This PDF file contains the editorial “Problem Solving” for OE Vol. 41 Issue 07

A new type of gain-flattening filter using microbending long-period fiber gratings is presented. The fabrication and characterization of the device are described and its use in an erbium-doped fiber amplifier to flatten the amplified spontaneous emission (ASE) spectrum is demonstrated. The result shows that the ASE variation is less than 1 dB over 33 nm at two different pump powers of 45 and 68 mW.

Flexible ordered bundles of infrared transmitting silver-halide fibers were fabricated by multiple extrusions from single crystals. We have developed flexible bundles of outer diameter 1 mm and lengths up to 2 m, which included 30 to 100 individual elements and the diameters of each element were between 40 to 70 ?m. The transmission losses of individual elements in the bundles were of the order of a few decibels per centimeter. The modulation transfer function of the bundles was determined and the resolution was found to be a few lines per millimeter. The crosstalk was of the order of 30%. Such thin, long, and flexible bundles would be extremely useful for fiber-optic thermal imaging in science, medicine, and industry.

This paper presents a method to achieve an amplitude-modulated optical wave in the range of telecommunication frequencies for optical components bandwidth tests. The principle is based on the optical heterodyning of two free distributed feedback lasers. The frequency tunabil- ity of the modulated wave reaches 275 GHz with high stability. These characteristics constitute the originality of our work. Our laboratory has developed an experimental fusion setup that enables the realization of numerous optical fiber components. Component bandwidth measurement enables their validation for high-rate data transmissions. We present in this paper results obtained for a 57-GHz beat signal in a fast photodiode and we describe bandwidth optical components tests in the 0 to 40 GHz range.

An idea of an equivalent refractive index for the ray tracing calculation of hollow core waveguides is proposed. A virtual, complex refractive index is uniquely found by minimizing the difference between reflectivity of a virtual monolayer material and a metal substrate coated with a dielectric film. By using this technique, transmission losses of a delivery system consisting of a hollow fiber and a tapered hollow waveguide are calculated.

A well-known fabrication problem with aspheric optical surfaces lies in high-frequency surface irregularities inherent in the figuring process. Optical grinding and polishing tools can smooth these ripples, yet retain the flexibility required to fit the aspheric surface. An f/0.52, paraboloidal, 17-in. convex surface is produced with conventional rigid tools. A transmission ronchigram is obtained showing high- spatial-frequency errors of large magnitude. After four hours of grinding with a semi-flexible multiple-segment ring tool, almost all high-frequency error is removed. This shows good potential for smoothing finished aspheric optics. Flexible tools can also be involved in the figuring process itself.

Mass customization is a new manufacturing trend in which mass-market products (e.g., apparel) are quickly modified one at a time based on customers' needs. It is an effective competing strategy for maximizing customers' satisfaction and minimizing inventory costs. An automatic body measurement system is essential for apparel mass customization. This paper introduces the development of a body scanning system, body size extraction methods, and body modeling algorithms. The scanning system utilizes the multiline triangulation technique to rapidly acquire surface data on a body, and provides accurate body measurements, many of which are not available with conventional methods. Cubic B-spline curves are used to connect and smooth body curves. From the scanned data, a body form can be constructed using linear Coons surfaces. The body form can be used as a digital model of the body for 3-D garment design and for virtual try-on of a designed garment. This scanning system and its application software enable apparel manufacturers to provide custom design services to consumers seeking personal-fit garments.

A Gabor filtering method for texture-based classification of color images is presented. The algorithm is robust and can be used with different color representations. It involves a filter selection process based on texture smoothness. Unichannel and interchannel correlation features are computed. Two types of color representations have been considered: computing chromaticity values from xyY, HIS, and RGB spaces; and using the three channels of the perceptually uniform color spaces L*a*b* and HSV. The RGB space universally used in image processing can be used for color-texture-based classification by computing the rgb chromaticity values, which yield higher classification accuracies than the direct use of R, G, and B values. The wavelet transform methods have been extended to perform color image classifications with additional features. The two approaches, Gabor filtering and wavelet transform methods, are compared in terms of classification accuracy and efficiency. The pyramid wavelet transform (PWT) performs well with all color spaces. The tree-structured wavelet transform (TWT) is more suitable for smaller classification problems. The best color spaces for higher class problems with wavelet methods are L*a*b* and HSV spaces. The HSV space is found to be the best for application of both of these texture-based approaches. Computationally the Gabor method followed by PWT is fast and efficient.

This work is concerned with the applications of Bragg-grating fiber-optic filters. First, on the basis of recently pointed out discrepancies between different formulations of the coupled-mode theory, we revisit the design process of devices intended for amplified spontaneous emission noise filtering in DWDM systems. Next, the use of these same basic filter structures in temperature sensing is discussed. The proposed device employs a dual-wavelength measurement scheme of the optical power being transmitted by the Bragg filter, thereby tracking temperature variations while simultaneously canceling spurious readings due to random optical power fluctuations.

A Brillouin-scattering-based distributed sensing system has been used for insitu strain measurement during the curing of AS4-3501 composite panels. It has a pulse length of 1.5 ns, which is equivalent to spatial resolution of 15 cm and read-out resolution of 5 cm. Distributed Brillouin scattering sensors have been used to monitor either temperature or strain for concrete structures, dams, and fiber cables by NTT Transmission System Labs (Japan) and Ecole Polytechnique Federale de Lausanne (EPFL) groups with spatial resolutions >1 m. We report the first application for a Brillouin scattering sensor in a composite curing process. The challenges for this application are the requirements of centimeter spatial resolution due to the small size of the composite material; simultaneous temperature and strain measurement during the fast temperature changing process of the curing, and short signal processing times. We present experimental results on strain measurements from optical fiber embedded in the eight-ply panels during the heat-up, isotherm, and cool-down stages of the cure process. The materials studied are 177°C cure thermo-set materials used extensively in the manufacture of composite parts for the aerospace industry. The distributed sensor can detect the reaction advancement by measuring the cure shrinkage at the gelation and vitrification stages. Shrinkage is then correlated with the degree of cure data from a differential scanning calorimeter (DSC). The thermal response of the solidified composite during cooling is profiled. Details of the data processing of the Brillouin-scattering- based distributed sensor to obtain the strain variations as a function of cure temperature, time, and location are explained.

Four-band fiber optic radiometry is a method for noncontact temperature measurement of a sample with an unknown emissivity, assuming the emissivity is uniform in all four spectral bands. The method corrects the temperature calculation automatically, according to the emissivity calculation, during continuous measurements on the sample. This method may be particularly useful in determining the temperature of a material that undergoes a transition in which both temperature and emissivity change, such as phase transitions or exothermic processes. We report a radiometric four-band measurement in the mid-infrared region, for determining the true temperature of an epoxy resin during its curing.

it is economically important for manufacturers of high- temperature machines to be able to measure creep so they can predict residual service life more accurately. This paper describes and refines an image analysis method for evaluating creep in laboratory test pieces. it is a preliminary study of how to extract relevant information for creep measurement by counting cavities. Sample preparation for quantification by image analysis is an important step determining the further development of the image analysis technique. Grain-boundary extraction, which involves directional information, is the major problem to be solved before measurement can be automated. The search for a crest-line extraction filter by the Canny method has led to the development of a directional wavelet transform filter. The results of this innovative filtering method are applied here.

An algorithm is described for simultaneously compressing and denoising images. The algorithm is called tree-adapted wavelet shrinkage and compression (TAWS-Comp). TAWS-Comp is a synthesis of an image compression algorithm, adaptively scanned wavelet difference reduction (ASWDR), and a denoising algorithm, tree-adapted wavelet shrinkage (TAWS). As a compression procedure, TAWS-Comp inherits all of the advantages of ASWDR: its ability to achieve a precise bit rate assigned before compressing, its scalability of decompression, and its capability for enhancing regions of interest. Such a full range of features has not been available with previous compressor-plus-denoiser algorithms. As a denoising procedure, TAWS-Comp is nearly as effective as TAWS alone. TAWS has been shown to have good performance, comparable to state of the art denoisers. In many cases, TAWS-Comp matches the performance of TAWS while simultaneously performing compression. TAWS-Comp is compared with other combined compressor/denoisers, in terms of error reduction and compressed bit rates. Simultaneous compression and denoising is needed when images are acquired from a noisy source and storage or transmission capacity is severely limited (as in some video coding applications). An application is described where the features of TAWS-Comp as both compressor and denoiser are exploited.

A new method of nonlinear interpolative vector quantization for wavelet-transform image coding is proposed. Training and quantization are carried out by classifying the coefficient vectors in the wavelet- transform domain according to their directional activity energy. A product codebook is used during encoding and decoding to allocate bit assignments among these classes adaptively. Compared to the conventional interpolative vector quantization technique, the new method gives superior compression performance and also reduces the complexity of codebook training and quantization. The PSNR improvement is about 1 dB at the compression ratio of 50.

A new hardware field programmable gate array (FPGA) structure suitable for the realization of soft color image morphological operations is presented for the first time. Data is processed in a pipelined array that performs fast soft vector erosion and dilation. The proposed hardware structure can process two-dimensional 24-bit resolution color images up to 4096 pixels wide, but it can be easily modified to accommodate larger images. MAX+ PLUS II software of the Altera Corporation is employed for design and simulation. The FPGA used is the EPF10K130VG599-3 Altera device. The typical clock frequency of operation is 40 MHz.

MR images and remotely sensed images share similar image structures and characteristics because they are acquired remotely as image sequences by spectral channels at different wavelengths. As a result, techniques developed for one may be also applicable to the other. In the past, we have witnessed that some techniques that were developed for magnetic resonance imaging (MRI) found great success in remote sensing image applications. Unfortunately, the opposite direction is yet to be investigated. In this paper, we present an application of one successful remote sensing image classification technique, called orthogonal subspace projection (OSP), to magnetic resonance image classification. Unlike classical image classification techniques, which are designed on a pure pixel basis, OSP is a mixed pixel classification technique that models an image pixel as a linear mixture of different material substances assumed to be present in the image data, then estimates the abundance fraction of each individual material substance within a pixel for classification. Technically, such mixed pixel classification is performed by estimating the abundance fractions of material substances resident in a pixel, rather than assigning a class label to it as usually done in pure-pixel-based classification techniques such as a minimum-distance or nearest-neighbor rule. The advantage of mixed pixel classification has been demonstrated in many applications in remote sensing image processing. The MRI experiments reported in this paper further show that mixed pixel classification may have advantages over the pure pixel classification.

We present an experimental identification of the local and mean Nusselt number from a rotating TGV brake disk model in the actual environment and exposed to an air flow parallel to the disk surface. This method is based on the use of a heated thermally thick disk combined with the technique of temperature measurement by infrared thermography. The local and mean convective heat transfer coefficient from the disk surface is identified by solving the steady state heat equation by a finite difference method using the experimental temperature distribution as boundary conditions. The experimental setup is constituted of a model disk with all the representative parts of the actual TGV brake system. The disk and its actual environment are inside a wind tunnel test section, so that the rotational disk speed and the air flow velocity can be varied. Tests were carried out for rotational speeds w between 325 and 2000 rpm (rotational Reynolds number Re between 88,500 and 545,000), and for an air flow velocity U ranging between 0 and 12 m.•s-1 (air flow Reynolds number Re0 between 0 and 153,000).

We introduce an algorithm for wavefront fitting of interfero- grams with Zernike polynomials. The algorithm is based on a singular value decomposition (SVD). SVD is superior to other reconstruction algorithms to some extent. We also introduce the optimum Zernike mode number. With the optimum number of modes, wavefronts can be expressed more exactly. The F-test has been widely used in many fields, and we introduce it to optical wavefront analyzing. We also do computer simulations of this algorithm.

Interferometric shape measurements of technical objects are hampered by stochastic phase variations in the micro region caused by the surface roughness of the test object. Without special precautions, low-contrast interference patterns or even totally stochastic fringe patterns may be observed. However, illuminating the surface under test at grazing incidence reduces the phase variations because it increases the effective wavelength by a factor of 1/cos ?, where (? is the angle of incidence. Here, an interferometer for testing plane objects is described where the test wavefront is provided by means of diffractive optical elements (DOEs). A pair of identical DOEs is used to do the necessary beam splitting and beam shaping. The resulting fringes, showing the surface deviations of the test object from the ideal plane, are evaluated by means of phase-shifting interferometry. Serial measurements on different rough surfaces were made to find out the limits of grazing- incidence interferometry when technical objects are measured. To do this, different incidence angles have been realized by choosing a suitable set of diffractive elements. The contrast of the interference fringes has been taken as the evaluation criterion for the measurability of the shape of the rough surfaces within the grazing incidence interferometer.

We present four receiver designs for a ladar system, based on an optical parametric amplifier, that is designed to collect returns from glint targets. After coupling the return energy into periodically poled lithium niobate, the target backscatter is detected with either an infrared camera or a CCD array. Assuming reasonable detector and system characteristics, the sensitivity of each design is then evaluated by setting the receiver SNR detection threshold equal to one and using the minimum transmitted energy as the figure of merit. Through numerical analysis, we show that an upconversion receiver followed by a visible CCD array offers the best trade-off between sensitivity and practical design for airborne ladar applications.

Two measurement techniques are used to measure the range and apparent elevation of surface targets. In one set of experiments, a survey station measured the apparent elevation angle, and Global Positioning System data were used to determine the range. In the other experiments, a lidar measured both elevation and range. Two different analysis techniques (one analytical and one involving an optimal estimation model) are used to characterize the shape of the temperature profiles. The six case studies discussed are equally split between stable and unstable cases. In four cases, data from a survey station are analyzed and include measurements of boats, buoys, and a tower. In two cases, lidar measurements of the sea surface were made. Elevation and range data are derived from a nonlinear model of the surface return. For the analytical model, the errors between the derived and the measured range are minimized; typically the range errors are less than 100 m. For the optimal estimation technique, the errors in derived and measured heights are minimized; the height errors are typically less than 1 m.

We have constructed a 1-m-diam holographically corrected membrane mirror telescope for optical imaging. Several thousand waves of surface error were removed using a corrective hologram, resulting in near diffraction-limited performance. A detailed discussion of the mirror, the corrective process, and the performance of the final telescope are included.

Polymer-dispersed liquid crystals (PDLCs) are formed by microdroplets of liquid crystal embedded in a flexible matrix and sandwiched between transparent electrodes. Large area units (several square meters) can be easily prepared. Opaque, transparent, and intermediate light transmission states can be achieved by applying appropriate electric fields. These features allow their use in active windows for illumination, greenhouse regulation, and privacy, both on buildings and vehicles. An electro-optic system based on a microcontrolled driver was implemented for on-line control of PDLC windows. The system may self- regulate daylight or may be used as remote control.

A simple technique is devised to measure the angles of tetragonal prisms without using expensive spectrometers, or autocollimators and angle gauges.

A novel approach to a measuring algorithm and calibration method for 3-D coordinate noncontact measurement has been developed. This algorithm introduces variables that relate the output digital image directly to the viewed 3-D object. A simple and efficient calibration procedure is achieved using the least-squares technique. It takes advantage of a 2-D coordinate mapping relation, along with dual CCD cameras and a 3-D calibration grid consisting of a series of characteristic points with known coordinates in real space. The space coordinates of these characteristic points are compared to their image coordinates in the CCDs. A 3-D mapping function between the actual space and its image planes in the dual CCD cameras is then constructed. The relationship between the two is determined accordingly and can be applied to extract 3-D information from a tested object sitting in a specific space with satisfactory accuracy. This algorithm not only significantly reduces the 3-D measurement computations, but also increases the measurement precision and speed. Current experimental results show that the measurement standard deviation is under 0.06 mm in a bounded space of 80 x60x60 mm. This result reveals that this measurement algorithm has the potential to be an effective positioning method for rapid and precise noncontact coordinate measuring systems.

A genuinely triple-security-level encryption module for optical communication systems is presented. The approach is practical and feasible supporting real-time encryption of data rates from 155.52 Mbit/s up to 2.4 Gbit/s, based on existing, commercially available electronic technologies. The system deploys a nonrestricted symmetric-key block algorithm with dynamic parallel-processing width according to the input data rates. The algorithm enables the system to support different key strengths according to dynamic key length and processing block size assignment. The operation principle is based on time-to-frequency translation of differentially coded data blocks from the input data stream after optical-to-electronic conversion and utilization of the resulting wideband multitone radio-frequency (RF) signal to modulate a laser source being fed back to the optical fiber network. An identical scheme implemented in time-reversed order is established during the decryption process to recover the wideband multitone RF signal from the optical fiber and decode the original data, based on a synchronized code generation scheme, before it is transferred back to the optical fiber. The total system bandwidth depends on data rates at input and expected encryption strength and can vary from 2020 MHz for an OC-3 link up to 12,900 MHz for an OC-48 link.

With fiber optical telecommunication systems penetrating into metropolitan and access networks, planar waveguide technology is increasingly being considered a solution to the bottleneck of cost-effective manufacture of passive components. Being recognized for their high thermo-optical coefficients, ease of fabrication, cost-effectiveness, and compatibility with other materials, polymers as a platform technology for waveguide devices are gaining more and more commercial acceptance. Fully exploiting the potentials of the polymeric materials demands comprehensive understanding of both the specific device applications and various polymer systems. The right choice of materials is often the key to the success of component development. Unfortunately, since extensive study of polymeric materials and devices operating at 1.55 mm began just recently, few ideal materials have so far been made commercially available. From the polymer chemistry point of view, it is possible to tailor the materials to meet specific and strict requirements for optical waveguide devices. This is a review of the most promising fluorinated polymers and silicone resins and their demonstrated device applications. The paper is designed to provide a guide to both polymer scientists who want to develop novel high-performance materials for waveguide applications, and optical engineers who need to gain insight into the materials.

A novel structure has been designed for bit synchronization in a terabit-per-second optical interconnection network with adjustable optical delay lines. The two-stage network consists of 1024 optical links connected to a central switching module. A distributed optical clock network is designed for bit synchronization at each node computer. Zero link setup time can be achieved by precisely matching the phases of each pair of clock and signal. The link-to-link switching time can be reduced to less than 20 ns in the two-stage network. Even in a three-stage network that can accommodate 32,768 optical links, the link switching time can be maintained below 30 ns. At the same time, a total bidirectional bandwidth of 98.304 Tbit/s can be obtained. Since bit synchronization is achieved entirely in the optical domain, this structure is independent of link transmission rates.

The availability of high resolution spatial light modulators (SLMs) and modern digital signal processors (DSPs) allows a convenient hybridization of digital and optical image processing to produce a small and compact correlator system. The hybridization allows the optical Fourier transform lens system for a correlator to be compact, such that it will fit inside a conventional PC. We describe the design of the optical system.

We describe a design for a modified, cat's-eye retroreflector. The design is catadioptric, containing a single concave mirror and several lenses. This retroreflector design exhibits a unique combination of performance characteristics. It is diffraction limited over a large field angle while operating with a large aperture and numerical aperture. There is little vignetting of the optical beam, even at large field angles, providing good light return at all angles of incidence. It brings the light beam to a focus and allows access to the light near the focal plane, thus decoupling the size of the aperture from the size of devices used with the retroreflector. This final feature makes the design appealing for use with spatial light modulators, or other optical or electro-optical components.

A compact mid-wavelength infrared zoom camera with a zoom range of 20:1 and automatic athermalization has been developed. The moving groups for a wide zoom range are only two lens groups and the loci of zoom are very simple, which allows easy access of optomechanical and electromechanical design. In addition, the microscanning technique was introduced to improve resolving power. We acquired increased resolving power of 7.6 cycles/mrad, superior to the original Ny- quist frequency of 4.5 cycles/mrad. The temperature range, being -30 to + 55°C, in which the camera is to operate, is rather wide. The minimum resolvable temperature difference of the zoom camera is 0.3°C at 6 cycles/mrad.

An Ag-Sb-Te alloy and its films are prepared as a new optical recording amorphous crystalline (a?c) phase transformation material. The crystallization process of Ag-Sb-Te films is systematically studied through measurement of recording characteristics to solve the trade-off problem between data (amorphous) stability and erasing sensitivity. phase change optical recording disks demonstrate long thermal stability of the amorphous recording marks. The crystallization process of Ag- Sb-Te material was studied using differential thermal analysis (DTA), and the nature of the material was studied by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The films were studied for both cases of before and after annealing. It was concluded that the alloy (Ag-Sb-Te) could be used as a phase change optical memory material.

The analog-to-digital (A/D) interface is generally considered to be the most critical part of any signal acquisition and processing system. Because of the difficulty in achieving high-resolution and highspeed A/D converters, this interface has been a barrier to the realization of high-speed, high-throughput systems. Recently, there has been renewed interest in new and innovative approaches to A/D conversion, with a significant emphasis on photonic techniques. Interleaving is a common approach applied to high-speed photonic A/D conversion; it reduces the wide-bandwidth input signal to one that can be converted using conventional high-speed A/D converters. The high-speed sampled input is interleaved to Nindividual channels with each channel operating at 1/N of the sampling rate. These channelization techniques are known to suffer from performance degradations due to channel-to-channel mismatch. Within the electronic A/D converter community, temporal oversampling and spectral noise shaping have become common practice in high-fidelity audio applications. Here, a low-resolution quantizer is embedded in a feedback architecture in an effort to reduce the quantization noise through spectral noise shaping. A large error associated with a single sample is diffused over many subsequent samples, and then linear filtering techniques are applied to remove the spectrally shaped noise, thereby improving the overall SNR of the converter. The approach to photonic A/D conversion described here leverages the 2-D nature of an optical architecture to extend the concept of spectral noise shaping to include 2-D spatial noise shaping. The proposed approach uses a mode- locked laser to generate the optical sampling pulses, an interferometer to modulate the electronic analog signal onto the optical pulses, and a 2-D smart-pixel hardware implementation of a distributed error-diffusion neural network.

Photothermal spectroscopy of gases is explored using optical detection of the thermal expansion of the gas through the use of photorefractive dynamic holography. The photorefractive effect in bismuth silicon oxide is exploited to demodulate the optical phase shift of a signal beam traversing a gaseous environment and coincident with a tunable chopped excitation beam. Molecular absorption at the excitation wavelength produces heat that causes local expansion and subsequent acoustic wave radiation. A test cell, although unnecessary for this technique that can perform measurements in a containerless or open environment, was used to provide known gas mixtures to be tested. A model of the photoacoustic absorption and the optical phase detection process has been developed at low frequencies, where the process can be approximated as photothermal expansion. Measurement and modeling results are presented that illustrate the ability of the method to detect water vapor and hydrogen fluoride concentrations in nitrogen atmosphere backgrounds near 800 nm, currently producing sensitivities in the 20 to 1000 ppm range. The effects of buffer gas concentrations, excitation frequencies, and the ability to measure temporal changes of trace gas concentrations are illustrated. Limitations of the technique and methods for extending to ppb sensitivities with infrared excitation are also discussed.

The stability of soliton propagation in a system with spectral filtering and linear and nonlinear gain is investigated numerically, assuming various input waveforms. Our results show that merely giving a linear frequency chirp to the initial pulse is not effective in suppressing the background instability in bandwidth-limited soliton transmission. However, it should be possible to achieve relatively stable pulse propagation over long distances by the use of a suitable combination of linear and nonlinear gains. Truly stable propagation of arbitrary-amplitude solitons can be achieved only in a system with purely nonlinear gain. A new soliton compression effect is demonstrated both for fixed-amplitude and for arbitrary-amplitude solitons. For fixed-amplitude solitons there is an optimum propagation distance for maximum pulse compression, whereas for arbitrary-amplitude solitons the pulse width remains practically constant after some oscillations in the initial compression stage.

We analyze the use of laser reflection near the critical angle for different refractometric measurements. We consider using dynamic reflectometry to obtain direct measurements of the angle differential of the reflectivity. From the angular profile of the differential reflectivity it is possible to obtain measurements of the absolute values of the real and imaginary parts of the refractive index (RI). For sensing purposes we consider monitoring the reflectance and/or its angular derivative near the critical angle for high-resolution sensing. We give simple and general formulas to estimate the resolution of the different measurements and give some estimates based on our experimental system. We show that a single instrument may be able to measure the real and imaginary parts of the RI with an uncertainty in the order of 10-6. In sensing variations of the RI, the same instrument may yield a resolution of the order of 10-7 for slow variations. However, if the RI is modulated harmonically in time, the same instrument might detect an amplitude of modulation as low as ~ 10-10. In addition, we study the applicability of the technique to turbid media consisting of particles in suspension. We illustrate the potential of the technique for that application with a few experiments on liquid milk and on water suspensions of polystyrene particles. We give experimental evidence showing it is possible to sense the physical state of particles in solution even when the particles have a diameter as large as the wavelength. We also show that the RI of a liquid sample can be monitored through a physical or chemical process even if the liquid is vigorously stirred.

A new independent component analysis (ICA) method is proposed and a new ICA algorithm making use of the higher order statistics is introduced accordingly. We call it the joint cumulant ICA (JC-ICA) algorithm. Its application in synthetic aperture radar (SAR) and airborne visible infrared imaging spectrometer (AVIRIS) images are discussed. The results show the potential usage in image processing problems.

Two-photon absorption of photosensitive media can produce interference fringes with double spatial frequency. This requires the employment of multiple-frequency beams, which interfere with one another to produce a stationary image with double spatial resolution. The required beams were produced by frequency filtering of broadband radiation from a cw mode-locked femtosecond Ti:sapphire laser (? = 790 nm) in a dispersion-free pulse shaper. Then the two multifrequency rays converged from opposite edges of a lens, focusing on Kodak commercial film. The laser intensity was high enough to produce a two-photon exposure. The doubling of the spatial frequency of the interference pattern has been observed, but the contrast ratio of the pattern was limited by competition from the more usual one-photon absorption. Laser pulse parameters for a single-pulse two-photon exposure have been estimated.

Optical waveguide thin films have been prepared from composites of TiO2 and organically modified silane (ormosil) at low temperature by the sol-gel technique, using ?-glycidoxypropyltrimethoxysilane and tetrapropylorthotitanate as precursors. Atomic force microscopy and ellipsometry have been used to characterize the morphologies and properties of the waveguide thin films deposited on compound- semiconductor and silicon substrates. These results indicated that a dense and porosity-free waveguide film could be obtained at 100°C. The refractive index of the film could be varied from 1.44 to 1.55 at 633 nm by varying the titanium content. It was experimentally demonstrated that a channel waveguide and grating structures could easily be fabricated for these composite thin films by etching and embossing. It was found that this method is specifically useful for the fabrication of diffractive grating and optical planar waveguides on sol-gel-derived glass films coated on temperature-sensitive substrates such as III-V compound semiconductors. By this means, channel waveguides have been fabricated using laser writing and reactive ion etching or wet etching, and gratings with a period of 1.102 ?m and depth of 57.2 nm have also been obtained by embossing.

A new and simple method is proposed to analyze the profiles of glass surface layers based on their reflectance for p-polarized light. By measuring the angle spectrum ?(?i) (?=Ia/Ib), where Ia and Ib are the intensities of reflection from the front and the back surface of the glass, and fitting the results with theoretical relations, the refractive index ns and extinction coefficient ks of plane glass surfaces can easily be obtained. Experimentally, glass samples subjected to different cleaning treatments have been analyzed. The results show that ns and ks of the glass surface increase exponentially with the depth into the surface layers, and an etched glass sample has a smaller extinction coefficient on its surface. This is confirmed by atomic-force-microscope observation and by laser-damage testing. In addition, the influence of the glass surface layers on the properties of films coated on one side or both sides of the glass substrate is analyzed. Dip-coated polymethyltriethoxysilane (PMTES) films and spin-coated PMMA films have been measured. It is found that the optical parameters of PMTES films and azo-doped PMMA films are in agreement with the experimental results only if the glass surface layers are considered.

A simple scheme to generate wavelength tunable picosecond pulses by the external injection seeding of a gain-switched Fabry-Perot laser diode (FP-LD) is demonstrated. The key parts of the system consist of a FP-LD, a fiber Bragg grating (FBG), and a 3-dB coupler. The peak wavelength can be tuned from one longitudinal lasing mode to another by changing the Bragg wavelength of the FBG. The wavelength tuning range achieved is about 10 nm and the overall side mode suppression ratio (SMSR) is better than 15 dB.