This PDF file contains the editorial “Covering Ground” for OE Vol. 46 Issue 03

To the best of our knowledge, this paper demonstrates the first hybrid analog-digital design fiber-optic spectrum processor that can simultaneously provide spectrum gain slope adjustment as well as independent channel equalization attenuation controls.

Ambient light in a scene can introduce errors into range data from most commercial three-dimensional range scanners, particularly scanners that are based on projected patterns and structured lighting. We study the effects of ambient light on a specific commercial scanner. We further present a method for characterizing the range accuracy as a function of ambient light distortions. After a brief review of related research, we first describe the capabilities of the scanner we used and define the experimental setup for our study. Then we present the results of the range characterization relative to ambient light. In these results, we note a systematic error source that appears to be an artifact due to a structured light pattern. We conclude with a discussion of this error and the physical meaning of the results overall.

The transmission system must simultaneously determine transmission rates that balance total profit from bit allocation with transmission costs. The objective is to maximize the present value of the satisfaction derived from progressive transmission over the future, subject to constraints imposed by the initial conditions and the prioritization and transmission technologies. The analytical results demonstrate an oscillatory behavior over time of the rent component of profit depending on the incremental cost of cumulative transmission and not the current marginal transmission cost. This oscillatory behavior of rent over time necessitates devising a stabilizing scheme for its optimal use in the transmission.

An array of light-emitting diodes (LEDs) assembled upon a spherical surface can produce a wider angle distribution of light than a typical array (i.e., an array assembled by mounting LEDs into a flat surface). Arranging each single LED into an optimal placement, the uniformity of the illumination of a target can be improved. We derive approximate formulas and equations for the optimum LED-to-LED angular spacing of several spherical arrangements for uniform far-field irradiance. These design conditions are compact and simple tools that incorporate an explicit dependence on the half-intensity viewing angle (half width half maximum angle) of LEDs.

A short-focal-length double-Fourier-transform-lens system for holographic storage was designed. The system is minimized by propagating signal beam and reference beam along a common optical axis. In order to obtain a compact configuration and high information density, the front and the rear Fourier-transform lens have short focal length (33 and 30 mm, respectively). The multiconfiguration optimization method is adopted to correct aberrations of the object region and meet the requirement of reference beam. The designed system, whose total numerical aperture for signal beam and reference beam is 0.60, achieves diffraction-limited imaging quality.

This research describes the derivation of a 50% probability of identification cycle criterion (N₅₀) for a set of underwater mines. Various levels of blur were applied to eight underwater mines and four nonmine objects with nine aspects in the visible spectra. Results were analyzed as a function of blur level and target size to give identification probability as a function of resolvable cycles on target. The results are applicable to underwater mine target acquisition estimates for visible electro-optical imaging systems and for laser-illuminated, range-gated imaging systems. This research provides for the design and analysis of electro-optical systems in the identification of underwater mines.

Many published research works regarding digital image correlation (DIC) have been focused on the improvements of the accuracy of displacement estimation. However, the original displacement fields calculated at discrete locations using DIC are unavoidably contaminated by noises. If the strain fields are directly computed by differentiating the original displacement fields, the noises will be amplified even at a higher level, and the resulting strain fields are untrustworthy. Based on the principle of local least-square fitting using two-dimensional (2D) polynomials, a 2D Savitzky-Golay (SG) digital differentiator is deduced and used to calculate strain fields from the original displacement fields obtained by DIC. The calculation process can be easily implemented by convolving the SG digital differentiator with the estimated displacement fields. Both homogeneous and inhomogeneous deformation images are employed to verify the proposed technique. The calculated strain fields clearly demonstrate that the proposed technique is simple and effective.

Molecular contamination of optical surfaces from outgassed material has been shown in many cases to proceed from acclimation centers and to produce many roughly hemispherical "islands" of contamination on the surface. The mathematics of the hemispherical scattering is simplified by introducing a virtual source below the plane of the optic, in this case a mirror, allowing the use of Mie theory to produce a solution for the resulting sphere in transmission. Experimentally, a fixed wavelength in the vacuum ultraviolet was used as the illumination source and scattered light from the polished and coated glass mirrors was detected at a fixed angle as the contamination islands grew in time.

A multi-end-pumped nonplanar ring laser with a two-mirror resonator has been demonstrated. Two important parameters of the beam loops are described, and simulation diagrams of the beam loops are shown. Using an output coupler with a fan-shaped distribution in reflectivity, multi-end-pumped operation with the output beam from a single point has been achieved, and the experimental results show that the laser could be multi-end-pumped to reduce thermal effects and multiply the fracture limit of the laser crystal.

We have experimentally investigated a stably tunable multiwavelength erbium-doped fiber ring laser based on phase modulation of a semiconductor optical amplifier with lasing wavelength and spacing tunability at room temperature. Stable multiwavelength operation is obtained by direct modulation of the bias current in the amplifier. We demonstrate such a laser incorporating a tunable Lyot-Sagnac filter with two segments of polarization-maintaining fibers with wavelength-spacing and lasing wavelength tunability. We successfully achieved 1.0-nm-spacing eight-channel and 0.8-nm-spacing ten-channel lasing wavelengths at room temperature. The lasing wavelength of the laser can be also controlled by a polarization controller.

We report on an improved numerical method for hysteresis compensation within a hybrid piezoelectric fiber optic voltage sensor. The previous technique relied on laborious and relatively inaccurate individual curve-fitting procedures to create multiple sets of lookup functions used for the final derivation of the measured voltage. The new technique uses an aggregate 3-D arrangement of the captured hysteresis loops and applies a surface-fitting algorithm based on the Levenberg-Marquardt method to create two 3-D lookup functions, which are then used to derive the instantaneous value of the measured voltage. Furthermore, a more advanced algorithm for selecting top and bottom hysteresis-loop parts has been applied to eliminate errors associated with the incorrect selection at low voltage levels by the previous algorithm. The proposed enhancements greatly simplify the calibration process and significantly reduce measurement errors. The technique, implemented using a real-time signal-processing system, was tested and its effectiveness evaluated experimentally. The new algorithm provided complete phase error compensation, from approximately 7 to 0 deg, and magnitude error compensation down to 0.15% (full-scale output)—an improvement of more than 3 times over the previous technique.

Based on the theory of optical fiber coupling and Fabry–Perot interference, a model of extrinsic Fabry–Perot interferometric (EFPI) optical fiber sensor for measuring strain is analyzed, and the theoretical model is demonstrated. A formula of white-light interfered EFPI optical fiber strain sensor is obtained. The system of the optical fiber sensor is designed using an LED source as light source to obtain reflected spectrum. The experimental results show that it coincides well with the computational simulation of theoretical model, which means the model is accurate.

We discuss a novel whole-field optical strain sensor termed the moiré interferometric strain sensor (MISS) for simultaneous measuring of multipoint strains and whole-field contours of in-plane displacement. A high-frequency grating, attached to the surface of a specimen, is used as the displacement and strain-sensing unit. When illuminated by two collimated beams at a prescribed angle, the interference of the diffracted beams gives the whole-field deformation contours. If, on the other hand, each of the individual beams is separately imaged using a multilens CCD sensor similar to a wavefront sensor, the separation between the spot centroids for each microlens is directly proportional to the normal or shear strain component at the corresponding position on the specimen. Applications are demonstrated for uniform rotation and simulated in-plane strains.

Isolation of a new structural acousto-optic switch based on an integrated optical polarization-independent quasi-collinear acousto-optic tunable filter (AOTF) is studied in detail. First, we demonstrate the structure and operation principle of this optical switch. Second, factors that influence isolation of the optical switch are analyzed, and expressions of the isolation are deduced. Finally, the optical switch is fabricated, and its isolation is measured in experiment. We show that the isolation mainly depends on the TE/TM mode intensity ratio when the radiofrequency signal is absent, but the isolation mainly depends on the mode conversion efficiency when the radiofrequency signal is present.

We propose and demonstrate a novel polarization- and wavelength-independent digital electro-optic switch in Ti:LiNbO₃ with switching voltages of ±32 at 1.55 μm wavelength. This 2×2 integrated optic switch is characterized by a steplike response to the applied voltage. Switching is achieved through adiabatic mode evolution in an asymmetric waveguide junction. An average insertion loss of ~4.5 dB and polarization-independent switching with average cross talk of −12 dB are achieved. The demonstration of 1×4 polarization-independent switches with digital optical switch elements is eventually reported.

We evaluate the performance of the externally modulated optical minimum shift keying (MSK) signal. The tolerance against fiber dispersion, linear crosstalk, self phase modulation (SPM), and stimulated Brillouin scattering (SBS) threshold for MSK is studied and compared with that of 50% duty cycle return-to-zero differential phase shift keying (RZ-DPSK) and 50% duty cycle RZ on-off-keying (RZ-OOK), via both experiment and simulation. Simulation results show that the MSK signal has the widest tolerance against SPM and the smallest linear crosstalk among these formats. Experimental results agree with the simulation on negative penalty against SPM tolerance and show widest tolerance against dispersion tolerance for MSK. The measured SBS threshold of both MSK and RZ-DPSK is more than 10 dB higher than that of an RZ-OOK system.

Typically, the region of interest (ROI), in the JPEG2000 standard, is manually defined, and then wavelets are used to compress the ROI at a higher bitrate than the rest of the image. The wavelet decomposition in JPEG2000 also lends itself to texture and edge extraction for segmentation and classification purposes. In this paper, a semi-automatic ROI generation algorithm for images is presented, where the texture and edge information provided by the first level of the wavelet decomposition is used to segment the wavelet coefficients. This first-level decomposition provides enough edge and texture information for image segmentation, allowing computational savings. A mask that outlines the ROI is determined based on the entropy calculation of the segmented regions. The advantage of this method is that the segmentation process is entirely performed in the wavelet and not in the pixel domain, therefore offering additional computational efficiency. The resulting ROI is coded using the MAXSHIFT method. The algorithm was applied and successfully demonstrated in several images.

For the first time a practical control analysis of semiconductor optical amplifier (SOA) dynamics has been conducted with implementation design considerations, based on a newly developed SOA electrical Spice model. It can be used for optimal design of SOA driver and control circuits within any existing optical network. In addition, a novel dual-control loop is designed and simulated. The two control loops overcome both fast (transient) and slow (steady-state) phenomena. Simulation results of the controlled SOA dynamics are presented, with an implementation example of an SOA high-speed optical switch driver. For optical switching the SOA driver uses a pre-impulse-step injected current as the switching signal. The developed Spice SOA electrical model is used for the predistortion optimization. The simulation results show that an ultrafast driver and control circuit seems feasible. It is also shown that the SOA can operate as a variable attenuator. A dynamic time constant of ~1–2 nsec is achievable using the predistortion scheme. The design and analysis here show that the SOA technology can be used as the switch-fabric core of a bufferless optical network with switching on packet timescales.

A simple experimental technique for the measurement of refractive index profiles of axially symmetric optical fibers is presented. This method is based on image phase gradients introduced into a transmitted optical field by a fiber sample. An image of the phase gradients is obtained using a technique based on bright field microscopy. Then a relative refractive index profile of optical fibers is reconstructed by means of the inverse Abel transform.

This paper presents complete models for various fiber dispersions and their mitigations using electronic equalizers. A novel constant-modulus-adaptive (CMA) blind equalization technique is proposed for fiber dispersion compensation. Simulations demonstrated effective compensation means for chromatic and polarization mode dispersions using least-mean-square (LMS) and CMA blind equation techniques. Also proposed is the adaptive fiber transceiver architecture with eye-open-detect circuits that can be used to provide the feedback controls for adaptive thresholds and high-speed analog equalizers.

A virtual Mueller matrix method is proposed to measure the first- and second-order polarization mode dispersion (PMD) vectors in optical fibers. This method not only can use a large frequency step to attain low-noise PMD vector data, but also does not require knowledge of the input polarization states. Our measurement method has a simpler setup and is more accurate than the traditional Mueller matrix method.

The effects of annealing on the properties of hermetically carbon-coated optical fibers are investigated. The hermetically carbon-coated optical fibers are prepared by the plasma enhanced chemical vapor deposition method using methane and hydrogen as the precursor gases. The annealing temperatures are selected at 100, 200, 300, 400, and 500°C, respectively. The thickness, optical band gap, and microstructure of carbon films are measured. Meanwhile, the water-repellency and low-temperature surface morphology of carbon-coated optical fibers are evaluated. The results indicate that the thickness and defect content of the carbon films decrease with increasing the annealing temperature, while the degree of structure order and the amount of microcrystalline graphite in the carbon films increase. When the annealing temperature is over 300°C, the carbon films acquire enough thermal energy to cause a great amount of hydrogen and hydrocarbon to be released, and the carbon films transform to the graphitelike structure. Additionally, based on the evaluation of water-repellency and low temperature–induced break or delamination of carbon films, it is found that the carbon film annealed at 300°C is the best one for use as the hermetic optical fiber coating.

Logic gates are the fundamental building blocks in any digital data-processing circuit. We propose a new method to implement a NAND logic operation controlled from a long distance. To realize it we consider two types of Gaussian pulses: the control beam and the signal beam. The signal beam propagates first through an electro-optic modulator and then through a nonlinear waveguide, whereas the control beam propagates directly through the optical fiber. Controlling the applied voltage of the electro-optic modulator, we can change the phase relation between the pulses. Hence a NAND switching operation can be realized at a very long distance in the optical fiber.

We present a simple and effective method for estimating arbitrary phase steps in the presence of noise in phase-shifting interferometry. The method is based on the linear prediction property of the sinusoids. Simulation and experimental results obtained from a holographic interferometry prove the efficiency and feasibility of the proposed method.

A technique for determination of the thickness of a plane-parallel transparent plate using a lateral shearing interferometer (LSI) is discussed. With this technique, the parallel plate whose thickness is to be determined is used to introduce a change in the collimation of the retro-reflected beam from an optical setup in which a corrected lens focuses an expanded collimated laser beam on the surface of a plane mirror placed at the back focal plane of the lens. The thickness of the plate is calculated by measuring the defocusing caused by the plate, which is inserted in the beam path between the collimating lens and the plane mirror, with an LSI. Results obtained for a parallel plate are presented.

In order to evaluate the performance of laser-based applications in maritime environments, laser beam propagation studies over the Baltic Sea during the period of about half a year have been carried out. A slightly slanted path with the laser system located 18 m above the sea level and corner cube targets and separate receivers placed on islands at 2.5-, 5.5-, and 16.5-km distance were used. From the registered laser signals irradiance fluctuation parameters for different beam offsets relative to the beam's center, temporal, and amplitude signal distributions, we derived the probability and mean time of fade. Results and statistics from single and double propagation paths are compared.

In radiometric interferometry from space [e.g., European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission, Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) interferometer], apodization or tapering functions can be used to ensure that ground resolution elements (synthetic antenna footprints) have equal area and shape. If resolution can be made uniform, subsequent processing—which relies on multiangular processing of the information from the same point of the Earth but seen from different directions, from snapshot to snapshot-is less prone to errors. This is called strip adaptive processing. We describe how to use two-dimensional (2D) Kaiser apodization functions to achieve the strip adaptive process and describe its relation to conventional Blackman nonadaptive apodization. In particular, we show how to compute the Kaiser parameters from sampling specifications within the complete field of view (FOV) of the instrument. This is achieved by setting up suitable regressions between a Kaiser parameters–dependent function, the array function half width at half maximum (HWHM), and the nominal elliptical footprints' area and eccentricity. The performance of the suggested methodology is compared to the baseline, based on nonadaptive Blackman apodization.

We describe an experimental setup to determine temperature distribution of an optical fiber with an object-space resolution of less than 0.01 mm. Determination of temperature distribution with a spatial resolution of about 7 μm as a function of position and time is reported here for the first time, to the best of our knowledge. We apply the high-resolution measurements to confirm the feasibility of using the rare-earth-doped silica as an infrared (IR)-to-visible converter.

A new small-target detection method in forward-looking infrared images (FLIR) is proposed. The goal is to identify target locations, with low false alarms, in thermal infrared images of a natural battlefield. Unlike previous approaches, the proposed method deals with small-target detection in low-contrast images, and it is able to determine centers of targets accurately. The method comprises three distinct stages. The first stage is called center-surround difference, whose function is to find salient regions in an input image. In the second stage, local fuzzy thresholding is applied to the region of interest that is chosen from the result of the first step. The extracted binary objects are potential targets that will be classified as valid targets or clutters. Finally, using size and affinity measurements, the potential targets are compared with target templates to discard clutters in the third stage. In the experiments, many natural infrared images are used to prove the effectiveness of the proposed method. The proposed method is compared to previously reported approaches to verify its efficiency.

A secure coding concept for pairwise images is revealed and implemented by the use of the proposed fractal mating coding scheme, in which the domain pools consist of the domain blocks selected from the pairwise images to explore both the intra- and interimage similarities. In addition to the pairwise relation, the mating ratios denoting the percentages of the domain blocks selected from both images are utilized. Further encryption can be achieved by the use of block mean permutation and mating of the fractal codes. The security level is high because the jointly coded images cannot be correctly reconstructed without all the required information. The computer experiments show that the coding performance can be greatly improved from conventional fractal coding schemes, and the intersecured purpose for pairwise images is successfully achieved.

This paper introduces a new class of weighted median filters. The proposed algorithm employs a spatial distance weighting function, which is based on the conclusion that the degree of similarity between two stimuli can be quantified as a simple exponential decay function of a normalized distance in a psychological space. Depending on two parameters of the weighting function, the proposed approach can provide filtering performance ranging from an identity operation to that of the standard median filter (SMF), and by adaptively adjusting one of the parameters, the best possible filtering effect may be achieved. The experimental results show the supriority of the proposed solution to the SMF and some other median filtering methods in terms of both the objective measures and the perceptual visual quality.

Edge detection plays an important role in image analysis systems. We present a color selective edge detection technique, which consists of two image processing steps. The first step represents pixel-based color detection and the second progressive block-oriented edge detection. The combination of these two steps defines a selective edge detection technique, which enables fast and simple processing of those images captured using arbitrary cameras in complex scenes with nonstandard illumination. The proposed method was implemented for the detecting of skin color objects and tested on real scene images.

Multiple-description (MD) transmission results in nonlinear value-metric (value-scaling) distortion in the case when some of the sent descriptions are not received. An acceptable image can still be received in the above situation due to the characteristic of MD. However, it is quite damaging to the traditional quantization-based watermarking technique for payload detection. To overcome the problem, a straightforward approach would be to increase the quantization step size in watermark embedding so as to keep the distortion within the tolerant range. However, as a larger quantization step size in watermark embedding would result in a worsened watermarked image, it is not feasible to adopt it without further consideration. The proposed multirate lattice quantization index modulation (MRL-QIM) encodes two watermark bits into each of the four co-set points of a lattice (so-called multirate). Compared to traditional vector-based quantization encoding or combined spread spectrum-quantization encoding, it significantly increases the payload (capacity) and enhances the robustness of watermark detection while preserving the fidelity of the watermarked image. Comprehensive experiments have confirmed that the overall performance and the effectiveness of the proposed scheme are superior to previous approaches.

Template matching is the search for a known object, represented by a template image, at an arbitrary location within a larger image. The local measure of match is often desired to be invariant to certain transforms, such as rotation and dilation, of the template. Although a variety of solutions have been proposed, most are designed to provide invariance to a specific transform or set of transforms, and often involve significant computational demands. When invariance to "small" transformations of the template (e.g., rotation by a small angle) is sufficient, local linear approximations to these transforms may be used to allow template matching with invariance to arbitrary transforms, without significantly increased computational requirements.

This paper presents a combination of a color- and a morphological-based method for face localization in color images. The skin-color segmentation technique is first applied using a color space, YCg′Cr′, specifically proposed for face detection, in order to obtain the skin regions inside the image. Then, a combination of morphological operators and algorithms is used for completing the segmentation masks. In a second phase, the location of the facial features, based on the elliptical shape of the face, is performed for finding the face candidates. Therefore, the definition of a simple face model based on the best-fit ellipse determined by the position of the eyes and mouth is proposed, and its application to the segmentation mask yields the localization of the face. Finally, the segmentation mask based on this ellipse is used for detecting the face in the image under test.

This paper presents an approach for gesture recognition based on curvature scale space (CSS). First, for each gesture sequence, hands are segmented from gesture images into binary silhouette images, and then the binary hand contours are computed. Next, CSS features of hand contours and four-directional chain codes between two successive hand contours are extracted to characterize hand shapes and trajectories of gestures, respectively. In particular, a feature-preserving algorithm is proposed to allocate the extracted CSS features to a one-dimensional feature vector with a fixed size for the input of hidden Markov models (HMMs). Finally, the HMM is applied to determine hand-shape and trajectory transitions in different gestures for gesture identification. Results show that the proposed approach performs well in recognizing the gestures and is more accurate than the previous methods that were based on conventional features.

A common problem in vanishing-point-based camera calibration methods is that they are ill-conditioned when the parallel lines for calibration in the world coordinates also appear to be parallel or near-parallel in the image coordinates. Then the camera parameters are not calculable or incur substantial error because one of the vanishing points approaches infinity. We propose a new method that overcomes the ill-conditioning. Using lane markings on a road to form a rectangular calibration pattern with two vanishing points, we first derive a set of equations based on one vanishing point without assuming any extrinsic parameters being known a priori. Because only one vanishing point can approach infinity at any one time, ill-conditioning can be avoided completely by selecting the vanishing point that does not suffer from it. This enables us to produce a set of extrinsic camera parameters that maps the world and image coordinates accurately. Simulation and real image tests show that the proposed method conquers ill-conditioning successfully and is insensitive to pixel offset on the calibration pattern.

We present designs of a diffractive polarizer having low zeroth-order reflectivity that is compact, potentially mass-producible and cost-effective, and compatible for high-power applications. It consists of subwavelength grating structures superimposed over a diffraction grating. Using rigorous coupled wave analysis, we optimized the parameters of multilevel grating structures to achieve antireflection for both incident TE and TM polarization states with one polarization passing in the zeroth order and the other into the first and higher orders. We focused on polarizer designs for the 1.31-μm wavelength range, and the theoretical values for zeroth-order reflection were calculated to be 0.01% for TE and 0.29% for TM modes. The zeroth-order transmission efficiencies were 97.2% for TE and 0.01% for TM modes. A prototype of one design was fabricated and tested to verify the functionality of the device, and the zeroth-order reflection was determined to be 1.2% and 3.5% for the two modes.

We numerically investigated subwavelength imaging in a silver nanorod of 50-nm height and 20-nm diam buried in dielectric background (SiO₂) with a finite-difference time-domain (FDTD) method in the three dimensions. The near-field components of the Gaussian incident beam were plasmonically transferred through the input end of a silver nanorod to reproduce the light distributions of the incident wave at the output end. The field distributions were calculated at the different sectional planes of the rods, and it was found that the spatial resolution was less than 40 nm given by the rod size, which is much beyond the diffraction limit of the conventional imaging system. The field intensity in the image plane was well resolved due to the collection of surface plasmon polaritons. The behaviors of the three components of field distribution at entrance and exit from the nanorod and the influences of the optical field distribution generated by some factors are also discussed in detail. The proposed structure possesses a deep transfer of super-resolution image and can be used with image transfer.

This PDF file contains the errata for “OE Vol. 46 Issue 03 Paper 2721993” for OE Vol. 46 Issue 03