This PDF file contains the editorial “Retiring” for OE Vol. 43 Issue 06

We compare the penetration rate of water in sapphire and silica optical fibers at elevated temperature and pressure, which are usually the conditions in harsh environment sensing applications. The water penetration rate is studied by measuring the transmission attenuation rate at 1390 nm for both fibers. The experimental result shows sapphire fiber is much more water-resistant in such an environment compared to silica fiber, which suggests that it is a strong candidate for harsh environment fiber sensing.

A simple and robust method for expanding the dynamic range of the readout circuits for uncooled microbolometer infrared sensor arrays is presented. The proposed method allows a reduction in the number of bits required for microbolometer nonuniformity compensation, significantly reducing the complexity of the pre-integration compensation circuity.

It is known that the reflection at a dielectric/metal boundary exhibits anisotropy at nonzero incidence angles. This is of concern for high-performance reflecting telescopes where the low F# can lead to a non-negligible polarization dependent amplitude and phase apodization. After calculating the phase apodization as a function of incidence angle, we describe an effective compensation technique using form birefringence.

A simple adaptive thresholding circuit for visual attention and tracking applications is presented. When used in visual attention systems or in tracking systems, the circuit allows "background" inhibition and false alarm reduction. A test chip with an eight-element adaptive thresholding circuit has been implemented in a standard 0.5-μm CMOS process and is operated via a 3.3-V supply in the saturation region. The circuit architecture is discussed, operation in saturation and subthreshold regions are explained, and measurements from a test chip are reported.

An adaptive algorithm that implements a saturation-based ordering (for color image morphology) is introduced. The adaptation is achieved using a tradeoff parameter in the form of a nonlinear function of the local saturation. To evaluate the performance of the proposed algorithm, a deigned psychophysical experiment is used to derive a metric denoted as the average value for the psychophysical evaluation in percent (APE%). Results of implementing the proposed APE show that an APE=73 to 96% can be achieved for basic morphological operators, i.e., dilation, erosion, opening, and closing. APE value depends on the size and shape of the structuring element as well as on the image details. The proposed algorithm has also been extended to other morphological operators, such as image smoothing (noise suppression), top hat, gradient, and Laplacian operators. In the case of a smoothing operation, an average peak signal-to-noise ratio (PSNR)=31 to 37 dB is achieved at various structuring elements and applied noise variances, while good results are achieved with the proposed top-hat operators.

Basic probability assignment (BPA) definition remains a difficult problem to apply Desmpter-Shafer evidence theory to practical applications such as in image processing. A new iterative approach of multisensor data fusion based on the Dempster-Shafer framework is proposed. BPAs, modeled by a Gaussian distribution, are estimated iteratively and in an unsupervised way using the fused image and the source images. Data fusion is performed at the pixel level. Results on synthetic and real images are presented to illustrate the effectiveness of the proposed fusion scheme. Limitations of the method are discussed.

We propose two computational techniques for extracting object cross-sectional shape information from digitized in-line Fresnel holograms that do not require prior knowledge of object depths but recover relative depth information along the way. The first algorithm is applicable to hologram segments involving a single particle only. It is based on estimating Fourier transform magnitude and phase of the particle from those of the hologram segment. The second algorithm conducts a joint inverse filtering and depth search procedure so as to minimize (or maximize) a binariness (or a concentration) criterion defined over the output object function. It is applicable to multiple-particle, multiexposure holograms as well. The proposed techniques are illustrated on synthesized holograms and their practical limitations are discussed.

We describe a diode-laser-based system capable of generating >50 mW of single-frequency radiation near 425 nm. An alternative extended cavity diode laser injection seeds a semiconductor amplifier, whose output is frequency doubled by a nonlinear crystal placed in a power enhancement cavity. This system is particularly well suited for high-resolution spectroscopy in the blue region, and for laser cooling and trapping of calcium atoms.

The laser properties of Nd:Y_xGd_1–xVO₄ crystal are investigated. The continuous-wave (cw) laser performance of diode-pumped Nd:Y_xGd_1–xVO₄ crystal operating at 1.06 μm and that intracavity second-harmonic generation (SHG) with a KTP crystal into the green spectral range at 532 nm are studied. Also the pulse characteristics of Nd:Y_xGd_1–xVO₄ laser crystal are reported. A diode-pumped compact and efficient Cr⁴⁺:YAG passively Q-switched Nd:Y_xGd_1–xVO₄ laser formed by a simple flat-flat resonator is demonstrated. The dependence of the operational parameters on the pump power is also investigated experimentally.

An MPEG-based image contrast enhancement algorithm for people with low vision is presented. Contrast enhancement is achieved by modifying the inter- and intra-quantization matrices in the MPEG decoder during the decompression stage. The algorithm has low computational complexity and does not affect the MPEG compressibility of the original image. We propose an enhancement filter based on the visual characteristics of low-vision patients, and report the results of preference experiments with 24 visually impaired subjects. Subjects favored low to moderate levels of enhancement for two of the tested video sequences, but favored only low levels of enhancement and rejected higher enhancement for two other sequences that had fast motion.

A linear CCD image sensor is used to electrically scan a measuring point and measure 1-D step-profiles in real time with a sinusoidal wavelength-scanning (SWS) interferometer using double feedback control. In this interferometer, the optical path difference (OPD) and the amplitude of the SWS are controlled so that a ruler marking every wavelength and a ruler with scales smaller than a wavelength are generated. These two rulers enable us to measure an OPD longer than a wavelength in real time. Two different step profiles with step heights of 1 and 20 μm, respectively, are measured with a measurement error of less than 8 nm. Measuring time for one measuring point is 0.04 s.

An all-optical Boolean XOR gate implemented with a semiconductor-optical-amplifier (SOA)-based Mach-Zehnder interferometer (SOA-MZI) is numerically simulated at 10 Gbits/s to extract simple design rules. If the control and clock energies and the small signal gain are properly selected, the metrics that define the quality of switching can be optimized for high gate performance.

A high-speed FM demodulation method for measuring mechanical vibration using a homodyne laser interferometer is presented. The Doppler frequency, which represents the surface velocity of a vibrating object, is obtained by using the homodyne laser interferometer, and converted to a voltage signal by using the proposed analogue FM demodulation circuit. The dc offsets of the interference signals obtained from the homodyne laser interferometer are eliminated by using simple subtraction. The new method for compensation of the asymmetry of each interference signal is presented. The laser power variation of the interferometer is normalized by using an auto-gain controller (AGC). The performance of the proposed FM demodulation algorithm is proved by using theoretical methods, and validated by using experimental results. In the experiments, the proposed FM demodulation algorithm is compared with a conventional demodulation method.

A diode-laser pumped blue intracavity frequency-doubled self-Q-switched microchip laser of a chromium Cr4+ Neodymium Nd3+ codoped yttrium aluminum garnet crystal (Cr4+Nd3+:YAG) combined with a potassium niobate (KNbO3) crystal is developed. By coating the cavity mirrors with the films that suppress the 1064-nm operation and enhance the 946-nm laser, the 4F3/24I9/2 transition of the Nd3+ ion is facilitated to achieve the 946-nm laser oscillation. The 946-nm laser of the Cr4+Nd3+:YAG is self-Q-switched due to the saturable absorption of the Cr4+ ion. The full width at half maximum (FWHM) of the laser pulse at a 964-nm wavelength is about 5.07 ns. A self-Q-switched 473-nm laser pulse is sequentially obtained through intracavity frequency doubling of the 946-nm laser by the KNbO3 crystal, whose FWHM is about 4.30 ns. The 473-nm intracavity doubled laser has a good fundamental transverse TEM00 mode, because the self-Q-switched 946-nm laser has a good TEM00 mode that results from the absorption bleaching established by both the 808-nm pump laser and 946-nm oscillating laser. The constant FWHM results from both the response time of the self-Q-switching and the establishing time of the oscillating laser being much faster than the accumulated time of the pump laser energy. The constant peak power of the 946-nm self-Q-switched laser mainly depends on the modulation ability of the self-Q-switching.

Measurement of the modulation transfer function (MTF) to quantify the quality of an imaging system proves to be very important in the context of Earth observation satellites. In particular, this measurement is essential to carry out the focusing of the telescope, or to implement a deconvolution filter whose goal is to enhance the image contrast or to reduce the noise. Its knowledge also allows us to compare the characteristics of different known and unknown satellites. We suggest an univariate MTF measurement method using nonspecific views. First of all, the landscape has to be characterized to discriminate ground structure information from MTF information. Once this separation is carried out, landscape structure information can be extracted, allowing a classification between very uniform scenes and more structured ones. Then the MTF, which is described by a bidimensional analytical physical model, can be assessed using an artificial neural network. The principle is to use the artificial neural network to learn the MTF of simulated or perfectly known images, and then use it to assess the MTF of totally unknown images. We show that this method is robust even if noise is taken into account. As a result, maximum MTF assessment errors are less than 10%. This enables us to suggest further developments, including a general scheme for assessment of image quality.

Two advanced nondestructive evaluation systems are developed for imaging surface-breaking cracks in aerospace materials. The systems use scanning heterodyne interferometry and frequency-translated holography principles to image ultrasonic displacement fields on material surfaces with high resolution and sensitivity. Surface-breaking cracks are detected and characterized by visualizing near-field ultrasonic scattering processes, which in turn results in local intensification of ultrasonic displacement fields in the immediate vicinity of a crack. The local intensification permits cracks to be easily distinguished from background levels, and creates unique displacement field images that follow the contours and morphology of the cracks with microscopic precision. The interferometric and holographic imaging approaches each provide noncontact and near optical-diffraction-limited measurement capabilities that are essential for probing ultrasonic displacement fields in the immediate vicinity of cracks. Several representative crack-imaging results are provided, along with detailed descriptions of both experimental techniques, and the capabilities and limitations of each method. The resulting systems provide simple yet very powerful tools for evaluating surface-breaking cracks in detail.

We discuss the effect of apodization on an entrance pupil of projection optics in a light valve (LV) projector. During a design process of projection optics, uniform illumination on its entrance pupil is generally assumed. However, actual illuminance distribution on the entrance pupil is not uniform due to an influence of a lamp and reflector construction. The numerically calculated angular intensity distribution from a light source is converted to an illuminance distribution on the entrance pupil. By considering the illuminance distribution as an apodization function, an image of projected pixels as well as the modulation transfer function (MTF) and encircled energy function (EEF) are calculated. By the pixel image simulation, apodization is found to improve the separation between pixels and the edge blur of a pixel. Considering the apodization, the MTF at the second harmonic of the fundamental frequency increases by 1.39 times and the radius of the EEF at 50% energy decreases by 0.66 times. As the pixel density of LV increases, this simulation method becomes more useful for improving the evaluation accuracy of design and manufacturing tolerance of high-resolution projection displays with stringent optical specifications.

We report a systematic investigation on the imaging behavior of an optical system consisting of a lens made of a uniaxial birefringent crystal sandwiched between two linear polarizers with the introduction of prespecified off-axis aberrations such as astigmatism and coma. The intensity point spread function is used as the image assessment parameter. It has been shown that for a point object, the proposed system has higher tolerance to the presence of off-axis aberrations than an imaging system formed by an ordinary glass lens. Some specific cases are computed and illustrated graphically.

Wide-dynamic-range wavefront control is demonstrated with a programmable 2-D high-resolution liquid-crystal-based phase modulator system. This system operates with 307,200 independently addressable phase modulating elements, a phase modulation range that exceeds 2π, and phase resolution that is better than one-tenth of an optical wavelength at 532-nm wavelength. A demonstration of large aberration compensation via modulo-2π phase profiles shows high-fidelity wavefront compensation with less than one-quarter-wave peak-to-peak residual aberration. The results of a numerical model of diffraction efficiency via discrete phase-step approximations to modulo-2π tilt functions are presented. The model treats the effects of a variable fill factor and an interpixel influence function associated with this technology. Optimization of the diffraction efficiency includes computational compensation for nonlinearities in the system phase response. Measured diffraction efficiencies are compared to calculated values, and show operation near the theoretical limit for this system.

Line-of-sight (LOS) stabilization systems form part of modern surveillance and fire control systems (FCSs). Conventional controller designs are dependent on the accuracy of the mathematical model of the plant, which usually ignore high order dynamics. Further, plants are linearized around the operating point. Although robust controllers can be designed to overcome uncertainties in plant parameters as well as nonlinearities, the resultant controller becomes complex for implementation. Fuzzy-knowledge-based controller (FKBC) design presents a good methodology to overcome above difficulties. A fuzzy control system is implemented to control inertial rate of LOS. We present the design and implementation aspects of this particular FKBC and the performance achieved with respect to the performance achieved using a conventional controller.

The resolution of many imaging systems is often restricted by the limited spatial resolution of its sensing device rather than by diffraction limits related to the optical system. This spatial resolution of the sensor, when limited by the pixels' dimensions, is coined as "geometrical resolution." Earlier, we suggested a technique for overcoming this limit, thus obtaining "geometrical superresolution." The proposed approach is based on capturing a set of images after attaching special predesigned mask to the detector plane, interlacing the pixels of the captured images, and eventually applying inverse filtering over the interlaced image. Experimental results are presented to demonstrate the ability of the suggested technique.

The joint transform correlator (JTC) technique has been found to be suitable for real time pattern recognition applications. Among the various JTC techniques proposed in the literature, the fringe-adjusted JTC has been found to yield better correlation output for target detection. We propose a generalized fringe-adjusted JTC (GFJTC) based algorithm for efficient detection and tracking of a target in a video sequence. The proposed algorithm has been found to be suitable for near real-time recognition and tracking of a static or moving target, while accommodating the detrimental effects of background variations as well as other artifacts. The performance of the proposed technique has been verified with real life forward looking infrared (FLIR) image sequences.

A series of experiments are performed to benchmark the performance of a target identification classifier trained on synthetic forward-looking infrared (FLIR) target signatures. Results show that the classifier, when trained on synthetic target signatures and tested on measured, real-world target signatures, can perform as well as when trained on measured target signatures alone. It is also shown that when trained on a combined database of measured plus synthetic target signatures, performance exceeds that when trained on either database alone. Finally, it is shown that within a large, diverse database of signatures there exists a subset of signatures whose trained classifier performance can exceed that achieved using the whole database. These results suggest that for classification applications, synthetic FLIR data can be used when enough measured data is unavailable or cannot be obtained due to expense or unavailability of targets, sensors, or site access.

Nonlinear filtering for state estimation has been recently advanced to include more accurate and stable alternatives. The extended Kalman filter (EKF), the first and most widely used approach (applied as early as the late 1960s and developed into the early 1980s), uses potentially unstable derivative-based linearization of nonlinear process and/or measurement dynamics. The unscented Kalman filter (UKF), developed after around 1994, approximates a distribution about the mean using a set of calculated sigma points. The central difference filter (CDF), or divided difference filter (DDF), developed after around 1997, uses divided difference approximations of derivatives based on Stirling's interpolation formula and results in a similar mean, but a different covariance from the EKF and using techniques based on similar principles to those of the UKF. We compare the performance of these three approaches to the coordinate conversion problem within the larger problem of ballistic missile tracking under various sensor configurations, target dynamics, measurement update/sensor communication rates, and measurement noise. The coordinate conversion problem here specifically deals with conversion of spherical measurements to Cartesian estimates and vice versa. The importance of filter stability in some cases is emphasized as the EKF shows possible divergence due to linearization errors and overconfident state covariance, while the UKF shows possibly slow convergence due to overly large state covariance approximations. The CDF demonstrates relatively consistent stability, despite its similarities to the UKF. The requirement that the CDF and UKF expected state covariances be positive definite is demonstrated to be unrealistic in a case involving multisensor fusion, indicating the necessity for the reportedly more robust and efficient square-root implementation. Strategies for taking advantage of the strengths (and avoiding the weaknesses) of each filter are proposed.

A new optical monitoring method for thin-film coating is described. Based on the admittance loci theory of Macleod, the traditional turning point method, and level monitoring method, the new method is able to compensate the coating error and break the transfer of error to the subsequent layers. The method can be applied to monitor deposition of quarter-wave optical thickness films or nonquarter-wave optical thickness films. Theoretical analysis shows the method can effectively improve the monitoring accuracy.

To improve variable optical attenuator (VOA) performance, the object of this study is to know the most appropriate structure of a polymer-dispersed liquid crystal (PDLC) composite. In fact, using its electro-optical properties, PDLC can be inserted between two optical fibers for adjusting the coupling efficiency. We utilize here the photopolymerization-induced phase separation (PIPS) method as a fabrication process. Furthermore, we present a theoretical analysis of light scattering by anisotropic spheres based on the anomalous diffraction approach (ADA) and compare this model with some experimental results. The optical attenuation (OA) range is optimized in the case where LC, the droplets nematic directors, are randomly oriented. If the nematic directors have a quasiprivileged direction, we demonstrate that PDLC transmission depends on the incident light polarization. Polarization-dependent loss (PDL) and OA behaviors are then studied at the 1.55-μm wavelength according to several parameters such as PDLC thickness, LC droplet size, and the device driving voltage.

An analysis of erbium-doped titanium indiffused LiNbO₃ channel waveguides under 1484-nm excitation is presented in terms of a complex refractive index profile under 1484-nm excitation (pump) to obtain modal gain and loss for signal and pump powers, respectively. Closed form analytical expressions for modal fields are used to define the complex index profile. The calculated gain characteristics are compared with available experimental results.

This PDF file contains the errata for “OE Vol. 43 Issue 06 Paper 1751133” for OE Vol. 43 Issue 06