconstruction technique (ART), and multiplicative algebraic reconstruction technique (MART). A novel basis function was adopted to improve the performance of SIRT, ART, and MART. These algorithms were used to reconstruct phantom data with realistic levels of noise from a number of different imaging geometries. The phantoms, the imaging geometries, and the noise were chosen to simulate conditions encountered in typical OCT measurement. In most cases, the ME and MART algorithms give better reconstruction results than the other two algorithms for the situations studied here.

We propose a coherent lightwave neural network system whose behavior can be controlled by the lightwave carrier frequency. We utilize the dependence of homodyne interference on the lightwave frequency by choosing the optical path lengths of the signal and the reference to be slightly different. The synaptic weights consist both of the optical path difference and variable delay realized by a spatial light modulator. A frequency-dependent learning is performed by a novel complex-valued Hebbian rule associated with the carrier frequency. Experiments demonstrate that the associative memory recalls different complex-amplitude patterns according to the carrier frequency variation successfully.

We propose and discuss several fabrication processes for a complex-shaped mirror, which is a fly-eye mirror, used in an extreme ultraviolet lithography (EUVL) illumination system. The mirror has a complex reflective surface consisting of many concave mirror elements that are sections of a sphere; the top of each element is arc shaped. In the present study, we focus on one process in which all elements are fabricated individually and then arranged side-by-side to form the mirror. Thus, as the first step in this process, we fabricate the arc-shaped elements made of invar with electroless nickel plating. The resultant reflective surfaces have a peak-to-valley (PV) surface accuracy of 0.31 μm. The surfaces have the rms roughnesses of about 0.23 and 0.35 nm in areas of 110×140 μm and 1×1 μm, respectively. The slope accuracies of the surfaces relative to the bottom surfaces are –166 and 43 arc-sec in the y and x directions, respectively. Thus, the mirror elements for the fly-eye mirror can be fabricated very accurately with smooth surfaces, although the mirror elements have a special shape compared to that of general optics.

We demonstrate a chip-scale micro-optical interconnect. The prototype system for this demonstration consists of optoelectronic transmitter and receiver multichip modules. A diffractive optical element (DOE) is used for optically interconnecting the multichip modules and in establishing a point-to-point link. The link length, as measured from the optical source of the transmitter to the detector plane of the receiver, is only 2.332 mm. The transmitter and receiver module dimensions as well as the integrated system volume are comparable to very large scale integration (VLSI). The design, fabrication, integration of this system, and experimental results are presented.

Two-dimensional angular optical scattering (TAOS) is an experimental technique by which the azimuth- and polar-angle-dependent intensity patterns of monochromatic light scattered by isolated material particles are collected within a limited aperture. TAOS patterns are analyzed with the ultimate goal of estimating the class of shapes and the set of complex refractive indices to which the scatterer may belong. The implemented heuristic reconstruction procedure integrates experimental techniques and methods of data analysis. It consists of three stages: (1) TAOS patterns are classified by cluster analysis and classification is validated by principal components analysis, (2) prior information about the shape and refractive index of the particles is acquired from analytical electron microscopy and drives the computation of TAOS patterns by a T-matrix code, and (3) cluster analysis is repeated by including one computed pattern at a time and ranking it according to distance to a cluster. As a result, some simulated scatterers are found, the computed patterns of which merge with clusters of experimental patterns. Whereas the reconstruction of each individual scatterer from the available TAOS pattern would not have been feasible, possible scatterer classes are identified. The investigated material is tire dust from laboratory abrasion tests of aerodynamic size <10 μm.

The refractive indices of crystalline phase-change films are usually obtained by thermal-induced crystallization. However, this is not accurate, because the crystallization of phase-change film in rewritable optical disks is laser induced. In this study, we use the initializer to crystallize the phase-change films. The dependence of the refractive index n and the extinction coefficient k of the phase-change films on the initialization conditions are studied. Remarkable changes of the refractive indices (especially k) are found when the initialization laser power density is 6.63 mW/μm2 and the initialization velocity is 4.0 m/s. At the same time, the structure changes of the phase-change films are also studied. This dependence is explained by the structure change of the films. These results are significant in improving the accuracy of optical design and the thermal simulation of phase-change optical disks, as well as in the study of phase-change optical disks at shorter wavelengths.

The height profile of information pits (or bumps) on the data surface of an optical disk is assumed in most publications to be either rectangular or trapezoidal. Currently, when bumps or pits on an optical disk are produced by using a photoresist technique, their height profile is inevitably "rounded." Moreover, in plastics molding practice, edge rounding and sloping sidewalls of pits are advantageous factors to avoid the sharp edge becoming damaged when the replica is removed from the mold. A new expression for the height profile of pits is obtained that provides a convenient description of the geometry of the pits with different degrees of edge rounding and sloping of sidewalls. We find that λ/4-deep pits are optimum for the readout signal only when the pits have rectangular profile and are illuminated by a focused uniform beam. The optimum pit depth increases to an even greater value when edge rounding and sloping of sidewalls are taken into account.

We propose a novel dual-amplitude pulse interval modulation (DAPIM) for optical wireless communications. We present the symbol structure, spectral properties, and the expression for bandwidth requirement, slot, and packet error rates, and optical power requirements of DAPIM in the absence of and in the presence of multipath dispersion. DAPIM shows a better compromise between bandwidth and power requirements compared with on-off keying, pulse-position modulation, and pulse interval modulation. DAPIM with M = 5 and M = 6 is attractive in terms of optical power and bandwidth requirements on a nondispersive channel.

We propose a new method for measuring 3-D facial shapes. This method uses two point light sources with red filters and a slit pattern projector with a blue filter. The time required for sampling data can be reduced by switching the color light rays at high speed, and hence a series of 3-D shapes of a human face at each instant can be measured. First, two facial images projected by both the red light rays and the blue light stripes are taken by the video camera, and these images are separated into two shading images and one slit image using the color information. Next, the normal vectors at points on the face are computed from the two shading images using the photometric stereo method. The 3-D coordinates of the points on the light stripes are measured from the slit image using the stereo vision algorithm. Finally, all the computed results are blended and the 3-D shape of the face is computed. Experimental results are shown to demonstrate the feasibility of the proposed method.

We analyze the number and range accuracy of pixels required to distinguish vehicle types in ladar imagery. A simulation is used to generate large numbers of synthetic ladar images of a set of vehicles, and an automated geometry-matching algorithm used to identify vehicles is presented with each of these images. By systematically varying the range resolution, cross-range resolution, and other parameters, we can estimate joint ladar-algorithm system performance as a function of the sensor parameters. The results of these studies will provide guidance to ladar researchers and designers in setting fundamental system parameters for specific applications or operational scenarios. We describe the simulation, identification algorithm, and experimental results.

Algorithms are considered for searching wide-area forward-looking infrared imagery for military vehicles. Wide-area search has typically been handled by using a simple detection algorithm with low computational cost to search the entire image or set of images, followed by a clutter rejection algorithm that analyzes only those portions of the image that are marked by the detection algorithm. We start with a feature-based detector and an eigen-neural-based clutter rejecter, and examine a number of architectures for combining these modules to maximize joint performance. The architectures considered include a clutter rejection threshold method and a nonlinear learning-based combination. The performance of the architectures is compared using a set of several thousand real images.

Area correlation tracking is an effective solution for tracking targets that have neither prominent features nor high contrast with the background. We address the implementation of a real-time area correlation tracker (ACT) for an on-board application based on the absolute difference algorithm. The ACT consists of a very high speed and very large scale field programming gate array (FPGA), which contains a special structure of parallel pipelined systolic array to accomplish the correlation computation, and a float-pointed digital signal processor (DSP), which does the parabolic interpolation. The configuration and the timing of the ACT are presented and the ACT is applied successfully in a real-time tracking system. The computation latency for correlation tracking is about 2 ms, which can satisfy the demand of the tracking system used for detecting the solar pore's motion induced by atmospheric turbulence and the mechanical vibrations of the solar telescope.

. The performance of an optical wavelength-division multiplexed system is analyzed taking the four-wave mixing (FWM) effect into account. The efficiency of FWM generation is observed to greatly reduce if the spacing between channels is made unequal. However, the scheme requires an increased bandwidth, which can further be reduced by utilizing another scheme called repeated unequal channel spacing. Also the FWM efficiency is found to reduce in the presence of chromatic dispersion. Use of nonuniform chromatic dispersion along the fiber length, which reduces the FWM effect while maintaining a low value of dispersion is investigated. Various other dispersion management mechanisms are also considered in addition to different channel spacing schemes to achieve a comparative study as to which scheme gives the best solution to the FWM problem.

In digital holography, holograms are digitally recorded by a CCD camera and stored in a computer’s memory. The reconstruction of the recorded wave field from the digitally stored hologram is realized numerically by the computer.123 This technique has found numerous applications, especially in 3-D microscopy,45 encryption,6 pattern recognition,7 and surface contouring.8 The process of digital holography by recording an interference pattern with a CCD array, and storing it in a computer to numerically reconstructing it can be interpreted as a coherent optical imaging system, which can be analyzed by Fourier optics and can be characterized by a point spread function (PSF). Such an analysis has been reported in recent publications written by Kreis.910 In this work, we indicate some shortages in Kreis’ analysis. At the same time we describe our new treatment and some new analytical results.

This PDF file contains the editorial “Response to "Effect of the fill factor of CCD pixels on digital holograms: comment on the papers 'Frequency analysis of digital holography' and 'Frequency analysis of digital holography with reconstruction by convolution'"” for OE Vol. 42 Issue 09