This PDF file contains the editorial “Hands-off Experiences” for OE Vol. 47 Issue 10

We demonstrate an improved fiber-stretcher-based scanning delay line in terms of both the scanning speed and the delay range. Such a module is essential to optical coherence tomography (OCT) applications. An 18-mm delay range at 2-kHz scanning frequency is achieved and verified in a dental OCT setup. Repeatability and reliability of the new design are further evaluated through multiple trials within days. The results confirm its feasibility into practical applications.

We have developed and demonstrated a high-duty-cycle asynchronous InGaAsP-based photon counting detector system with near-ideal Poisson response, room-temperature operation, and nanosecond timing resolution for near-infrared applications. The detector is based on an array of Geiger-mode avalanche photodiodes coupled to a custom integrated circuit that provides for lossless readout via an asynchronous, nongated architecture. We present results showing Poisson response for incident photon flux rates up to 10 million photons per second and multiple photons per 3-ns timing bin.

We present a method for improving a flash system for retroreflective beacon detection in CMOS cameras. Generally, flash systems are designed in such a manner that makes them suited for beacon detection in a small range interval. We strive to increase the flash system range interval by exploiting the directional properties of the retroreflector. Thus, light sources placed relatively far away from the optical axis of the camera will contribute only when the retroreflector is far away. This fact can be used to compensate for the 1/distance² dependency of optical power. We present underlying theory and formulae, then describe a flash system consisting of several light-emitting diodes that was designed by considering the presented method. Simulations show that the usable flash range of the improved system can be almost doubled compared to a general flash system. Tests were performed indicating that the presented method works according to theory and simulations.

We describe an evolutionary algorithm for the design of an imaging triple-étalon Fabry-Perót interferometer (MFPI), which gives a solution to the multidimensional minimization process through a stochastic search method. The interactions between design variables (the étalon reflectances, interétalon ghost attenuator transmittances, and spacing ratios) are complex, resulting in a fitness landscape that is pitted with local optima. Traditional least-squares and gradient descent algorithms are not useful in such a situation. Instead, we employ a method called evolution strategies in which several preliminary designs are randomly generated subject to constraints. These designs are combined in pairs to produce offspring designs. The offspring population is mutated randomly, and only the fittest designs of the combined population are passed to the next iteration of the evolutionary process. We discuss the evolution strategies method itself, as well as its application to the specific problem of the design of an incoherently coupled triple-étalon interferometer intended for use as a focal plane instrument in the planned National Solar Observatory's Advanced Technology Solar Telescope (NSO's ATST). The algorithm converges quickly to a reasonable design that is well within the constraints imposed on the design variables, and which fulfills all resolution, signal-to-noise, throughput, and parasitic band suppression requirements.

A low-cost, robust, versatile digital shadowgraph visualization system is presented that provides a fast nonintrusive diagnostic for unsteady high-speed flows. The technique is particularly designed for real-time automated tracking of shock positions, enabling high-speed active shock control. The visualization system is based on a high-intensity white LED light source combined with a CMOS-imaging sensor, providing the system with three modes of operation: (1) high-resolution overall instantaneous visualization; (2) high-resolution visualization showing spatial-temporal variations in the flow field, allowing direct identification of areas where changes occur; (3) adjustable windowed visualization at reduced resolution at high frame rate (currently up to 980 Hz). Experimental results are presented together with numerical simulations based on the high-accuracy NTS Navier-Stokes solver and Roe's flux difference splitting method. The flow studied is an adjustable underexpanded jet flow coming from a nozzle that is placed in a counterflowing Mach-2 flow. The interaction of the two flows results in a complex shock and expansion pattern, providing a challenging configuration for the numerical flow solver. By modulating the jet, high-frequency changes are induced in the interaction pattern, allowing simulation of shock movement in a supersonic inlet. Good correspondence between measured and numerical shock position and angle is found.

A new kind of optical remote camera with wide field of view in space is presented, which can work in broad wave bands with all-reflective optical systems. Microelectromechanical system deformable mirrors create a variety of image quality across the entire field of view and dynamically correct all the geometrical aberrations at any field angle. After correction, the performance of the system is close to the diffraction limit. The off-axis optical system with three mirrors is introduced to simplify and miniaturize the optical system. The variable resolution is used to reduce the difficulty of the high-resolution large-aperture reflective system and to minimize the bandwidth requirements for data transmission.

The Dyson spectrometer form has the potential to deliver good imaging performance, high throughput, and low distortion in a compact configuration suitable for cryogenic infrared applications. The three main requirements for a practical implementation—availability of the required concave diffraction grating, availability of the Dyson lens material, and clearance for slit and focal plane packaging—are now within the state of the art, opening the Dyson form to serious consideration. Several high-performance Dyson designs for the long-wavelength infrared are presented.

There is a need to ensure that light-emitting diodes (LEDs) are performing well as they are increasingly used in demanding applications. In this work, we present an adapted usage of flatbed scanners to inspect LEDs. This offers the benefits of lower cost and higher measurement throughput than specialized equipment like goniophotometers. The technique is demonstrated to detect LED encapsulation defects, to identify low illumination and misaligned LEDs in arrays, and to evaluate angular color nonuniformity of white LEDs.

This work shows the design, fabrication, and optimization of a silicon sensor with an extended sensing range toward the UV region. The main characteristic of this detector is the enlargement of the common silicon detection range to the ultraviolet region (240 to 400 nm). The fabrication process of this detector is compatible with complementary metal oxide semiconductor (CMOS) silicon technology, which makes it cheaper than commercial UV detectors.

The net gain (G_net) and internal noise figure (INF) characteristics of an Er³⁺–Yb³⁺ doped waveguide amplifier (EYDWA) with a bidirectional pump scheme and double-pass configuration (DPC) were analyzed both experimentally and numerically. A propagation-rate-equation model was used to describe the eight-level EYDWA system. The EYDWAs used in the experiments were fabricated on an Er³⁺–Yb³⁺ doped phosphate glass waveguide of 3.8-cm length using a two-step ion exchange. In comparison with the backward-pump-only scheme (BPOS), the equalizing bidirectional pump scheme (EBPS) achieved a 4.1-dB enhancement in G_net, while the forward-pump-only scheme (FPOS) achieved a 6.0-dB reduction in the INF with a single-pass configuration (SPC) and 90-mW total pump power. About a 100% enhancement in the G_net and 50% less deterioration in the INF were obtained in DPC compared with SPC under saturation pump power. A 4.1-dB enhancement in G_net and a 1.4-dB reduction in INF were observed by applying EBPS instead of BPOS or FPOS with 150-mW total pump power in DPC. Both the experiments and simulations showed that a significant improvement in the performance of the EYDWA can be achieved by optimizing the amplification configuration and pump scheme.

The first results are reported from a repetitively pulsed, electron-beam-pumped angularly multiplexed krypton fluoride (KrF) laser system. This laser system, called Electra, was constructed at the U.S. Naval Research Laboratory. The technologies developed on Electra are scalable to a full-size fusion power plant beam line and should meet the inertial fusion energy (IFE) requirements for durability, efficiency, and cost. As in a full-size fusion power plant beam line, Electra is a multistage laser system that consists of a commercial discharge laser, a 175-keV electron-beam-pumped (40-ns flat-top) preamplifier, and a 500-keV (100-ns flat-top) main amplifier. Angular multiplexing is used in the optical layout to provide pulse length control and to maximize laser extraction from the amplifiers. The laser system initially demonstrated 452 J in a single shot and 1.585 kJ total energy in a one-second, 5-Hz burst. The preamplifier alone produces a 25-J KrF output with two angularly multiplexed beams. Extraction volumes were calculated for both a single-pass and a double-pass angularly multiplexed amplifier. A standard ray trace must be used to calculate the extraction volumes for the double-pass amplifier with focusing elements.

Starting with ray tracing through a single-mirror beam steering system, an analytical approach is developed to predict the scan pattern on a flat screen. The results for a single-mirror system are then extended to a two-mirror/two-axis beam scanning system, which paves the way to a performance comparison of single-mirror and two-mirror/two-axis laser projection systems. Some fundamental aspects of mirror-based laser projectors of different configurations are investigated, such as maximization of image size on screens of different format, the effect of pixel rotation and size changes on image resolution, and the relationship of angular motion of the mirror in a single-mirror system and the two mirrors in a two-mirror/two-axis system.

In a photoconductive semiconductor switch (PCSS) ultrawideband (UWB) array system, optical fiber bundles with several hundreds or more fibers are employed to distribute laser beam energy and yield multiple laser beams with the same energy and coherence. Ideally, if each fiber of the bundle is the same length, the laser beams transmit in the fiber and may arrive to the PCSS simultaneously in the UWB array system. In this work, the coupling rate of the fiber bundle is estimated. The experimental result of dividing laser energy shows that multiple lasers can be obtained by using a fiber bundle. Specifically, the experiments on the 17-PCSS antenna array radiation indicate that the pulses generated by PCSS give up only several picoseconds of time jitters, thus demonstrating that laser distribution by fiber bundle can be used for UWB coherent combination.

A fully integrated complementary metal-oxide-semiconductor 16×1 linear photodetector array with on-chip signal processing for applications in laser Doppler blood flow imaging has been designed and tested. Analog circuitry is used to provide the transimpedance amplifier, amplification of the modulated (ac) component, and antialiasing filter at each pixel, and also multiplexing of the 16 pixels. Digital signal processing is used to implement the lowpass, bandpass, and frequency-weighted filters, along with the signal averaging conventionally implemented as discrete components in laser Doppler blood flow imaging. The sensor demonstrates good agreement with simulation at each design stage. The measured optical performance of the array is demonstrated using modulated light signals and a vibrating test structure. As the first iteration of the chip, this 16×1 linear array provides an opportunity to evaluate the feasibility of full-field laser Doppler blood flow imaging with on-chip processing, with a view to providing a design that is scalable to a larger linear array or 2-D imaging array.

A complementary metal oxide semiconductor (CMOS) readout integrated circuit (ROIC) for an uncooled focal plane array (UFPA) is presented. The circuit consists of four major parts: a bolometer current direct injection (BCDI) input circuit, a column-shared capacitor feedback transimpedance amplifier (CTIA) integration circuit, a column-shared correlated double sampling (CDS) circuit, and a common output circuit. By applying the BCDI input circuit and the column-shared integration circuit of CTIA structure, high performance (high gain, a highly stable detector bias, and high photon current injection efficiency) is realized with a small pixel size. Moreover, the CDS circuit is utilized to reduce or eliminate noise of the readout circuit, and the common output circuit is utilized to improve the speed performance of the ROIC under low power dissipation. An experimental readout chip for a 45-µm-pitch 64×64 element VO₂-based UFPA has been designed and fabricated. The measurement results of the fabricated readout chip at 298 K with 5-V supply voltage have successfully verified the readout function and the performance improvement. The proposed CMOS ROIC can be applied to a micro bolometric UFPA.

A new code for a spectral-amplitude coding optical code-division multiple-access (OCDMA) system called the random diagonal (RD) code is proposed. This code is constructed using code segment and data segment. One of the important properties of this code is that the cross correlation at data segment is always zero, which means that phase intensity–induced noise (PIIN) is reduced. The performances of a RD realistic optical direct-sequence OCDMA network have been evaluated with PIIN and avalanche photodiode (APD) receivers. The comparison carried out included realistic system parameters, such as chromatic dispersion, multiple-access interference (MAI), receiver noises (dark current shot noise, and receiver thermal noise), and photodiode bandwidth. We analyze the error probability of the network taking into account these parameters. Bit-error rate (BER) performance is compared to Hadamard and modified frequency hopping (MFH) codes. It is shown that the system using this new code matrix not only suppresses PIIN, but also allows a larger number of active users compared to other codes. Simulation results show that using point-to-point transmission with three encoded channels, RD code using APD has better BER performance than PIN photodiode. Also, it is found that at 20 km the BER is 1.28×10^-20 and 6.26×10^-13 for APD and PIN photodiodes, respectively.

Spatial domain multiplexing is a new dimension in fiber optic multiplexing with the potential to greatly increase the data-carrying capacity of optical fiber communication systems. Spatially multiplexed channels follow helical paths inside the fiber and do not interfere with each other as optical energy from individual channels is distributed in the radial direction at different distances from the origin of the fiber. The resultant output appears as concentric circles when projected on a screen. The experimental setup and results for two spatially modulated analog channels of the same wavelength over approximately 600 m of standard 62.5/125-μm multimode fiber, using 635-nm pigtail laser sources and 30-MHz sinusoidal modulation, are reported here.

The performance of a free-space optical interconnect (FSOI) system based on a super-Gaussian-Bessel (SGB) beam is evaluated. The signal-to-noise ratio (SNR) is used as the performance measure. The SNR of an FSOI system utilizing SGB is analyzed, evaluated, and compared with the SNR of the system that uses a Bessel beam. We show that for large interconnect distance the achievable SNR of the former is superior to that of the latter.

High-spectral-resolution lidar (HSRL) is a powerful tool for atmospheric remote sensing, which consists of many complicated optics and optoelectronics components. In order to provide a convenient signal source for testing the amplifier following the PMT, the analog-digital converter card, the performance of the data-processing software, and the debugging display software, a novel electronic simulator is developed to generate an electronic signal resembling the atmospheric backscattering signals of HSRL. The amplitude, detection range, and aerosol characteristics of the simulated signals can be adjusted.

Spectral remote sensing has evolved considerably from the early days of airborne scanners of the 1960's and the first Landsat multispectral satellite sensors of the 1970's. Today, airborne and satellite hyperspectral sensors provide images in hundreds of contiguous narrow spectral channels at spatial resolutions down to meter scale and spanning the optical spectral range of 0.4 to 14 µm. Spectral reflectance and emissivity databases find use not only in interpreting these images but also during simulation and modeling efforts. However, nearly all existing databases have measurements of materials under pristine conditions. The work presented extends these measurements to nonpristine conditions, including materials contaminated with sand and rain water. In particular, high resolution spectral reflectance and emissivity curves are presented for several man-made surfaces (asphalt, concrete, roofing shingles, and vehicles) under varying amounts of sand and water. The relationship between reflectance and area coverage of the contaminant is reported and found to be linear or nonlinear, depending on the materials and spectral region. In addition, new measurement techniques are presented that overcome limitations of existing instrumentation and laboratory settings. These measurements enable simulation of optical images with objects in the scene attributed with realistic surface reflectance and emissivity spectra.

This paper focuses on applying an interval recursive least-squares (RLS) filter to a video target tracking problem. This is to circumvent the potential limitation of a RLS filter due to its sensitivity to variations in filter parameters and disturbances to state observations. Such sensitivity can make the solutions invalid in practical problems. In particular, in the application of video target tracking using a RLS filter, inaccurate parameters in the affine model may result in noticeable deviations from true target positions sufficient to lose a target. An interval RLS filter is proposed to produce state estimation and prediction in narrow intervals. Simulations show that an interval RLS filter is robust to state and observation noise and variations in filter parameters and state observations, and it outperforms an interval Kalman filter. Using an interval RLS filter, a video target tracking algorithm is developed to estimate the target position in each frame. The proposed tracking algorithm using an interval RLS filter is robust to noise in video sequences and errors in the affine models, and outperforms that using a RLS filter. Performance evaluations using real-world video sequences are provided to demonstrate the effectiveness of the proposed algorithm.

This paper suggests a simple scheme of block motion estimation in which the search pattern is divided into a number of sectors. By employing prediction the motion vector search is constrained to a small area. In the first step five neighboring blocks are searched for finding the predicted motion vector. The vector thus obtained is chosen as the initial search center. This predictive search center is closer to the global minimum and thus decreases the effect of the monotonic error surface assumption on the motion field estimates. Secondly, the prediction information is used to obtain the direction of the predicted motion vector. Based on that direction, the search area is divided into eight sectors. Thirdly, the magnitude of the predicted motion vector is used to define the motion content of different blocks. Thus the final search pattern dynamically uses the motion magnitude and motion direction information and significantly reduces the computational complexity. Experimental results show speed improvement of the proposed algorithm over other fast search algorithms; in addition, the image quality measured in terms of peak SNR also shows good results.

In this paper, we propose an image denoising method based on nonseparable wavelet filter banks and two-dimensional principal component analysis (2D-PCA). Conventional wavelet domain processing techniques are based on modifying the coefficients of separable wavelet transform of an image. In general, separable wavelet filters have limited capability of capturing the directional information. In contrast, nonseparable wavelet filters contain the basis elements oriented at a variety of directions and different filter banks capture the different directional features of an image. Furthermore, we identify the patterns from the noisy image by using the 2D-PCA. In comparison to the prevalent denoising algorithms, our proposed algorithm features no complex preprocessing. Furthermore, we can adjust the wavelet coefficients by a threshold according to the denoising results. We apply our proposed technique to some benchmark images with white noise. Experimental results show that our new technique achieves both good visual quality and a high peak signal-to-noise ratio for the denoised images.

A novel 3-D terrain matching algorithm is presented for a passive aircraft navigation system. Stereo matching of a pair of overlapping images can yield a recovered digital elevation model (DEM), which can be matched to the airborne reference DEM of the 3-D terrain. The two DEMs can be represented by the compact representation of contour maps, so the terrain matching is converted to contour-map matching. A contour-map matching algorithm using a combination of Fourier transform and polar transform is then proposed to estimate the associated translation and rotation parameters, which provide the desired position and orientation of the aircraft. Experimental results with real terrain data demonstrate that the proposed algorithm is insensitive to large noise and distortion compared to the previous state of the art, and also has the merits of high reliability and accuracy.

An optical extensometer was tested using artificially deformed images with a known strain field. A real image series from a tensile test was used to obtain realistic deformation parameters, including spatial and temporal strain characteristics, changes in tonal pixel properties due to deformation, and the effect of nonuniform illumination. These parameters are used to artificially deform a real image taken from an object with a random speckle pattern. The signal-to-noise ratio of the resulting artificially deformed images is varied by applying a blurring pillbox filter and additive Gaussian noise to them. The optical extensometer uses digital image correlation to track homologous points of the object, and further to measure strains. The strain measurement algorithm includes a heuristic to dynamically control the template size in image correlation. Furthermore, several other methods to improve the accuracy-complexity ratio of the algorithm exist. The effects of different parameters and heuristics on the accuracy of the algorithm as well as its robustness against blur and noise are studied. Results show that the proposed test method is practical, and the heuristics improve the accuracy and robustness of the algorithm.

Background subtraction is a method commonly used to segment objects of interest in image sequences. By comparing new frames to a background model, regions of interest can be found. To cope with highly dynamic and complex environments, a mixture of several models has been proposed in the literature. We propose a novel background subtraction technique derived from the popular mixture of Gaussian models technique (MGM). We discard the Gaussian assumptions and use models existing of an average and an upper and lower threshold. Additionally, we include a maximum difference with the previous value and present an intensity allowance to cope with gradual lighting changes and photon noise, respectively. Moreover, edge-based image segmentation is introduced to improve the results of the proposed technique. This combination of temporal and spatial information results in a robust object detection technique that deals with several difficult situations. Experimental analysis shows that our system is more robust than MGM and more recent techniques, resulting in less false positives and negatives. Finally, a comparison of processing speed shows that our system can process frames up to 50% faster.

A detailed view into the problematics of diffraction on the structure of synthetic stereogram of dot-matrix type is presented. The diffractive structure, composed of elementary diffractive gratings, is described, as well as the process of diffraction. The distribution of the diffracted light is very important because the diffracted wave in fact provides the observation of the stereogram, and is thus crucial for its spatial perception. The relations between a behavior of the diffracted wave and parameters of the stereogram structure are calculated. The presented results help to find the optimal balance between the conception of a designer and the technical possibilities for synthetic stereography.

A novel pattern recognition technique is proposed that employs shifted phase-encoded fringe-adjusted joint transform correlation (SPFJTC) for efficient real-time application and synthetic discriminant function for invariance to distortions. The proposed technique produces a single delta-like correlation for a potential target in the input scene, and performs successfully even in a noisy environment. The simple architecture of the proposed technique can be implemented easily on optoelectronics for very high-speed operation. Computer simulation results verify the efficient performance of the technique under different variations of the input scene and the environment.