This PDF file contains the editorial “Plagiarism” for OE Vol. 43 Issue 01

In order to optically vary the magnification of an imaging system, continuous mechanical zoom lenses require multiple optical elements and use fine mechanical motion to precisely adjust the separations between individual or groups of lenses. By incorporating active elements, such as liquid crystal spatial light modulators or deformable mirrors, into the optical design, we can eliminate the need to change the spacing between lenses and create an imaging system with variable optical magnification that has no macroscopic moving parts.

An experiment is conducted that successfully reduces the amount of aberration detected in a reconstructed hyperspectral datacube. The correction alleviates an unnoticed rotational misalignment in the calibration of our near-infrared flash hyperspectral imager. This alteration induces a spherical-aberration-like artifact as well as spectral defects because of the three-dimensional hyperspectral quality of the data. This artifact, which would cause severe misregistration of the spatio-spectral information, is correctable post detection.

We propose a unique approach for calibrating robot tool center point (TCP) using a noncontact sensor such as a laser displacement sensor. This approach is based on nonlinear least square optimization algorithms with geometry constraints. Experimental results show that calibration algorithm is robust and the calibration error is reduced significantly compared to that of a conventional method.

We demonstrate a simple circuit that generates control signals from single or multiple flag pulses to control all-optical gates at 40 GHz. It is comprised of a Fabry-Perot filter to produce an amplitude-modulated sequence of pulses and a semiconductor optical amplifier used for pulse amplitude equalization.

An investigation is made of the characteristics of the formation of a self-controlled volume discharge for pumping CO2 lasers, i.e., self-sustained volume discharge (SSVD), which involves the preliminary filling of a discharge gap by an electron flux from an auxiliary discharge plasma. It is determined that this method is suitable for large interelectrode gaps, that distortion of the electric field in the gap by the space charge of the electron flux plays an important role in the formation of the discharge, that the electrodes could be profiled dynamically during propagation of an electron flux through the discharge gap, and a SSVD could form in systems with a strongly inhomogeneous field. High-power SSVD-based CO₂ laser systems are created and investigated. Another type of self-controlled volume discharge without preionization, i.e., a self-initiated volume discharge (SIVD), in nonchain HF lasers with SF₆–C₂H₆ mixtures is also investigated. It is established that, after the primary local electrical breakdown of the discharge gap, the SIVD spreads along the gap in directions perpendicular to that of the electric field by means of the successive formation of overlapping diffuse channels under a discharge voltage close to its quasi-steady-state value. As new channels appear, the current that flows through the channels formed earlier decreases. The volume occupied by the SIVD increases when the energy deposited in the plasma increases, and, when the discharge volume is confined by a dielectric surface, the discharge voltage increases simultaneously when the current increases. The possible mechanisms that explain the observed phenomena, namely the dissociation and electron attachment of SF₆ molecules, are examined. A simple analytical model is developed that makes it possible to describe these mechanisms at a qualitative level. High-power SIVD-based HF(DF) lasers are developed and tested.

A color interpolation algorithm is necessary to acquire color images of good quality with a digital still camera in which a single CCD image sensor is equiped. Since a color filter array is adopted, we must interpolate missing color values in the red, green, and blue channels at each pixel location. Inappropriate color interpolation causes color artifacts, resulting in degraded color resolution. A color interpolation algorithm with a variable color ratio is proposed by incorporating cross-channel correlation. The color ratio of weighted sums in the color channels is utilized instead of the weighted sum of color ratios, and the color ratio varies with respect to the cross-channel correlation at each pixel. The proposed algorithm is composed of initial color interpolation step and the updating step of the green channel, where the updating step is carried out iteratively. Furthermore, a minus weighted sum based on the external divide, a modified edge indicator function, and a 5×5 support region for the red and blue channels are also adopted to improve the performance. With the proposed algorithm, we have achieved improvements in the image quality especially along the edges and in the detail of the image by removing color artifacts. The superiority of the proposed algorithm is demonstrated in the experimental results.

We present a new vertex selection scheme for polygon-based contour coders. When contour coding is used for image coding applications, it is argued that partitioning a contour into partial segments and then approximating these segments with a variable accuracy is required to efficiently handle their relative significance. Accordingly, a decision rule for the allowable maximum distortion (d_max) is derived based on interregion contrasts, and an adaptive vertex selection method is proposed based on the derived rule. Experimental results are presented to show the superiority of the proposed method over conventional methods.

Based on the interpretation of the restoration process as a backward diffusion process, an optimal filtering in terms of the spectral characteristics of an image is proposed. The restored image is given as the solution of the optimal backward diffusion equation (BDE). Several aspects of applying the backward diffusion on the blurred image are given. The ill-posedness of the backward diffusion is subdued by regularizing the speed at which the high-frequency components should be recovered by the use of a decreasing function. In manipulating the recovery speed, the spectral characteristics of an image are taken into account. The proposed scheme uses an exponentially decreasing speed function on the recovery of high-frequency components. The use of the decreasing function makes it possible to include more significant low-frequency components without failing the system, and thus yields results of better quality than the constant bounded scheme.

We present an improved signal processing algorithm for the max-min scanning method for phase determination in speckle and other interferometric techniques. After recording the intensity changes at one point on the image for a range of arbitrary phase shifts, curve fitting is employed to obtain the intensity waveform corresponding to this position. The phase value is derived from this curve, and the sign of the computed phase angle is determined using the derivative of the fitting curve. The phase map is obtained by performing the same procedure throughout the entire image. This improved algorithm requires neither sorting the recorded image signals nor additional sign images. Experimental examples are provided.

A reference transmission sphere is an important device to measure the spherical surfaces, and it offers a high-quality spherical wave and a reference spherical surface with peak-to-valley less than λ/20 (λ=0.6328 μm). For this value, only the manual alignment is difficult to manage it. Thus, we study the computer-aided alignment (CAA), which can provide a guide to align the individual lens. We describe the following works concerning the CAA with no iteration for the transmission spheres: (1) by analysis of the relationship between the Zernike polynomial coefficients and the sensitive variables, for example, the air thickness error, the tilt, the decenter of the transmission sphere with an analysis program written in Zemax program language, only several Zernike coefficients that change linearly with these sensitive variables are chosen; (2) the magnitude and direction of the correction are found using the Moore-Penrose generalized inverse matrix; and (3) a numerical simulation and successful alignment are processed for a 4-in.-diam, f/5 transmission sphere.

The temporal phase modulation class of interferometers is known to be able to provide high-accuracy measurements of displacement. The resolution of this class of interferometers is studied in relation to its application in atomic force microscopy under the static deflection mode. Interferometric sensing is advantageous if atomic force microscopy is implemented for array cantilevers. The sources of error are assumed to be either additive (due to noise in photodetection and in the electrical circuitry) or multiplicative (due to ambient effects). We reveal that minimum detectable displacements in the 0.1-nm range are possible under very careful design and measuring conditions.

We develop a new implementation of a Michelson interferometer designed to make measurements with an uncertainty of less than 20 pm. This new method uses a tunable diode laser as the light source, with the diode laser wavelength continuously tuned to fix the number of fringes in the measured optical path. The diode laser frequency is measured by beating against a reference laser. High-speed, accurate frequency measurements of the beat frequency signal enables the diode laser wavelength to be measured with nominally 20-pm accuracy for the measurements described. The new interferometer design is lightweight and is mounted directly on an ultra-high vacuum scanning tunneling microscope capable of atomic resolution. We report the simultaneous acquisition of an atomic resolution image, while the relative lateral displacement of the tip along the sample distance is measured with the new tunable diode laser Michelson interferometer.

The laser-induced damage threshold (LIDT) and damage morphology of antireflection (AR) coatings on quartz and sapphire are investigated. A very interesting phenomena is found in the measurement. In the case of a single pulse laser, the LIDT of the AR coatings on quartz is higher than that of sapphire. On the contrary, for a free-pulse laser, the LIDT of AR coatings on sapphire is higher than that of quartz.

We outline a novel imaging approach in tiling liquid-crystal panels seamlessly in a photoluminescent liquid-crystal display (PLLCD) architecture. The optical design of a tiled PLLCD is presented in this paper. The key advantages of the front-face and rear-face optical design are reviewed.

A new fuzzy Hopfield-model net based on rough-set reasoning is proposed for the classification of multispectral images. The main purpose is to embed a rough-set learning scheme into the fuzzy Hopfield network to construct a classification system called a rough-fuzzy Hopfield net (RFHN). The classification system is a paradigm for the implementation of fuzzy logic and rough systems in neural network architecture. Instead of all the information in the image being fed into the neural network, the upper- and lower-bound gray levels, captured from a training vector in a multispectal image, are fed into a rough-fuzzy neuron in the RFHN. Therefore, only 2/N pixels are selected as the training samples if an N-dimensional multispectral image was used. In the simulation results, the proposed network not only reduces the consuming time but also reserves the classification performance.

A new optodigital multiple information hiding and real-time extraction system is suggested. In the proposed system, multiple information is hidden in a cover image by using stego keys, which are generated by combining use of a random sequence and Hadamard matrix. This hidden data is then extracted in real time by using an optical nonlinear joint transform correlator (NJTC)-based decoding system. In the experiment, 50, 30, and 20 combination sets of three letters selected from the English and Korean alphabets, and Arabic numerals are used as the multiple message data and they are formulated into 8×8 blocks, in which each block is composed of 64×64 pixels and multiplied with the corresponding stego keys of 64×64 pixels one by one. Then, by adding these modulated data to the cover image of Lena of 512×512 pixels, a stego image is generated. To extract the hidden data from this stego image in real time, an optical NJTC is employed, in which optical correlation between the stego image and each of the stego keys is performed, and from these correlation outputs the hidden data can be extracted. Computer simulations and optical experiments reveal that the proposed optical NJTC-based decoding system has a highly improved signal-to-noise ratio (SNR) performance of the correlation outputs under the condition of k<0.6, and these experimental results finally confirm the feasibility of the NJTC-based optical decoding system.

In optical arithmetic, logic, and algebraic operations, spatial input encoding and decoding techniques have already been established as successful candidates for conducting parallel operations with optics. Many authors around the world proposed various schemes for successful conduction of the operations with different methodologies. We propose a new method of coding of any number input variables at a time with the use of spatial light modulators. In various places of information processing with optics, this method can be exploited successfully. We use this coding principle of any number of input variables in all optical multiplexing/demultiplexing processes.

Arithmetic operation is an essential task in any type of computing system. In high-speed all-optical computations, arithmetic operations are used to exploit the highest capability of optical performance. We propose a binary addition and subtraction scheme with proper accommodation of optical nonlinear materials. It is also shown that this circuit can perform these operations with the inherent parallelism of optics.

Equipping all nodes of a large optical network with full conversion capability is prohibitively costly. To improve performance at reduced cost, sparse converter placement algorithms are used to select a subset of nodes for full-conversion deployment. Further cost reduction can be obtained by deploying only limited conversion capability in the selected nodes. We present a limited wavelength converter placement algorithm based on the k-minimum dominating set (k-MDS) concept. We propose three different cost-effective optical switch designs using the technologically feasible nontunable optical multiplexers. These three switch designs are flexible node sharing, strict node sharing, and static mapping. Compared to the full search heuristic of O(N³) complexity based on ranking nodes by blocking percentages, our algorithm not only has a better time complexity O(RN²), where R is the number of disjoint sets provided by k-MDS, but also avoids the local minimum problem. The performance benefit of our algorithm is demonstrated by network simulation with the U.S Long Haul topology having 28 nodes (R is 5) and the National Science Foundation (NSF) network having 16 nodes (R is 4). Our simulation considers the case when the traffic is not uniformly distributed between node pairs in the network using a weighted placement approach, referred to as k-WMDS. From the optical network management point of view, our results also show that the limited conversion capability can achieve performance very close to that of the full conversion capability, while not only decreasing the optical switch cost but also enhancing its fault tolerance.

A novel multiplexed sensor system using two tunable lasers for simultaneous precise measurement of pressure and temperature is designed, tested, and simulated. The sensors are optical Fabry-Pérot cavity-based pressure sensors similar to those fabricated by us using microelectromechanical systems (MEMS) techniques. This system follows a different philosophy than that followed by other multiplexing systems. We use light sources in two wavelength regions for each sensor and two interrogation methods for pressure and temperature measurement. Sensors are designed to operate over the pressure range of 0 to 30 psi and the temperature range of 25 to 90°C. This multiplexed sensor system has the potential of multiplexing about seven sensors with temperature sensitivity of 0.15 nm/°C.

An optical temperature-sensing instrument based on broadband polarimetric differential interferometry (BPDI) technology is developed for ultrahigh temperature measurement in chemically corrosive environments. A single-crystal sapphire disk with inherent birefringence is employed as a sensing element in this instrument. Absolute temperature information is obtained and the measurement results are relatively immune to optical source power fluctuations and transmission fiber losses. Other advantages of this sensor are its mechanical simplicity, high resolution, and great accuracy over a wide measurement range.

We present a design and simulations for a low power consumption, low manufacturing tolerance optical switch for integrated photonic lightwave circuits (PLC). The design is applicable to all material platforms, including silicon, lithium niobate, indium phosphide, glass, polymer, sol-gel, and gallium arsenide, just to name a few. The mechanism is one that functions by altering the index of refraction in the multimode slab region of a multimode interference splitter via the thermo-optic or electro-optic effect. The design is unique in that the electrodes are distributed to maintain multimode behavior, which 1. renders the device easy to manufacture via very loose tolerances, and 2. allows the multiple modes to vary adiabatically so as to maintain efficient optical power switching. The power consumption is only about four times higher than that of a Mach-Zehnder interferometer switch, and it requires much less tolerance in manufacturing.

A photometer used for on-line product testing of light-emitting diode (LED) buoy lanterns is calibrated for illuminance responsivity using two different methods. The first method is based on absolute calibration of the photometer with the CIE standard illuminant A light source, combined with spectral correction factors (SCFs) calculated from the measured spectral responsivity of the photometer and relative spectra of the LED lanterns. The second method is based on direct comparison with a characterized reference photometer using the LED lanterns as light sources. Comparison of the resulting correction factors shows that both methods agree within 1%. However, the second method includes geometrical aspects and LED characteristics that caused problems. These problems are discussed and the reasons for recommending the first method are given.

A photonic crystal (PhC) horn is presented. Using the finite-difference time-domain method with the perfectly matched layer (PML) absorbing boundary condition (ABC) in inhomogeneous media, we optimize the mismatch factor between a PhC horn and waveguide to achieve high radiation efficiency. The optimized PhC horn is then used to construct an horn array of 25 elements, whose element weights and element positions are optimized using simulated annealing (SA) to acheive continuous beam steering in the range between –15 and 15 deg with the maximum side lobe of less than –10 dB.

Optical characteristics of magnetic fluid are described and measured with the Mueller matrix. Furthermore, the optical activity of the magnetic fluid is discovered. A theory to determine the rotation ratio of two retarders of the Mueller matrix imaging polarimeter is also presented. To measure the entire Mueller matrix elements, due to the sampling theorem, the rate of 5 is chosen as the best and most unique for this polarimeter.

Polarized laser light microscopy is used to investigate single isolated wood fibers. The macerated Scots pine (Pinus sylvestris L.) fibers are fixed for laser-optical and x-ray experiments perpendicularly on sawn slits of glass support plates. In the optical experiments, the fibers are fixed under a laser microscope and the electric field vector is rotated together with a linear polarizing plate. The images of the fibers, taken through our polarizing microscope, are grabbed consecutively with a charge-coupled device (CCD) camera, and they are recorded in the memory of a PC for analysis. The calculated values of microfibril angles from the responses of the laser polarimetry data correlate to the orientation of the microfibril angles.

The accuracy of Fourier transform profilometry (FTP) is greatly influenced by the aliasing between foundational spectra and zero and higher order spectra. We present a novel method in which a bicolor sinusoidal fringe pattern that consists of two interlaced RGB format base color fringe patterns with π phase difference is projected onto the object through a digital light projector and the deformed color pattern is captured by a color digital camera. Then the deformed color fringe pattern is decoded into two individual fringe patterns with π phase difference using a color-separating technique. After calibration of means and contrast, we subtract one of the two fringe patterns from the other to eliminate the aliasing between foundational spectra and zero and higher order spectra. We provide the calibration algorithm of means and contrast. Computer simulation and experiment verify that this method has an obvious advantage. Compared with the π-phase-shifting technique, only one fringe pattern is required to remove the zero spectra and raise the measurable slope of height variation nearly three times, and no phase-shifting device is required in the experimental setup.

Imagery data acquired in practice to support tactical surveillance and tracking missions in hostile environments typically suffer from a variety of degradations making it essential to subject the data to digital postprocessing aimed at restoration and superresolution before they can be used for any image exploitation tasks (visualization, target detection and characterization, etc.). A number of novel iterative techniques for resolution enhancement are presently being developed, with statistical optimization and set-theoretic estimation offering two popular approaches for algorithm design. The challenges posed by the processing needs of tactical imagery data often require greater capabilities than what the existing algorithms can offer, however, and typically require more enhanced procedures to achieve satisfactory restoration and superresolution. We outline three such enhancements: parallel projection implementation with adaptive relaxation, use of scene-derived information for constraint set design, and a hybrid statistical and set-theoretic estimation procedure. The restoration and superresolution performance of an iterative algorithm that incorporates these enhancements is illustrated by application to tactical imagery data [images acquired from state-of-the-art synthetic aperture radar (SAR) and passive millimeter-wave (PMMW) sensors].

We present a new channel wavelength selectable single/dual-wavelength erbium-doped fiber ring laser. The novelty of this device is the use of a specially designed fiber cascaded Mach-Zender interferometer (MZI) called a channel wavelength generator (CWG) that has a triangular-shaped transfer function. The lasing wavelength only occurs in the vicinity of the peak wavelengths of the transfer function of the MZI when tuning the filter, and this makes it channel wavelength selectable. With a compound cavity structure, gain balance between two different wavelengths is easily achieved by adjusting the overlapping positions of the passbands of the filters over the CWG pattern, and as a result two stable lasing wavelengths are achieved simultaneous. Each wavelength can be tuned individually. The deviation of the exact lasing wavelength from the desired channel wavelength is estimated to be less than 1/20 of the CWG's channel spacing (100 GHz). In addition, it is also observed that the use of the CWG reduces significantly the number of longitudinal modes, and hence improves the stability of the output power.

The aim of this work is to recall the key role of sampling theory in image sampling with 2-D solid-state image sensors. Sampling restrictions due to the actual sensor structure—especially in the case of noncontiguous sensitive pixels—are often neglected, although sampling principles are well known from a theoretical point of view. Our purpose is to present experimental results that demonstrate clearly what kind of ghost detection and spectrum overlapping may result if the spectral bandwidth of the incident image is not suited to the image sensor capabilities. These experimental results and image distortions are shown to be the direct consequences of the sampling principles that are recalled in the first part. Finally, a sequential acquisition procedure, based on image sensor scanning, is shown to fulfill sampling theory requirements and thus enlarges the bandwidth capabilities of any image sensor.

Analytical expressions for the intensity and frequency/phase noise of single-mode semiconductor lasers based on quantum-mechanical rate equations are derived. Correlated photons, electrons, and phase Langevin noise sources and their auto and cross-correlation relations are presented, along with a novel self-consistent normalized laser model that includes the laser's correlated noise sources. A symbolically defined device (SDD) is constructed using the proposed normalized model and implemented in Agilent's advanced design system (ADS) CAD tool. Dynamic laser characteristics are predicted using the SDD implementation for 1300-nm InGaAsP/InP lasers. The results of time domain dynamic simulations of photons, carriers, optical output power, and phase—with and without the effects of noise—are presented. Simulation results are used to show the effects of random noise on both the phase and optical power output of semiconductor lasers. Simulation results are analyzed to demonstrate the resonance frequency shift dependence on the bias current levels, the relation between the frequency response and the bias current, and the dependence of the laser line width broadening on the frequency fluctuations. Comparison between the presented results and other published results (simulations and measurements) show good agreement while achieving simulation time enhancement. The suitability of the proposed models for the study and characterization of the performance of complete systems in both circuit and system simulations is examined.

A novel and simple approach for fine-tuning a laser wavelength is proposed and demonstrated. The key element is a planarly aligned nematic liquid crystal (NLC) cell inserted between the grating and end mirror of an external-cavity semiconductor laser (ECL). Varying the voltage driving the NLC cell, causes its extraordinary index of refraction to change due to field-induced reorientation of the LC director. This is equivalent to tuning the laser cavity length. As a result, the laser wavelength can be continuously tuned. With a 35.5-µm-thick NLC cell, the output frequency of the present laser can be continuously tuned over 4 GHz. The root mean square driving voltage required is 1.5 V or less. The tuning range is in good agreement with the theoretical predictions.

We present an analysis of the different aspects involved with the sampling and reconstruction of Fresnel field distribution. Fresnel fields, describing a propagating optical wave, are digitally recorded in many optical applications. The recording process involves discretization of the continuous Fresnel field using a sampling sensor. Typical nonideal sensors induce degradation of the optical information due to finite spatial sampling rate, finite aperture size, and finite detector element size (finite fill factor). In this work, we investigate the condition and limitation of the reconstruction of Fresnel fields sampled with nonideal sampling sensors. We also analyze the propagation of measurement and reconstruction noise through the reconstruction process. In our analysis, we distinguish between continuous (optical) and numerical (computational) reconstructions. We focus on the different reconstruction conditions and limitations, depending on whether the reconstruction is performed in a continuous space or numerically.

The conventional Shack-Hartmann wavefront sensor (SH-WFS) has a fixed subaperture area that is determined by consideration of several parameters such as the average atmospheric coherence diameter r₀ at the telescope site. Its SNR can be severely degraded due to low-light conditions caused by increasing turbulence, strong atmospheric scintillation, or simply viewing a faint object. Typically, the integration time of the sensor is increased to improve the SNR. Unfortunately, a decrease in bandwidth causes an increase in residual wavefront error that reduces image quality. We show that an increase in subaperture area produces a smaller residual wavefront error than an equivalent increase in integration time. Furthermore, we show that the ability to reconfigure the subaperture area, in combination with control bandwidth adjustment, provides superior performance over a system with fixed subaperture area when r₀ is different than the design point. We present a reconfigurable Shack-Hartmann wavefront sensor (RSH-WFS) with adjustable subaperture area implemented using a phase-modulated liquid crystal device (LCD). Experimental results demonstrate that the RSH-WFS increases system dynamic range by increasing the subaperture diameter whenever the Hartmann spot irradiance falls below the threshold of operation of the conventional SH-WFS.

We apply principal component analysis (PCA) to the characterization of artifacts in a digital image-acquisition system containing image-compression algorithms. The method is successfully applied to web cameras. The classification done with the PCA method produces three processes. The pure spatial process retrieves the luminance distribution of a static object. The pure temporal process is directly related with the temporal noise of the system. An intermediate spatial-temporal process reveals the interaction between the compression algorithms and the spatial-frequency contents of the object. Without prior information, the PCA method is able to distinguish this interaction from the classical temporal noise. The analysis of the anomalous pixels also reveals the location in the scene where the compression algorithms work harder. An extension of this analysis identifies the origin of the anomalous behavior in terms of its spatial or temporal character.