This PDF file contains the editorial “Assigning Manuscripts” for OE Vol. 47 Issue 08

Simultaneous noninverted and inverted dual-wavelength conversion is proposed and demonstrated by using a cross-polarization modulation effect in a semiconductor optical amplifier (SOA). Experiments show that noninverted and inverted conversions lead to power penalties of 1.3 dB and 0.8 dB, respectively, at a bit rate of 10 Gb/s. Based on this scheme, we have demonstrated, for the first time to our knowledge, an all-optical modulation format conversion from amplitude-shift keying (ASK) to frequency-shift keying (FSK). A frequency-shift keying signal with flexible tone spacing and high bit rate can be successfully generated by using this method.

Based on the generalized Huygens-Fresnel diffraction integral theory and the stationary-phase method, we analyze the influence on diffraction-free beam patterns of an elliptical manufacture error in an axicon. The numerical simulation is compared with the beam patterns photographed by using a CCD camera. Theoretical simulation and experimental results indicate that the intensity of the central spot decreases with increasing elliptical manufacture defect and propagation distance. Meanwhile, the bright rings around the central spot are gradually split into four or more symmetrical bright spots. The experimental results fit the theoretical simulation very well.

We show an optimization method based on an evolutive algorithm to obtain the profile of a simulated machined aspherical surface starting from a set of noisy discrete Cartesian coordinates (x,y,z), where the experimental coordinates x and y are used to simulate the sagitta z of the analyzed surface. By minimizing an objective function, the proposed method fits the sagitta function to the set of noisy discrete coordinates; thus the geometrical parameters of the simulated surface under test, such as the paraxial radius of curvature, the conic constant, and the aspheric deformation constants, can be obtained. Numerical results show that our method can be successfully applied to retrieve the simulated machined surface profile.

We propose a flexible technique to easily calibrate a structured light system, in which only two patterns are required: a paper checkerboard printed on a laser printer and attached to a surface plane, and a pattern image of a checkerboard, which is projected on the surface plane by a LCD projector. We treat the projector as a second camera; then the problem of calibrating the structure light system becomes equivalent to calibrating a stereovision system. The key to our technique is to make the projector able to see the calibration checkerboard like a camera. For that purpose, two approaches are proposed, called the black-and-white checkerboard method and the red-and-blue checkerboard method. Both computer simulation and real data are used to test the proposed technique, and very good results are obtained.

Accurate laser damage measurements are more difficult to perform in nonlinear optical crystals than in glasses due to several effects proper to these materials or greatly enhanced in these materials. Before discussing these effects, we address the topic of error bar determination for probability measurements. Error bars for the measured damage probabilities are especially important when testing small and expensive samples like nonlinear crystals, where only few sites are used for each measurement. The mathematical basics for the numerical calculation of probability error bars corresponding to a chosen confidence level are presented. Effects that possibly modify the maximum light intensity obtained by focusing into a biaxial nonlinear crystal are mainly the focusing aberrations and self-focusing. Depending on focusing conditions, position of the focal point in the crystal, beam propagation direction, and polarization, strong aberrations may change the beam profile and drastically decrease the maximum intensity in the crystal. A correction factor based on former theoretical work is proposed for this effect. The characteristics of self-focusing are quickly reviewed for the sake of completeness, and a note on parasitic second harmonic generation is added at the end.

Scatterometry takes advantage of the sensitivity exhibited by optical diffraction from periodic structures, and hence is an efficient technique for lithographic process monitoring. Even though the potential of this technique has been known for many years, it is challenging to accurately and quickly extract the multilayer grating overlay from diffraction data. We propose a method to measure the overlay by selecting an optimal measurement design based on the theoretical modeling of differential signal scatterometry overlays. A set of two grating overlay targets are designed with an intentional offset difference between the top and bottom gratings, to maximize the differential signal measurement sensitivity and to minimize the response to the process noise. We model the measurement sensitivity to overlays of two layer gratings, at a fixed wavelength and with a range of azimuth incidence angles from 0 to 180 deg, by means of rigorous diffraction theory. We compare the optical response of the zero- and first-order diffractive overlays. We show that with the appropriate target design and algorithms, scatterometry overlay achieves improved accuracy for future technology nodes.

We analyzed graphically the resonator stability versus the thermal focal length and the thermally induced birefringence, as well as the fundamental spot size on the laser rod for a green laser of Nd:Y₃Al_5O12 (Na:YAG) on high-gray-tracking-resistivity KTiOPO₄ as a function of the pumping power for r- and θ-polarization beams. The spot size distribution of the second harmonic and the fundamental wave in the folded cavity are also presented. On the basis of the theoretical analysis, an average power of 185.21 W was obtained with an intracavity frequency-doubling two-diode-side-pumped Nd:YAG laser system with twin acoustic-optical modulators, and the pulse width was 180 ns at a repetition rate of 10 kHz when the pumping power of each laser diode module was 600 W. The optical-to-optical conversion efficiency was 15.4%, and the beam quality factors were M_x² =9.52, M_y²=9.86.

Results are presented on the influence of acoustic waves on the performance of high-repetition-rate TEA CO₂ lasers. It is shown that acoustic waves generated inside the laser cavity lead to nonuniform discharges, resulting in a deterioration of the laser beam quality, decreased output energy, and an increase in pulse-to-pulse energy variation. The effect of the gas mix on the acoustic behavior is investigated, and experimental results on laser performance across a range of gas mixtures are presented. Methods to reduce the effects of acoustic waves are presented together with experimental results. The influence of acoustic damping measures on laser gain are demonstrated, showing a significant improvement in gain and output power at high pulse repetition rates.

Pollution is known to play an important role in optical performances of high power laser components. In this study, we propose a method to characterize the particulate pollution and to evaluate its impact on the optical component lifetime. We applied it at the Ligne d'Intégration Laser (LIL), which is a prototype installation of the future Laser Mega Joule facility. By systematic examination of each particle, we obtain, in number, size, and chemical composition, an overview of pollution deposited on optical surfaces. Then we deduce the origin of the principal pollutions. After irradiation of the polluted samples, with conditions close to the maximum of the LIL operation, no laser-induced damage is observed. In contrast, irradiation induces an important cleaning.

TiCN/Ti reinforced layers were fabricated on titanium alloy by the laser cladding method, aiming at improving its surface properties. Ti and TiCN powders of size 60 and 3 μm, respectively, were mixed in a certain ratio, prepasted on a Ti-6Al-4V substrate, and irradiated with a pulsed YAG laser in nitrogen atmosphere. The temperature field was calculated with a semi-infinite model. The calculated results show that the laser scanning velocity should be in the range of 1.0 to 3.0 mm/s; the best value for metting the substrate and realizing the cladding process is about 1.4 mm/s. The cladded samples were analyzed with x-ray diffraction, optical microscope observation, and a microhardness test. The results indicate that the composition of the cladding layer changes with the ratio of Ti to TiCN. The optimum Ti:TiCN mass ratio is about 15:85. The optical micrograph revealed a TiCN film thickness about 100 μm. The hardness was increased from 330 HV in the substrate to nearly 1500 HV in the modified layers, due to the presence of the hard TiCN phase. It is found that the proper laser scanning velocity to obtain high cladding quality is about 1.5 mm/s, which agrees with the calculated result of 1.4 mm/s.

Experimental work on the study of erbium (Er) diffusion and the characterisation of Er-diffused layers required to form waveguide amplifiers in LiNbO₃ substrate is presented. The presence of active Er in the lattice of LiNbO₃ is investigated using x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) measurement, and spectroscopic reflectometry. The shift of XPS peaks confirms the oxidized state of Er. The PL spectrum shows a sharp peak at 1530-nm wavelength, indicating the ⁴I_15/2→⁴I_13/2 lasing transition of Er³⁺. The reflectance spectrum indicates absorption at ~980 and ~1500 nm.

The suppression of degenerate four-wave mixing (DFWM) between pumps is demonstrated. By choosing proper fiber length, pump wavelengths, and pump powers, the DFWM between pumps can be completely suppressed. The characteristics of the parametric gain are also investigated, and some important conclusions are drawn.

A fiber laser with two different polarization-dependent output states is demonstrated. On the one hand, intensity-dependent loss in a nonlinear optical loop mirror, induced by nonlinear polarization rotation, can balance mode competition of a homogeneous broadening gain medium, and thus allows for stable room-temperature multiwavelength generation. On the other hand, intensity-dependent transmission can act as an equivalent saturable absorber, which results in passive mode locking of the fiber laser. It may thus serve two functions: passive mode locking and multiwavelength emission.

Optical free-space communications involving moving parties require precise beam pointing and mutual tracking of communicating transceivers. The existing variety of tracking techniques is still the major limiting factor in free-space laser communications. Here we propose a technique for optical beam tracking that utilizes nonlinear optical properties of materials. In our proof-of-concept experiment, a thin layer of a nematic liquid crystal (NLC) with high thermal nonlinearity was used to produce a thermal lens induced by the incoming optical beam. That beam modulated the NLC refractive index. As the transmitted optical beam passed through the same layer, the beam intensity was modulated in the far field. A sharp intensity maximum was formed at the distant communicating party position. This tracking capability has been demonstrated for angular disturbances at a subkilohertz frequency. This tracking mechanism also offers adaptive capability of compensation of strong aberrations. Such compensation has been demonstrated experimentally; numerical modeling performed with the Fresnel integral technique showed very good agreement with the experiment.

We study the wavelength-division multiplexing implementation of the recently proposed temporal Fresnel transform encoding technique, in order to evaluate its potential application for secure data transmission in short-haul fiber optic links. The different signal broadenings produced by each stage of the encoding process are analyzed in a dual time-frequency domain by using the Wigner distribution function. Furthermore, the robustness of the proposed method is illustrated by comparing the signal-to-noise ratio between the input and the decrypted signals obtained by varying typical parameters of the encryption-decryption setup. Numerical simulations revealed good system performance. Finally, we consider experimental feasibility with current photonic technology.

We develop a common tunable optical encoder/decoder based on fiber delayers and a logical controller. A new tunable optical encoder/decoder for optical code division multiplexing is designed and analyzed by using a delayer based on photonic crystals (PhCs) and a logical controller. In this scheme, it is found that the optical pulse delay is flexibly controlled by using PhC-based delayers to achieve optical encoding and decoding with a logical controller. The reflection, transmission, time delay characteristic, and optical spectrum of the encoded and decoded pulses are studied by numerical calculation for waves tuned in the PhC band gap. Theoretical analysis and numerical results show that optical pulses are achieved with the proper time delay. The advantages of this scheme are integration, reconfigurability, and easy operation.

A simple method for optical buffering management without electronic processing is proposed. The buffer system consists of feedback-looped parallel fibers as first-in–first-out buffers and an optical buffer manager. The buffering system has multiple input ports and one output port. The buffer stores incident packets in order of arrival. The manager controls stored packets and output switches of the buffers by sensing packet flow at the output port. The buffer system can be operated autonomously without an external controller. The buffer performance is evaluated with numerical simulation.

Considering the state of the art of wavelength conversion technology, it is likely to dictate a more limited and sparse deployment of wavelength converters in optical burst switching (OBS) networks. Without wavelength conversion capabilities at optical switches, how to avoid burst contention becomes an important issue. This paper proposes an approach to controlling wavelength-division-multiplexing passive optical stars, consisting in queuing arrivals for delayed reception, to be used in OBS networks. This solution brings optical star networks a step closer to all-optical realization. In addition, it allows almost total utilization of the channels, obtainable until now only by time-division multiplexing control. The proposed solution does not suffer performance degradation under heterogeneous traffic conditions and increasing number of nodes. It is thus unique in offering the potential for an all-optical solution and providing at the same time high throughput, low delay, small buffer requirements, and robustness under all traffic conditions.

An artificial neural network (ANN)-based model for erbium-doped fiber amplifiers (EDFAs) is developed. For a network with two input neurons and a single hidden layer, the number of neurons in the hidden layers is optimized by monitoring the mean square error (MSE) at the outputs. The model is used to optimize the design of EDFAs for central signal gain, 3-dB bandwidth, and noise factors, taking into account the influence of amplifier constructional parameters in the amplifier performance. For a given pump power and pulse width that correspond to the two input neurons, the model computes the constructional parameters to obtain values of central signal gain, 3-dB bandwidth, and noise figures in optimal range. The effects due to intrapulse stimulated Raman scattering (ISRS) are also considered by varying pulse widths from subpicosecond regime to hundreds of picoseconds. Self frequency shift due to ISRS is found to be prominent in the subpicosecond regime. The proposed model generates results with a MSE of 1×10^-4, and needs relatively very low computational time compared to other analytical models.

We briefly discuss the characters of aerosol profiles in the marine atmospheric boundary layer (MABL) detected by a high spectral resolution lidar (HSRL) system developed by Ocean University of China. In this 532-nm lidar system, an iodine filter is used to reject the Mie scattering signal and extract the Rayleigh signal. We deduce the aerosol scattering ratio (R_b) by the HSRL and analyze the correlation index of R_b and relative humidity (RH) under different weather conditions. The height of MABL is derived from the R_b profiles using a wavelet method.

Relatively inexpensive laser-guided weapons were identified as an important threat to naval platforms operating in littoral environments. Countermeasures (CM) developed against laser-assisted threats in air and land environments are all based on accurate threat detection. Due to the limited performance of current laser detecting capabilities applied to large maritime platforms, there are no known CMs that can be used to protect them. We present a new far off-axis laser detection approach for minimizing the number of laser sensors required to protect a ship. A model predicting the detection of aerosol scattering in the spectral band used by laser threats, exploiting the high humidity conditions present in maritime environments, is described, as well as its limitations and some preliminary results regarding its validation.

Currently, no infrared scene projector technology has the ability to simulate the real-world, high-dynamic-range temperatures encountered by modern infrared imagers. This paper presents the application of type II W-quantum-well diode lasers used in a high-radiance mid-infrared-emitting object plane in an effort to develop the first truly high-dynamic-range infrared scene projector. The observed dynamic-range capability simulates 250 K apparent background temperature to 1100 K maximum apparent temperature for inband mid-wave 3 to 5 μm radiance. In the presented research a linear array of high-power continuous-wave mid-infrared diode lasers and a high-speed optical polygon scanner were used to produce two-dimensional projected images. The ability to simulate high apparent temperatures was the direct result of luminescent infrared radiance emitted by the diode lasers. The simulation of the low background apparent temperatures was obtained by enclosing the entire projector system in an environmental chamber operating at 232 K. The apparent temperature of the infrared scene projector was calculated analytically and compared with the measured results. Sample imagery from the high-dynamic-range infrared scene projector is also given.

Multiple occlusion target tracking is usually a difficult problem in video surveillance. But in many cases, traditional mean shift tracking algorithms fail to track occlusion targets robustly. In this work, we focus on improving mean shift tracking algorithms to model and track all kinds of occlusion targets in video surveillance scenes. Two primary improvements on traditional mean shift tracking algorithms are proposed. First, after we determine which target the overlapping patches belong to, the nonocclusion part of each occlusion target can be obtained and applied to the tracking algorithm. Second, all the related occlusion target states are iteratively estimated one after another to eliminate the occlusion effects during the tracking process. Furthermore, the contrast experiment results show that the improved algorithm can track multiple occlusion targets, whereas traditional mean shift tracking algorithms fail.

An all-purpose multifunctional optical microscope based on digital holographic microscopy (DHM) is proposed, which enables DHM to be practically suitable for bioimaging applications. With our simple suggested digital signal-processing scheme, we have demonstrated that various optical images, including pure amplitude, pure phase, dark-field, phase-contrast, and differential interference contrast images, can be obtained from a single DHM measurement. Because these images obtained with our method are essentially the same as those obtainable with conventional optical imaging instruments, biologists can easily analyze them with their previous knowledge and experience.

Though it is the first step of a real video surveillance application, detection has received less attention than tracking in research on video surveillance. We show, however, that the majority of errors in the tracking task are due to wrong detection. We show this by experimenting with a multi object tracking algorithm based on a Bayesian framework and a particle filter. This algorithm, which we have named iTrack, is specifically designed to work in practical applications by defining a statistical model of the object appearance to build a robust likelihood function. Likewise, we present an extension of a background subtraction algorithm to deal with active cameras. This algorithm is used in the detection task to initialize the tracker by means of a prior density. By defining appropriate performance metrics, the overall system is evaluated to elucidate the importance of detection for video surveillance applications.

We address the important product inspection application of contaminant detection on chicken carcasses. Detection of four contaminant types of interest (duodenum, ceca, colon, and ingesta) from chickens fed with three different feeds (corn, milo, and wheat) is considered. We consider feature selection algorithms for choosing a small set of spectral bands (wavelengths) in hyperspectral (HS) data for online contaminant detection. For cases when an optimal solution is not realistic, we introduce our new improved forward floating selection algorithm; we call it a quasi-optimal (close to optimal) algorithm. Our algorithm is an improvement on the state-of-the-art sequential forward floating selection algorithm. We train our algorithm on a pixel database using only corn-fed chickens and test it on HS images of carcasses with three feeds. Our new algorithm gives an excellent detection rate and performs better than other suboptimal feature selection algorithms on this database.

This paper presents a novel omnidirectional camera calibration technique based on automatic contour matching. In this proposed method, the initial parameters, translation and rotation, are computed using the epipolar constraint from matched feature points. By using a normal vector to a 3-D plane connecting the contour with the center of the camera projection model, after contour segmentation, candidate contours are chosen to establish a correspondence between views. In order to find the corresponding end points of the contours among the views, the initial parameters and their active matching windows are utilized. Finally, the camera motions are estimated by minimizing the angular error function between the epipolar plane and the backprojected end-point vectors of the corresponding contours. Experiments on synthetic and real images demonstrate that the proposed algorithm obtains more precise camera parameters than previous methods.

We model a recognition problem for connected hand gestures to find an optimal path through a hidden Markov model (HMM) directed acyclic graph. To determine this optimal path, an online graph search method is proposed that decodes the observed gesture pattern and evaluates the optimal graph node at each time frame of the continuously deepening HMM graph. The temporal characteristic of gesture recognition is subsequently handled by introducing a rejection threshold time that acts as a depth-wise sliding window for pruning unnecessary graph nodes. The functional depth of the graph is defined by this depth rejection threshold. Experimental comparison of our algorithm with other HMM-based search algorithms demonstrates the effectiveness and robustness of our method.

We propose a method based on a polar transformation between the source image and the output image for stereo image rectification. The proposed method solves several problems that arise from existing line-based methods. Many existing line-based methods require a different flow of rectifying process according to the different location of each epipole. Moreover, they inevitably require a recording table due to the lack of a transforming relationship. We solve these problems in a simple way by using a linear transformation that avoids the necessity of having different flows according to the locations of epipoles during rectification. Since all the epipolar lines use the same mathematical basis, look-up tables are not needed for referencing purposes. The proposed method consists of four steps, namely pretransform, common region detection, polar transform-based image rectifying, and epipolar line matching. The method is used on four pairs of stereo images obtained from three stereo structures. The results show good accuracy as well as good efficiency compared to existing methods in the literature.

Registration of multiresolution and multisensor images is a challenging problem in the research area of remote sensing. Conventional image registration algorithms have greatly suffered from control points selected manually. In this study, we propose a novel method for automatic affine image registration based on local descriptors, called automatic image registration by local descriptors (AIRLD). In this algorithm, we first apply the Harris-affine method to extract the interesting points of the given images, and then they are used to calculate the affine invariant feature descriptors. These descriptors are combined through the normalized mutual information (NMI) method to obtain robust matching control points in both reference and sensed images automatically. Finally, we show that subpixel registration accuracy can be reached by means of a least-square fitting of a paraboloid function to the surface generated by the NMIs of the control point neighbors. Experimental results show that our method can provide better accuracy than the conventional registration process.

This PDF file contains the errata for “OE Vol. 47 Issue 08 Paper 2979677” for OE Vol. 47 Issue 08