This PDF file contains the editorial “It has been a marvelous experience!” for OE Vol. 39 Issue 12

The coherent generation of synchrotron radiation by an electron storage ring is predicted for wavelengths equal to or longer than the electron bunch length. With typical bunch lengths of approximately 1 cm, diffraction and chamber-screening effects have so far blocked observation of coherent radiation from a conventional radiation beamline. In the low-energy, second-generation light source MAX-I, the magnet lattice has been tuned to a small momentum compaction factor, allowing rms bunch lengths as short as 1 mm. Here we report the coherent far-infrared emission observed from such a bunch. The paper discusses the origin of coherent synchrotron radiation for Gaussian and non-Gaussian electron bunches, and the procedure used to generate such bunches. The emission was characterized using the infrared beamline at MAX-I, including an interferometer, a liquid-helium-cooled bolometer detector, waveguide high-pass filters, and a conductive-grid polarization filter. The intensity of the coherent radiation is greater by a factor of 2x103 to 6x103 than normal incoherent synchrotron radiation, and is seen between 8 and 22 cm-1.

Iterating (composing) a sequence of window operators results in an operator defined over the window determined by the dilation of the component windows. Although a statistically designed iterative operator uses potentially all of the variables in the large dilated window, the design of each component operator requires design only over a much smaller component window, thereby resulting in a reduced estimation error. This means that designed iterative operators can perform better than designed estimates of fully optimal operators over the same effective window. While the optimal iterative and fully optimal operators may differ substantially in their logical structure, they may be probabilistically very close as operators on the random image processes under consideration. Thus, a precisely designed iterative operator can be closer to optimal than a less precisely designed unconstrained operator. We present three measures by which to compare iterative operators, with main interest focusing on the difference in their mean-absolute errors (MAEs), and discuss iterative design procedures and relationships between MAEs occurring from various procedures. A key aspect of design is the dependency on sample size. Increasing the number of iterations may in theory produce a better filter but, like using large windows, increasing the number of iterations increases the amount of data required for precise design. We pay particular attention to this issue. Using both restoration and recognition operators, we consider the best number of iterations and window size. We also consider the manner in which the training data should be split when designing the individual component operators. Iteration number, window size, and training method are all dependent on the filtering task, image characteristics, and amount of training data available.

Most edge-preserving image restoration algorithms preserve discontinuities that are larger than a prescribed threshold value, therefore noise components whose differences in neighboring pixels are larger than the threshold become amplified unintentionally. We propose a noise-adaptive edge-preserving image restoration algorithm based on a Markov random field image model. The proposed potential function is controlled by the weighting function to adaptively incorporate the discontinuities into the solution. To avoid undesirable amplification of the noise, we introduce a noise-adaptive threshold to each pixel difference. As a result, the potential function varies its shape from a quadratic form to a concave form according to the amount of noise added to each pixel. In doing so, high-frequency components caused by strong noise are relatively more smoothed as with the quadratic potential function used, while edge components that have a small noise intensity are well preserved. The smoothing functional to be minimized is formulated to have a global minimizer in spite of its nonlinearity by enforcing the convergence and convexity requirements. The effectiveness of the proposed algorithm is demonstrated experimentally.

Due to significantly improved spatial and spectral resolution, hyperspectral sensors can now detect many substances that cannot be resolved by multispectral sensors. However, this comes at the price that many unknown and unidentified signal sources, referred to as interferers, may also be extracted unexpectedly. Such interferers generally produce additional noise effects on target detection and must therefore be taken into account. The problem associated with this interference is challenging because its nature is generally unknown and it cannot be identified from an image scene. This paper presents an approach, called the target-constrained interference-minimized filter (TCIMF), which does not require one to identify interferers, but can minimize the effects caused by interference. It designs a finite-impulse-response filter that specifies targets of interest in such a way that the desired targets and undesired targets will be passed through and rejected by the filter, respectively; the filter output energy resulting from unknown signal sources is also minimized. More precisely, the TCIMF accomplishes three tasks simultaneously: detection of the desired targets, elimination of the undesired targets, and minimization of interfering effects. A recently developed technique, constrained energy minimization (CEM), can be considered as a suboptimal version of the TCIMF. Computer simulations and hyperspectral image experiments are conducted to demonstrate advantages of the TCIMF over the CEM.

We propose a block-based unsupervised image segmentation algorithm in which the basic unit for the region labeling is an image block rather than a pixel. Adopting the image block, we can introduce a new paradigm for the unsupervised image segmentation, namely, an edge-and-region based segmentation. That is, in our block-based segmentation, we exploit both edge blocks in the region boundary and the homogeneous blocks inside the region. The interior texture and monotone blocks are used to identify regions and the edge blocks are used to find an accurate contour. To obtain a pixel level segmentation, we divide the edge and unlabeled blocks into quadrant and relabel those subblocks to one of the neighboring homogeneous regions. We repeat this process until we obtain a pixel-based segmentation. Experimental results show that the proposed segmentation yields accurate segmentation for natural images even if they contain some texture regions as well as monotone regions.

This paper presents the application of two face recognition techniques to a manufacturing recognition problem, the detection of the presence of surface-mounted devices on printed circuit boards. Without assuming that the region of interest (ROI) is well framed, a preprocessing stage is developed to find and extract this subimage from a larger image using a series of image-processing steps. The eigenfaces and Fisherfaces feature extraction methods are then applied to the ROIs in order to project them onto lower-dimensional, optimal feature subspaces in which the classification is done. An important part of the work presented is the evaluation and selection of most discriminating eigenimage feature projections to use in classification. Experimental results on a testing database of 37,000 images yields 87% to 94% correctclassification rates for both feature extraction strategies.

We present a supervised method, developed for industrial inspections by artificial vision, to obtain an adapted combination of descriptive features and a lighting device. This method must be implemented under real-time constraints and therefore a minimal number of features must be selected. The method is based on the assessment of the discrimination power of many descriptive features. The objective is to select the combination of descriptive features and lighting system best able to discriminate flawed classes from defect-free classes. In the first step, probability densities are computed for flawed and defect-free classes and for each tested combination. The discrimination power of the features can be measured using the computed probability of error. In the second step, we obtain a combination that gives a low probability of errors. This leads us to choose each feature individually and then build a multidimensional decision space. A concrete application of this method is presented on an industrial problem of flaw detection by artificial vision. The results are compared with those given by classical multiple discriminant analysis (MDA) to justify the use of our method.

The problem of automated defect detection in textured materials is investigated. A new algorithm based on multichannel filtering is presented. The texture features are extracted by filtering the acquired image using a filter bank consisting of a number of real Gabor functions, with multiple narrow spatial frequency and orientation channels. For each image, we propose the use of image fusion to multiplex the information from sixteen different channels obtained in four orientations. Adaptive degrees of thresholding and the associated effect on sensitivity to material impurities are discussed. This algorithm realizes large computational savings over the previous approaches and enables highquality real-time defect detection. The performance of this algorithm has been tested thoroughly on real fabric defects, and experimental results have confirmed the usefulness of the approach.

SPICE models of optical interconnect systems that include optical devices as well as electrical circuits are implemented and their performance is analyzed using SPICE simulation. The relationships between such system parameters as bit rate and transmitter power dissipation are investigated.

A new fringe compensation method for holographic interferometry is presented. Virtual displacement fields are used to compensate the displacement of the investigated object. The virtual displacement fields are built from phase-shifted reconstructions of the recorded interferogram. The sensitivity of the measurement can be changed after the recording of the interferogram. The fringe density at highly deformed points of the object can be decreased; the defect detection can be simplified.

We present a new algorithm for processing the particular fringe patterns known as Newton’s rings. The first step of the algorithm consists of finding the common center of the circular fringes using a blockwise Fourier transform of the interferogram. The directions perpendicular to the fringes in several zones are thus detected and combined to give the circles’ center. As a second step, a specially adapted type of pixel intensity spectrum is calculated, in which the 2-D pattern is reduced to a 1-D profile showing a periodic structure of fringes. By further processing, the parameters of the initial interferogram can be easily extracted. The statistical nature of the algorithm (achieved by taking into account all pixel intensities) leads to a high accuracy and a good immunity to noise.

We deposit composite MgF2-ZnS and MgF2-Ta2O5 thin films by coevaporation, and explore their optical, mechanical and thermal properties. The refractive indices of pure MgF2, Ta2O5 and ZnS films are 1.37 to 1.38, 1.94 and 2.38 at 550 nm wavelength, respectively. In the MgF2-ZnS system, the measured refractive index is in good agreement with the Drude formula and the stress varied from tensile (3.5 x109 dyne/cm2) for pure MgF2 films to compressive (2.7 x109 dyne/cm2) for pure ZnS films. The transition from tensile stress to compressive stress occurs at a composition corresponding to 43% MgF2 by mole fraction. In the intermediate compositions of 40 to 75% MgF2, the system exhibits very low stresses. In the MgF2-Ta2O5 system, the measured refractive index agrees well with the Lorentz-Lorenz formula and all films have tensile stresses, in the range 0.7 to 3.1 x109 dyne/cm2, depending on the film composition. The film stress does not vary linearly with the film composition. We determine the thermal diffusivity of the films using the photoacoustic effect. The thermal diffusivities of the films increase with the mole fraction of MgF2. They are found to be an order of magnitude of 10-25 cm2/s.

Digital holography for micromeasurement is an active research topic. With respect to the requirement of realizing characterization of micro-scale structures in microelectromechanical systems with high resolution and accuracy, in-line configuration is studied in this paper as the fundamental structure of a digital holography system. A mathematical model based on Fourier optics is developed to analyze the digital recording mechanism and properties of the system in comparison with those of the commonly used off-axis arrangement. Theoretical analysis and experimental results demonstrate that in-line configuration is advantageous in enhancing the system performance. Besides the relaxed requirement of spatial resolution on the CCD sensors and the greater flexibility of the system, higher lateral resolution and lower speckle noise can be achieved.

We present simulations of coherent lidar wind measurements from space and from ground using telescope arrays. The theory as well as simulation results are presented

An alternative image multiplexing technique, employing a multiple-aperture pupil for each exposure, is presented. The aperture arrangement of the pupil is modified between exposures. A thick photorefractive BSO crystal is used as recording material. The technique allows concentrating the spectral components of a pair of images selectively into isolated spots in the Fourier plane. Two applications are proposed, in image processing and optical metrology.

Implementation of closed-loop microelectromechanical system (MEMS) control enables mechanical microsystems to adapt to the demands of the environment that they are actuating, opening a broad range of new opportunities for future MEMS applications. Integrated optical microsystems have the potential to enable continuous in situ optical interrogation of MEMS microstructure position fully decoupled from the means of mechanical actuation that is necessary for realization of feedback control. We present the results of initial research evaluating through-wafer optical microprobes for surface micromachined MEMS integrated optical position monitoring. Results from the through-wafer freespace optical probe of a lateral comb resonator fabricated using the multiuser MEMS process service (MUMPS) indicate significant positional information content with an achievable return probe signal dynamic range of up to 80% arising from film transmission contrast. Static and dynamic deflection analysis and experimental results indicate a throughwafer probe positional signal sensitivity of 40 mV/?m for the present setup or 10% signal change per micrometer. A simulation of the application of nonlinear sliding control is presented illustrating position control of the lateral comb resonator structure given the availability of positional state information.

An instrument using laser light scattering at non-normal incidence for measuring surface roughness of machined surfaces in a production environment is described. The Lasercheck instrument is noncontact, has a fast response time (10 samples/s), automatically corrects for sample vibration, and is optimized to measure surface roughness in the 0.2 to 20-min. (0.005- to 0.5-?m) average roughness range; the range can be extended to .40 ?in. (1 ?m) with careful surface alignment. Surface inspection can be close to 100%, and the repeatability is ±0.1%. Although no formal chain of traceability to the National Institute of Standards and Technology has yet been developed for optical roughness-measuring instruments, within its specified operating range the informal agreement of the Lasercheck with NIST traceable instruments and standards is equivalent to that of any commercial profilometer. Examples of the use of the instrument for measuring ground steel surfaces and polyurethane laser-printer rollers are described. The Lasercheck makes possible considerable improvement in surface-finish uniformity and tolerances, less wasted material, and more cost-effective use of production machinery.

We propose a wavelength-converting optical-space switch in a Mach-Zehnder interferometer based on the semiconductor optical amplifier (SOA), and its pulse characteristics in the time domain are investigated theoretically. The cross phase modulation and the gating function in the SOA are utilized for a wavelength converter and optical space switch. The output pulse shape of the switch is varied for different injection current patterns, current levels, and probe power levels. To reduce the switching time, exponential current injection into the SOA is preferred to the cw current. The influences of the probe power and bias current of the wavelength converter on the switching characteristic are investigated, including the dynamics of the wavelength converter.

A model of a new kind of luxmeter photometric head is designed. The luxmeter photometric head fitted with a new kind of element for spatial (angular) correction has a photometric efficiency 4.3 times higher than that of the photometric head used in current luxmeters. Thus high photometric efficiency of the photometric head enables measurements of very low illuminance. In the diffuser currently used for angular correction of the photometric head, the flux being measured is reflected and diffused several times. This type of luxmeter measuring head has a low photometric efficiency. Moreover, in this case, the light signal penetrating through the diffuser creates an electrical signal in the detector on a level comparable to its noise. This signal cannot be measured even using additional low-noise amplifiers.

In mercury-cadmium-telluride (HgCdTe) photoconductive detectors, the recorded signal is usually a nonlinear function of the incident photon flux. This nonlinear behavior must be taken into account when used in precision hardware applications such as IR radiometers. We investigate the effects of nonlinearity contributed by the illuminated and nonilluminated detector areas, which to our knowledge was not investigated earlier. The effects of wire resistance and detector preamplifier loading as well as the impact of off-centered detector illumination compared to the centered detector illumination on the nonlinearity are also investigated. Finally, it is shown that the nonlinearity can be estimated using a third-order polynomial. Computer simulation results that estimate the effect of nonlinearity are presented for a constant-voltage-biased detector circuit that minimizes the nonlinearity as compared to alternate detector bias and readout methods.

A scintillation model is developed for uplink-downlink optical communication channels applicable in moderate to strong fluctuation conditions that may arise under large zenith angles between transmitter and receiver. The model developed here is an extension of a recently published theory that treats irradiance fluctuations along a horizontal path as a modulation of small-scale scintillation by large-scale scintillation. For a downlink path the scintillation index is modeled like that of an infinite plane wave, and for an uplink path we consider a spherical wave model. In both cases the scintillation index agrees with conventional weak-fluctuation-theory results out to zenith angles of 45 to 60 deg. The covariance function of irradiance fluctuations is also developed under the same conditions as assumed for the scintillation index. On a downlink path under small zenith angles the implied correlation length is proportional to the Fresnel-zone scale. For zenith angles exceeding 85 deg, the downlink correlation length varies directly with the spatial coherence radius weighted by a factor that depends on changes in C n 2 the refractive index structural parameter with altitude.

A fiber optic voltage sensor is developed using the photoelastic effect in a single-mode optical fiber, excited with a 632.8-nm HeNe laser. The force due to the mechanical deformation of piezoelectric material induced by voltage produces the photoelastic effect in the optical fiber. It is found that the sensor output voltage is proportional to the applied voltage up to 300 V. The frequency of the output voltage is equal to that of the applied voltage. The use of a commercial single-mode fiber rather than a low-birefringence fiber gives an economic advantage. In addition, the sensor’s structure is designed to be installed easily on a coaxial cable and to reduce the temperature effect. Experimental results from a laboratory model show a temperature coefficient of 0.076%/K.

Results obtained from a new, 600-Hz, 624-subaperture, twodimensional optical wavefront sensor for fluid-optic interaction applications are presented. This instrument can operate at 1 kHz as currently configured and is extendable to 8 kHz with a minor modification. The new sensor uses the small-aperture beam technique for measurements in the streamwise direction, and scanned beams for cross-stream wavefront measurements. Wavefront aberrations produced by a low-speed, heated mixing layer are presented, and the accompanying measurement uncertainty is evaluated.

An optoelectronic arbitrary-waveform generator has been proposed and experimentally developed for radar applications. It permits generating predefined waveforms by driving the amplitudes and phase shifts of optically carried microwave signals. The optical architecture combines a high-resolution frequency shifter (for the heterodyne generation of the microwave signal) with spatial light modulators (for the parallel control of amplitudes and phases). Such a waveform generator can be attractive for target recognition or for target simulation.

We present a model to more accurately describe the optical properties of a twisted nematic liquid crystal display (LCD). In particular, we study the optical properties of molecules near either edge of the LCD that are unable to twist and tilt under the application of an external electric field. The properties of these layers can be deduced from the intensity transmittances measured using different configurations of the external linear polarizers. The agreement between the theoretical and the experimental transmittances is excellent, thus enabling the prediction of the actual modulation for any configuration of the polarizers.

A digital watermark is a perceptually unobtrusive signal embedded in a multimedia asset, such as an image, for copyright protection. In many cases watermark detection amounts to thresholding a correlation value between a watermark and a received image. Watermarking detection schemes can be classified into two types. Type 1 is based on a correlation process that is applied to the difference between an original image and an input image to be tested. Type 2 is based on a correlation process that is directly applied to an input image. Type 1 fails to prove the rightful ownership, while type 2 can do so. However, type 2 has a problem that type 1 does not have: The correlation between a watermark and an original image to be watermarked is too significant to be ignored, when it is normalized by the watermark’s energy. In this paper, based on an analysis of the correlation, we propose a novel watermarking scheme to minimize that effect. We verify the performance of the proposed scheme by experiments.

In the analysis that follows, the mechanical response of an optical glass fiber specimen subjected to a significant change in temperature is modeled. The objective of the analysis is to find out whether small lateral coefficient of thermal expansion (CTE) and Young’s modulus gradients (along one of the diameters of the fiber cross section) can contribute appreciably to the observed residual bow (‘‘curling’’) of the fiber. Although the communication does not contain experimental data, we show that these gradients have enough potential to be one of the contributing factors that could possibly lead to curling. As is known, large ‘‘curls’’ can make automatic splicing of fibers difficult.

This PDF contains the communications section including 'Comments on "Robust control of the multiple mirror telescope adaptive secondary mirror"'.