This PDF file contains the editorial “Editorial” for OE Vol. 41 Issue 06

In this paper, we describe an all-fiber Sagnac interferometer with a high-birefringence fiber loop that can be used to compensate the intensity spectrum of an optical source. The compensation is experimentally achieved by controlling the birefringence in the loop of the interferometer.

A simple, accurate model for the quantitative analysis of the Shupe effect in a Sagnac interferometer has been put forward and confirmed by experimental results. Based on the model, the thermal phase-drift contribution of each layer has been evaluated and a novel approach to reduce the Shupe effect by 60% has been developed.

This PDF file contains the editorial “Focal Plane Detector Array Developments” for OE Vol. 41 Issue 06

Currently operating optical superconducting tunnel junction (STJ) detectors, developed at the European Space Agency (ESA), can simultaneously measure the wavelength (?? = 50 nm at 500 nm) and arrival time (to within ~5?s) of individual photons in the range 310 to 720 nm with an efficiency of ~70%, and with count rates of the order of 5000 photons s~1 per junction. A number of STJs placed in an array format generates 4-D data: photon arrival time, energy, and array element (X,Y). Such STJ cameras are ideally suited for, e.g., high-time- resolution spectrally resolved monitoring of variable sources or low- resolution spectroscopy of faint extragalactic objects. The reduction of STJ data involves detector efficiency correction, atmospheric extinction correction, sky background subtraction, and, unlike that of data from CCD-based systems, a more complex energy calibration, barycentric arrival time correction, energy range selection, and time binning; these steps are, in many respects, analogous to procedures followed in high- energy astrophysics. We discuss these calibration steps in detail using a representative observation of the cataclysmic variable UZ Fornacis; these data were obtained with ESA's S-Cam2 6x6-pixel device. We furthermore discuss issues related to telescope pointing and guiding, differential atmospheric refraction, and atmosphere-induced image motion and image smearing ("seeing") in the focal plane. We also present a simple and effective recipe for extracting the evolution of atmospheric seeing with time from any science exposure and discuss a number of caveats in the interpretation of STJ-based time-binned data, such as light curves and hardness ratio plots.

Superconducting tunnel junctions (STJs) have been extensively investigated as photon detectors. They combine intrinsic energy resolution with good detection efficiency and high responsivity, thus providing energy-resolved photon-counting performance in a wide energy range, from the near iR (NiR) to the x rays. On this basis, STJs offer advantages with respect to alternative photon detection systems, such as an intrinsic energy resolution, higher speed, and when compared to wavelength dispersive systems, increased detection efficiency. We overview the STJ development in the Astrophysics Division of the European Space Agency (ESA), including STJ fabrication and operation, spectroscopic performance of single STJs, small arrays, and STJ-absorber structures in the UV-NIR and x-ray bands. As a first application we describe S-Cam, a cryogenic camera for ground-based optical astronomy exploiting a 6 x6 array of Ta STJs. This camera has undergone four campaigns at the 4.2-m William Herschel Telescope at La Palma (Spain), and future generations of the camera are under active development. For the soft x-ray band (50 to 3000 eV) a STJ-based instrument with an active area of 7 x 7 mm2 is proposed as part of the payload of the X-ray Evolving Universe Spectroscopy (XEUS) mission, which is currently under study at ESA. Future developments include devices based on lower Tc superconductors for improved energy resolution as well as larger format detector arrays combined with alternative readout schemes.

We review the performance of charge-coupled detectors (CCDs) currently planned for space science instrumentation during the coming decade. With focal planes of up to a billion pixels, we discuss technical challenges in transferring this technology from ground-based to space imagers and identify areas for future development to facilitate these missions. The current capabilities of CCDs for UV imaging are also summarized and future directions for the development of CCDs suitable for wide field UV imaging are highlighted.

A measurement program designed to investigate the variations in sensitivity of focal plane arrays on a subpixel scale has produced such information for a front-illuminated CCD device, results for which have been previously presented. New measurements have been made to provide information on sensitivity variations within a single pixel for a back-illuminated CCD. The measurements were made using a stable broadband light source and two high-precision translation stages. The pixel scans were obtained using four different spectral filters (three broadband and one narrowband). These results are compared to the pixel response functions of the front-illuminated CCD. These experimentally measured pixel response functions were used to determine their effect on photometric and astrometric measurements. The uncertainty introduced into such measurements, especially when using undersampled CCD data, is significant. it was also found that the form of the subpixel response is dependent on the wavelength of illuminating light. Therefore, the position of the optical center of weight of a pixel varies with wavelength.

We compare the performance of competing CCD and CMOS imaging sensors including backside-illuminated devices. Comparisons are made through a new performance transfer curve that shows at a glance performance deficiencies for any given pixel architecture analyzed or characterized. Called lux transfer, the curve plots SNR as a function of absolute light intensity for a family of exposure times over the sensor’s dynamic range (i.e., read noise to full well). Critical performance parameters on which the curve is based are reviewed and analytically described [e.g., quantum efficiency (QE), pixel nonuniformity, full well, dark current, read noise, modulation transfer function (MTF), etc.]. Besides SNR, many by-products come from lux transfer including dynamic range, responsivity (e-/lux-s), charge capacity, linearity, and International Organization for Standards (ISO) rating. Experimental data generated by 4 ?m, three transistor (3T) pixel digital video graphics array (DVGA) and a 5.6-?m, 3T pixel digital extended graphics array (DXGA) CMOS sensors are presented that demonstrate lux transfer use.

We present an empirical dark current model for CMOS active pixel sensors (APSs). The model is based on experimental data taken of a 256 x 256 APS chip fabricated via HP in a standard 0.5-?m CMOS technology process. This quantitative model determines the pixel dark current dependence on two contributing factors: the "ideal" dark current determined by the photodiode junction, introduced here as a stable shot noise influence of the device active area, and a leakage current due to the device active area shape, i.e., the number of corners present in the photodiode and their angles. This part is introduced as a process- induced structure stress effect.

We present an automatic method for region of interest (ROI) location in camera calibration used in computer vision inspection. An intelligent ROI location algorithm based on the Radon transform is developed to automate the calibration process. The algorithm remains robust even if the anchor target has a notable rotation angle in the target plane. This method functions well although the anchor target is not carefully positioned. Several improvement methods are studied to avoid the algorithm's huge time/space consumption problem. The algorithm runs about 100 times faster if these improvement methods are applied. Using this method fully automatic camera calibration is achieved without human interactive ROI specification. Experiments show that this algorithm can help to calibrate the intrinsic parameters of the zoom lens and the camera parameters quickly and automatically.

Inclusive noise and bit error rate (BER) analysis for optical code-division multiplexing (OCDM) using coherence techniques is presented. The analysis contains crosstalk calculation of the mutual field variance for different number of users. It is shown that the crosstalk noise depends deeply on the receiver integration time, the laser coherence time, and the number of users. In addition, analytical results of the power fluctuation at the received channel due to the data modulation at the rejected channels are presented. The analysis also includes amplified spontaneous emission (ASE)-related noise effects of in-line amplifiers in a long-distance communication link.

Even though the hue saturation intensity (HSI) color model has been widely used in color image processing and analysis, the conversion formulas from the RGB color model to HSI are nonlinear and complicated in comparison with the conversion formulas of other color models. When an RGB image is degraded by random Gaussian noise, this nonlinearity leads to a nonuniform noise distribution in HSI, making accurate image analysis more difficult. We have analyzed the noise characteristics of the HSI color model and developed an adaptive spatial filtering method to reduce the magnitude of noise and the nonuniformity of noise variance in the HSI color space. With this adaptive filtering method, the filter kernel for each pixel is dynamically adjusted, depending on the values of intensity and saturation. In our experiments we have filtered the saturation and hue components and generated edge maps from color gradients. We have found that by using the adaptive filtering method, the minimum error rate in edge detection improves by approximately 15%.

A new method is developed to precisely control the key parameters for fabricating high quality diffractive optical elements with any surface-relief profiles. For the case of direct laser writing on photoresists, these parameters include the laser intensity, laser writing speed, etc. A computer control system has been programmed to compute the corresponding laser intensity incremental step and the spacing of adjacent scans, and control of the optimized process for fabricating diffractive optical elements. The compensation due to the laser variation over time has also been considered in the program. An example of a fabricated diffractive optical element and its optical performance is also demonstrated.

In-core temperature measurement is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing capabilities of fiber Bragg grating (FBG) temperature sensors. Our experiments show that sensitivity to radiation depends on both the radiation field and the grating characteristics. For some FBGs installed in an aircooled graphite-moderated nuclear reactor the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the difference saturated after 30 h of reactor operation at about 5°C. To reach megagray per hour level gamma-dose rates and 1019 neutron/cm2 flu- ences, we irradiated multiplexed FBG sensors in a material testing nuclear reactor. At room temperature, FBG temperature sensors can survive in such radiation conditions, but at 90°C a severe degradation is observed. We evidence the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care under well- specified conditions.

We analyze the performance of a fiber optic temperature sensor in terms of the standard radiometric figures of merit to optimize its design from the measured fluorescence spectrum. We apply this analysis to an erbium-doped silica fiber employing the intensity ratio technique. We develop the expressions for the SNR, the responsivity in the band, and the noise-equivalent power to predict and optimize the performance of each spectral channel, considering a number of spectral subbands. Incorporating narrow bands for each transition, we improve the SNR by a factor of 5 for each channel. We evaluate and predict the sensitivity of the sensor, incorporating the intensity ratio, to be around 0.025 K-1. The temperature resolution of this highly linear sensor is estimated at a value of about 0.1 K for the temperature interval from below room temperature to above 500°C.

The effects of compression on image utility are assessed based on manual exploitation performed by military imagery analysts (IAs). The original, uncompressed synthetic aperture radar imagery and compressed products are rated for the Radar National Imagery Interpretability Rating Scale (NIIRS), image features and sensor artifacts, and target detection and recognition. Images were compressed via standard JPEG compression, single-scale intelligent bandwidth compression (IBC), and wavelet/trellis-coded quantization (W/TCQ) at 50-to-1 and 100-to-1 ratios. We find that the utility of the compressed imagery differs only slightly from the uncompressed imagery, with the exception of the JPEG products. Otherwise, both the 50-to-1 and 100-to-1 compressed imagery appear similar in terms of image quality. Radar NIIRS indicates that even 100-to-1 compression using IBC or W/TCQ has minimal impact on imagery intelligence value. A slight loss in performance occurs for vehicle counting and identification tasks. These findings suggest that both single-scale IBC and W/TCQ compression techniques have matured to a point that they could provide value to the tactical user. Additional assessments may verify the practical limits of compression for synthetic aperture radar (SAR) data and address the transition to a field environment.

We theoretically and experimentally examine the selection of spatial frequencies of optical beams carried out by acousto-optic methods. Control of the optical rays is provided in two orthogonal directions with the help of a single acousto-optic cell fabricated of a birefringent paratellurite single crystal. The investigation is performed using a cell of a tunable acousto-optic filter illuminated by coherent radiation of a He-Ne laser at optical wavelength ? = 633 nm. The filter is capable of a 2-D selection of optical beams propagating all over the range of spatial frequencies |vx|= |Vy| = ?300 cm-1 in the spectral passband ?v = 50 to 100 cm-1. Optimal regimes of the acousto-optical interaction characterized by high efficiency of the filtration are considered during the research. New results on processing of optical images formed by divergent optical rays are presented and regular trends of the processing are discussed.

An interference microscope based on a wavelength-to-depth encoding technique is analyzed. The wavelength-to-depth encoding is achieved with a diffractive lens and a wavelength-tunable laser. The theoretical depth resolution is consistent with the experimental result. The technique offers a comparable depth resolution as traditional depth scanning. With the rapid advances in microfabrication and wavelength- tunable lasers, the system is promising for fast, noncontact, and high- resolution three-dimensional imaging.

A technique is developed for microscope autofocusing, which is called the eccentric light beam approach with high resolution, wide focusing range, and compact construction. The principle is described. The theoretical formula of the eccentric light beam approach deduced can be applied not only to an object lens whose objective plane is just at the focal plane, but also to an object lens whose objective plane is not at the focal plane. The experimental setup uses a semiconductor laser device as the light source. The laser beam that enters into the microscope is eccentric with the main light axis. A defocused signal is acquired by a symmetrical silicon photocell for the change of the reflected light position caused by differential amplification and processed by a microprocessor. Then the electric signal is power-amplified and drives a dc motor, which moves a fine working platform to an automatic focus of the microscope. The result of the experiments shows a ±0.1-?m precision of autofocusing for a range of ±500-?m defocusing. The system has high reliability and can meet the requirements of various accurate micro measurement systems.

A good knowledge of the evolution process of a crack or cavity is necessary for a better understanding the mechanical behavior of the damage and failure of materials and structural components. Before damage begins, the size of the initial or prescribed cracks and cavities are usually very small. These small cracks and cavities can be conveniently used as the diffraction apertures of the coherent light. By analyzing the diffraction fields, the characteristic parameters of these special diffraction apertures can be obtained and, hence, the deformation fields. Experimental tests are conducted to demonstrate the reliability and accuracy of the laser diffraction technique in detecting the evolution and propagation of a small crack or cavity. The relationships between the coherent diffraction patterns and the crack and cavity geometric parameters are established for the simple shape of the crack or cavity aperture. Moreover, the autocorrelative method and the spectral iterative technique are introduced to retrieve the small crack and cavity apertures for the complex shapes.

A simple and reliable all-fiber-optic component for a first-order polarization mode dispersion compensator is presented and tested in a 10-Gbit/s transmission system. The proposed component incorporates tunable differential group delay (DGD) provided by several pieces of polarization maintaining (PM) fibers of different DGD that were spliced together with short lengths of single-mode fibers between them. The total DGD of the compensator is determined by the alignment of fast and slow axes of the PM fiber sections. Applying this component as a first-order polarization mode dispersion (PMD) compensator, we demonstrate that a component with seven pieces of PM fiber segments could possibly compensate the first-order PMD up to 63.5 ps with 1-ps steps and a switching time of 300 ms. We also optimize the number of PM fiber sections in this component for a first-order PMD compensation in a 10Gbit/s system based on experimental results.

The design of tightly jacketed double-coated optical fibers to minimize long-term hydrostatic-pressure-induced microbending losses is investigated. Microbending loss in these fibers is dominated by compressive radial stress at the interface between the glass fiber and the primary coating, which is a function of the polymeric materials properties and their thicknesses. To minimize the long-term hydrostatic-pressure- induced microbending losses, one should decrease the Young's modulus, Poisson ratio, and relaxation time of the primary coating as well as the radius and Poisson ratio of the secondary coating, and increase the Young's modulus and relaxation time of the secondary coating as well as the radius, Young's modulus, Poisson ratio, and relaxation time of the jacket. Alternatively, the radius of the primary coating has its optimum value.

A viscoelastic theory is adopted to analyze the optical effects in double-coated polymeric optical fibers subjected to thermal loadings. A closed-form solution of the microbending loss and refractive index changes obtained by using the Laplace transformation method is presented. To obtain the solution, the compatibility conditions of displacement at interfaces and Lame's formula for the stress components are used. The results are initially identical to those obtained by the elastic analysis, and then decrease gradually as time progresses. To minimize the microbending loss, the polymeric coatings were suitably selected, so that the Young's moduli, Poisson's ratios, and viscosities of the primary and secondary coatings would decrease. Furthermore, selecting suitable primary and secondary coatings with lower Young's moduli, Poisson's ratios, and viscosities reduce the changes of refractive index in the glass fiber.

We present a real-time method for recognizing threedimensional (3-D) objects with scale invariance. The 3-D information of the objects is codified in deformed fringe patterns using the Fourier transform profilometry technique and is correlated using a classical convergent correlator. The scale invariance property is achieved using two different approaches: the Mellin radial harmonic decomposition and the logarithmic radial harmonic filter. Thus, the method is invariant for changes in the scale of the 3-D target within a defined interval of scale factors. Experimental results show the utility of the proposed method.

We propose an algorithm that can extract the depth location of a target from its complex hologram directly in optical remote sensing applications. The target is located in a 3-D space and its complex hologram is obtained by optical scanning holography. Once the target's depth has been extracted, we can recover the image of the target from its hologram.

We describe an optical sensor using photonic quantum ring lasers and resonance-enhanced photodetectors for the purpose of counting object discrimination. Due to the angle-dependent peak wavelength shift characteristics of the photonic quantum ring laser, an emission at an angle of 0~30 deg from a perpendicular direction was reflected and absorbed in the resonance-enhanced photodetector, with a peak responsivity of 0.107 A/W at 757 nm. The use of the photonic quantum ring laser at an optimized angle in this monolithically integrated system, based on a distributed Bragg partial reflector structure, can lead to a compact, low cost, dust-free optical sensor and counter.

The modulation of a photorefractive index grating, ?n, can be estimated by counting the number of peaks in the curve for the diffraction efficiency as a function of time under the phase matching condition. The accuracy for this method is about 13%. Here a new method is proposed, in which we improve the measurement accuracy to about 1%, by adding the measurement for the diffraction efficiency as a function of the angle mismatch. This precise ?n measurement allows us to quantify photorefractive effects with greater accuracy.

We present a new algorithm that uses the randomness of the noise pattern to achieve high positioning accuracy by applying a modified averaging operation. Using the suggested approach, noise sensitivity of the positioning accuracy can be significantly reduced. This new improved algorithm can improve the performances of tracking systems used for military as well as civil applications. The concept is demonstrated theoretically as well as by optical experiment.

We present a general spectral model for target detection in the visible spectrum and a cluttered environment. Having the model, we derive a rectangular spectral filter to be attached to the camera’s aperture plane to improve the detection abilities. Simulations and experimental verification demonstrate the abilities of the suggested approach.

A new improved unwrapping algorithm based on the combination of modulation and phase fitting reliability is presented. In a 3-D sensing technology with structured light projection, for the measurement of a surface with complex reflectivity and a quasisanded object, noises such as saturation/cut-off and quasispeckle in the fringe images are important error sources. The phase unwrapping algorithm based only on fringe modulation is not enough to bypass these error points. The characteristics of the phase measurement errors caused by saturation/cut-off are analyzed, and a new phase reliability criterion that is sensitive to both modulation and some intensity noises is defined to reduce the unwrapping errors. With this new reliability criterion and "flood" unwrapping algorithm, the phase unwrapping can be implemented along a more reliable path and errors are limited in the minimum areas.

We present a novel algorithm based on a hybrid of the global and local treatment of a wrapped map. The proposed algorithm is especially effective for the unwrapping of speckle-coded interferogram contour maps. In contrast to earlier unwrapping algorithms by region, we propose a local discontinuity-restoring criterion to serve as the preprocessor or postprocessor of our hybrid algorithm, which makes the unwrapping by region much easier and more efficient. With this hybrid algorithm, a robust, stable, and especially time effective phase unwrapping can be achieved. Additionally, the criterion and limitation of this hybrid algorithm are fully described. The robustness, stability, and speed of this hybrid algorithm are also studied. The proposed algorithm can be easily upgraded with minor modifications to solve the unwrapping problem of maps with phase inconsistency. Both numerical simulation and experimental applications demonstrate the effectiveness of the proposed algorithm.

Phase retrieval is a nonlinear technique used to recover the phase in the Fourier domain using intensity measurements at the image plane and additional constraints. We describe a method to solve the phase retrieval problem using linear iterations near the solution, which provides both analytical insight into phase retrieval and numerical results. The algorithm finds the maximum a posteriori estimate of the phase using prior information about the statistics of the noise and the phase, and it was found to converge well in practice. When phase retrieval is performed on data from subdivided apertures, there is a loss of information regarding the relative piston terms of the subapertures. This error is quantified. We find that there is a smaller wavefront error when estimating the phase from a full aperture rather than from a subdivided aperture. Using a combination of intensity measurements from full and subdivided apertures results in a small improvement at very high photon levels only.

Optical element arrays act as retroreflectors and have the pseudoconjugate property. A bead array is one type of optical element arrays that is widely used in displays, ranging targets, and traffic signs. In view of the diffraction effect, larger glass beads should be better for far field than the smaller ones usually used. We use a CO2 laser beam to produce these beads. To increase the retroreflective efficiency and answer the concern in cases of the near and far field, we propose and realize the technology of inlaying small beads into large beads. The experimental results show that our new film is superior to existing commercial retroreflective films. It displays industrial prospects in related applications.

Moire interferometry is a powerful technique for high sensitivity in-plane deformation contouring. However, from an engineering viewpoint, the derivatives of displacement, i.e., strain, are the desired parameter. Thus there is a need to differentiate the displacement field. Optical and digital methods have been proposed for this differentiation. Optical methods provide contours that still need to be quantified, while digital methods suffer from drawbacks inherent in the digital differentiation process. We describe a novel approach of strain segmentation for the moire pattern using a multichannel Gabor filter. Appropriate filter design allows for user-specific segmentation, which is essentially in engineering design and analysis.

The basic technology for a robotic system is developed to automate the packing of polycrystalline silicon nuggets into fragile fused silica crucible in Czochralski (melt pulling) semiconductor wafer production. The highly irregular shapes of the nuggets and the packing constraints make this a difficult and challenging task. It requires the delicate manipulation and packing of highly irregular polycrystalline silicon nuggets into a fragile fused silica crucible. For this application, a dual optical 3-D surface mapping system that uses active laser triangulation has been developed and successfully tested. One part of the system measures the geometry profile of a nugget being packed and the other the profile of the nuggets already in the crucible. A resolution of 1 mm with 15-KHz sampling frequency is achieved. Data from the system are used by the packing algorithm, which determines optimal nugget placement. The key contribution is to describe the design and implementation of an efficient and robust 3-D imaging system to map highly irregular shaped objects using conventional components in context of real commercial manufacturing processes.

Bilayers of a thin, nonoxidized Al film over an Ir film are prepared. The bilayers are intended to enhance the reflectance of Al below the Al plasma wavelength (?p~83 nm). Above ?p the bilayers have a high reflectance comparable to that of single Al films. Ir films were deposited by electron gun evaporation on a heated substrate. Al films of various thicknesses are deposited by evaporation from a thermal source on the Ir film once it is cooled to room temperature. The bilayers are prepared under ultra-high-vacuum (UHV) conditions and their reflectance is measured in situ. Accelerated aging processes, which are performed by annealing the bilayers in UHV, show that the bilayers are stable after 1 h annealing at temperatures up to 540±625 K. The bilayers show no observable deterioration after long-term storage under normal atmosphere. The optical constants of Ir films are also obtained. In-orbit coating of optical surfaces with Ir/Al bilayers is proposed for extreme UV astronomy.

For an accurate scene analysis using monocular color traffic image sequences, a robust segmentation of moving vehicles from the stationary background is generally required. However, the presence of moving cast shadow may lead to an inaccurate vehicle segmentation, and as a result, may lead to further erroneous scene analysis. We propose an effective method for the detection of moving cast shadow. By observing the characteristics of cast shadow in the luminance, chrominance, gradient density, and geometry domains, a combined probability map, called a shadow confidence score (SCS), is obtained. From the edge map of the input image, each edge pixel is examined to determine whether it belongs to the vehicle region based on its neighboring SCSs. The cast shadow is identified as those regions with high SCSs, which are outside the convex hull of the selected vehicle edge pixels. The proposed method is tested on 100 vehicle images taken under different lighting conditions (sunny and cloudy), viewing angles (roadside and overhead), vehicle sizes (small, medium, and large), and colors (similar to the road and not). The results indicate that an average error rate of around 14% is obtained while the lowest error rate is around 3% for large vehicles.

Laser writing using conventional lasers has found widespread application in the fabrication of electronic components and the repair of integrated circuits. However, due to large thermal damage associated with the laser ablation process, these lasers are not suitable for microelectronic components with submicron feature size. To address these issues, a novel system is introduced for laser direct writing using femtosecond lasers. Here, an acousto-optic deflector-based scanning system has been developed for steering the femtosecond laser beam with high positional accuracy, resolution, and scan speed. The capability of the system to machine complex features with submicron line widths has been proved. This system has profound potential application in the electronic and microfabrication industry.

A new design scheme of a wavelength demultiplexer device structure using waveguide mirrors to generate a series of reflected light beams with wavelength dependent phases is proposed. The light propagated in the waveguide is reflected into a planar waveguide at multiple, partially reflecting mirrors, in which positions and a geometry of the waveguide are optimized to lower the filter response difference between the output ports. Methods to reduce the sidelobe level in the filter response are demonstrated.