This PDF file contains the editorial “SPIE with/and/or OSA?” for OE Vol. 37 Issue 04

This PDF file contains the editorial “Guest Editorial: Special Section on Integrated Optics” for OE Vol. 37 Issue 04

Waveguides have been fabricated by a hybrid organic- inorganic glass sol-gel process and ultraviolet-light exposure. The adjustment of chemical composition of the materials provides precise selection of the refractive index from 1.48 to 1.52 at the wavelength of 632.8 nm. The effect of ultraviolet-light exposure and heat treatment on the refractive index is studied. Ultraviolet-light inscription has been employed to produce ridge waveguides with smooth sidewalls.

We describe various structures of channel waveguides made through the sol-gel process and present numerical simulations to study and optimize the geometrical and physical parameters. The computations give the guiding modes and their field profiles for a set of values. By comparison with experimental observations, an analysis of parameters such as the depth of the waveguides and the refractive index is performed, and parameters for an optimized structure are established.

We present the fabrication process and the optical characterization of buried ion-exchanged channel waveguides made by lowtemperature direct bonding on Pyrex wafers. Multimode or monomode buried channel waveguides were fabricated, and the method provides very good control over the intensity distribution.

A simple and low-cost laser writing system that allows one to obtain micrometer features with high-quality borders is presented. After an overview of the complete fabrication process of masks to carry out the ion-exchange technique in glass in a selective way, a complete description of the experimental setup for direct laser writing is given, including details about such important parts as the control of the motorized stage and the system to perform focusing and alignment. A discussion of the way of focusing the laser beam in order to get the best performance is presented. Results of the characterization of the laser writing system and some examples of integrated optical elements and masks to fabricate them are shown.

The possibility of utilizing certain polymers, known as variable-index materials, as cladding in tapered optical fibers for gas sensing is explored. We have shown that any small variation in the refractive index of the cladding of a tapered single-mode fiber has a direct influence on the transmission characteristic of the tapered region. A 4% deviation in the refractive index of a certain polymer is obtained upon the exposure of the polymer to ammonia. Sensitivity, repeatability, and selectivity of several fabricated sensors are also illustrated.

The sol-gel process is an interesting alternative for making glass integrated-optics components. Reasonable performance can be reached with low-cost fabrication. We show a one-step method for imprinting gratings on a thin layer made through the sol-gel process, thus enlarging the field of application in integrated optics to filters, out-ofplane connections, etc. The efficiency of the grating is studied through diffraction measures and computations.

A vectorial coupled mode theory is used to analyze sol-gel ridge waveguides with UV-light-imprinted Bragg gratings. The effect of device parameters on the grating reflectivity is investigated. It is shown that 100% reflectivity for both TE and TM modes can be achieved using short (1 to 2 mm) gratings. The measured reflectivity in a ridge waveguide with a grating agrees with the theoretical predictions for TE and TM modes.

Deeply etched 1-D third-order Bragg reflectors have been used as mirrors for broad-area semiconductor lasers operating at 975-nm wavelength. From a threshold and efficiency analysis, we determine the mirror reflectivity to be approximately 95%. The design of the GaAs-based laser structure features three InGaAs quantum wells placed close (0.5 ?m) to the surface in order to reduce the required etch depth and facilitate high-quality etching. Despite the shallow design and the proximity of the guided mode to the metal contact, the threshold current density (Jth5220 A/cm2 for infinite cavity length) and internal loss (?=9±1 cm-1) are very low.

We study optical gain and laser emission from semiconducting conjugated polymers after strong optical excitation with femtosecond laser pulses. Stimulated emission from few-hundred-nanometer thin films results in a sharp increase in emission intensity and emission line narrowing. Very large optical gain due to the stimulated emission is measured directly by pump-probe spectroscopy. Using the large light amplification in these conjugated polymers, we demonstrated planar and ring lasers of very small sizes that are suitable for applications in integrated optics. Vertical cavity surface emitting lasers, distributed feedback through surface relief grating, and whispering-gallery mode emission from ring resonators are demonstrated.

The integration of metal-semiconductor-metal (MSM) photodetector arrays with multimode polyimide waveguides has been demonstrated. MSM detectors were fabricated using transparent indium tin oxide (ITO) interdigitated electrodes on semi-insulating GaAs substrates. Light at 830 nm was coupled efficiently from the waveguides into the underlying photodetectors through gaps in the SiO2 buffer layer. Absolute responsivities of the integrated MSM detectors were around 0.5 A/W, and 3-dB bandwidths of 4 to 6 GHz were measured. Division of the input optical signal between four detectors of increasing length integrated along a single waveguide was achieved. Four 1 x4 arrays were used to assemble a prototype hybrid 4x4 optoelectronic switch. Band- widths of around 1.2 GHz, limited by the electronic circuitry, were measured. The isolation was better than 35 dB, and the crosstalk was approximately ?35 dB. The advantages of polyimide waveguides for on- chip interconnection in switch matrices are discussed.

The passive mode-locking properties of large, monolithic semiconductor ring lasers have been studied both theoretically and experimentally. By using a computer simulation based on the traveling- wave model, the operating conditions under which mode locking may be achieved have been determined. Three different configurations of intracavity saturable absorber have been used, and the dual-absorber arrangement shown to be superior in having greater range of operation and shorter pulse width. Experimental confirmation of mode-locked operation has been obtained.

The design and fabrication of circular grating surface emitting lasers are presented. The influence of device parameters on the mode selection is described. The fabrication of an InGaAs/GaAs circular grating distributed Bragg reflector (DBR) laser is presented. Significant improvement is obtained by using a quantum-well structure with a multistack and an etch-stop layer. A threshold current as low as 15 mA and an output power in excess of 170 mW without saturation are obtained. The potential of circular grating DBR laser arrays for multiwavelength operation is demonstrated.

A microscopic theory for the interaction between semiconductor quantum-well structures and laser fields based on the semiconductor Bloch equations is applied to vertical-cavity surface-emitting lasers (VCSELs) with the inclusion of plasma heating. The semiconductor Bloch equations are reduced to a set of equations for the first and second moments of the carrier distribution functions. Plasma heating and many-body effects are then studied by solving this set of equations in steady state under the approximation of a single transverse and longitudinal mode. The transverse- and longitudinal-mode dynamics of VCSELs is analyzed by solving the full space-time-dependent problem.

Modeling results for Er3+-doped waveguide amplifiers fabricated by various ion-exchange processes are presented. The gain performance of these devices for 0.98- and 1.48-?m pump wavelengths are compared. Measured spectroscopic properties of a phosphate glass with Er3+ concentration of 3.7x 1020 cm-3 are used throughout the modeling, and waveguide propagation loss of 0.1 to 0.6 dB/cm is used. It is shown that gain coefficients exceeding 5 dB/cm are feasible utilizing optimized ion-exchanged waveguides in this glass. The modeling is performed for glasses doped with erbium only, excluding possible ytterbium codoping.

Characteristics of potassium-ion-exchanged channel waveguide amplifiers fabricated in a commercial phosphate glass with 1.65 wt% Er2O3 and 22 wt% Yb2O3 are reported. Theoretical investigations reveal that at 1.530-?m signal wavelength, 4.2 dB/cm can be achieved in potassium-ion-exchanged waveguides when 120-mW pump power at 974-nm wavelength is used. Theoretical studies predict 42% slope efficiency with 42.5-mW threshold pump power in a 1.0-cm-long potassium- ion-exchanged waveguide lasers in this glass.

We present a review of our research activities concerning the proton exchange technique on lithium niobate and lithium tantalate for integrated lasers and nonlinear frequency converters. In the first part we present the material characterizations that allowed us to establish the phase diagram of the proton exchanged layers on both crystals. In the second part we show how the knowledge of these phase diagrams allows us to understand and handle the problem of the reduction of the excited-state lifetime of the rare-earth ions in Nd- and Er-doped crystals. In the third part, we describe the influence of the different proton exchange processes on the nonlinearity and the domain structure of periodically poled crystals, which leads us to recommend the use of waveguides fabricated using low-acidity melts, which preserves both the nonlinear coefficient and the domain structure.

Siloxane-silica nanocomposites functionalized with Red 17 were synthesized via a sol-gel process. Thin films were thermally cured and poled to obtain high and stable nonlinear second-order coefficients of the order of 150 to 200 pm V-1 (? = 1.064 ?m). Radiated second- harmonic generation was observed in these planar waveguides with a conversion efficiency of 5.10 -4% W-1 cm-1.

Based on the construction and analysis of the mathematical model for a multidither adaptive optical system, the relative theory of automatic control is applied to researching the dynamics of a multidither adaptive optical system. The practical application of adaptive optical technology is also provided.

Most traditional shape-from-shading algorithms assume that an image contains only a single object with Lambertian surface and uniform albedo values. These algorithms perform poorly on images with nonuniform albedo values. In this paper, we first discuss a linear shape- from-shading method that employs discrete approximations for the surface normal using finite differences of the depth and linearizes the reflectance function in depth directly. Next we present a simple method to cancel the effect of albedo variation. In our approach, we first estimate the albedo values for each pixel, and segment the scene into regions with uniform albedo values. However, the estimated albedo values are not accurate at the points near edges, so we apply a modified median filter to improve the result. Then we adjust the intensity value for each pixel by dividing by the corresponding albedo value before applying the linear shape-from-shading method. Experimental results are presented for several synthetic and actual images.

The use of chloride-based deicing agents to help clear U.S. highways of roadway hazards leads to associated chemical related problems. Fouling of local rivers and streams due to runoff of the waterborne chlorides is significant and has contributed to local ordinances that are attempting to force state agencies to reduce, if not eliminate, the use of these chlorides (typically at the cost of increased driving hazards). With respect to the corrosion aspects of chloride application, cracks that occur in the roadway/bridge pavement allow water to seep into the pavement carrying the chloride to the rebar with the resultant increase in corrosion. The costs of this corrosion are considerable and have led to the widespread use of chloride/water impermeable membranes on roadways and especially within bridges. Fiber optic sensors have repeatedly been shown to provide measurement capabilities of parameters within such reinforced concrete structures. Development of fiber optic chloride sensors capable of being embedded within a roadway or bridge deck is reported.

Fiber optic switches have become more and more appealing components not only in the field of optical communication, but also in measurements systems, sensors, and data storage. We develop a number of concepts for fiber optic switches, all based on different types of special micro-optic components actuated by miniaturized mechanical systems, primarily piezoelectric actuators. We present micro-optic configurations and discuss their potential for the creation of different types of miniaturized switches.

A large area charge-coupled device (CCD) based fiber optic taper array detector (APS-1) is installed at the insertion-device beamline of the Structural Biology Center at the Argonne Advanced Photon Source x-ray synchrotron. The detector is used in protein crystallography diffraction experiments, where the objective is to measure the position and intensity of x-ray Bragg peaks in diffraction images. Large imaging area, very high spatial resolution, high x-ray sensitivity, good detective quantum efficiency, low noise, wide dynamic range, excellent stability and short readout time are all fundamental requirements in this application. The APS-1 detector converts the 2-D x-ray patterns to visible light images by a thin layer of x-ray sensitive phosphor. The phosphor coating is directly deposited on the large ends of nine fiber optic tapers arranged in a 3x3 array. Nine, thermoelectrically cooled 1024x 1024 pixel CCDs image the patterns, demagnified by the tapers. After geometrical and uniformity corrections, the nine areas give a continuous image of the detector face with virtually no gaps between the individual tapers. The 18 parallel analog signal-processing channels and analog-to-digital converters ensure short readout time and low readout noise. We discuss the design and measured performance of the detector.

Optical fiber distribution systems with intensity-modulation/ direct-detection and erbium-doped fiber amplifiers are designed for cable television (CATV) applications. Two types of system configurations are considered, i.e., the passive power splitter with optically preamplified receivers (PPS-OPR) scheme and the hybrid passive-and-active power splitter (HPAPS) scheme. The receiver sensitivity is calculated for various system parameters. We compare both schemes through the number of CATV subscribers and show that the HPAPS scheme is superior to the PPS-OPR scheme for large-scale CATV distribution applications.

The error and hence the residual nonuniformity in an IR focal plane array, after two-point linear nonuniformity correction, which results from measurement errors and nonlinearity, is calculated, yielding different results than have been obtained in the past.

We develop a method for encoding phase and amplitude in microscopic computer-generated holograms (microtags) for security applications. An 8 x 8 cell phase-only and an 8 x 8 cell phase-and- amplitude microtag design are fabricated in photoresist using an extreme ultraviolet (13.4-nm) lithography (EUVL) tool. Each microtag measures 80x 160 xm and contains features 0.2 xm wide. Fraunhofer-zone diffraction patterns can be obtained from fabricated microtags without any intervening optics and compare very favorably with predicted diffraction patterns [Descour et al. (1996)]. We present the results of a rigorous coupled-wave analysis (RCWA) of microtags. Microtags are modeled as consisting of subwavelength gratings of a trapezoidal profile. Transverse-electric (TE) and TM readout polarizations are modeled. The analysis concerns the determination of optimal microtag-grating design parameter values and tolerances on those parameters. The parameters are grating wall-slope angle, grating duty cycle, grating depth, and metal coating thickness. Optimal microtag-grating parameter values result in maximum diffraction efficiency, which is calculated at 16% for microtag gratings in air and 12% for microtag gratings underneath a protective dielectric coating, within fabrication constraints. TM-polarized readout illumination is diffracted with higher efficiency than TE-polarized illumination by microtag gratings.

We examine various physical characteristics of current-art acousto-optic tunable filters (AOTFs) for spectrally filtered imaging applications. Imaging spectroscopy, where the goal is to obtain simultaneously spatial and spectral information, is a topical application that effectively utilizes the attractive features of the AOTF. Key measures of performance include image blur, relative contribution from sidelobes, contribution from background illumination, broadband scattering and diffraction efficiency.

Spatial and temporal input-output transformations of a system CCD camera linked to a frame grabber-digitizer board are investigated. A realistic model for this type of system is proposed and applied to reconstruct a continuous input image from its digital sampling with remarkable accuracy. This reconstruction remains successful for an image with spatial frequencies up to the Shannon maximum frequency, i.e., the half sampling frequency.

A novel filtering algorithm is presented to restore images corrupted by impulsive noise. As a preprocessing procedure of the noise cancellation filter, an improved impulse detector is used to generate a binary flag image, which gives each pixel a flag indicating whether it is an impulse. This flag image has two uses: (1) a pixel is modified only when it is considered as an impulse; otherwise, it is left unchanged, and (2) only the values of the good pixels are employed as useful information by the noise cancellation filter. To remove noises from the corrupted image, we propose a new filter called a polynomial approximation (PA) filter, which is developed by modeling a local region with a polynomial that can best approximate the region under the condition of least squared error. Furthermore, an adaptive approach is introduced to automatically determine the orders of the polynomials. The proposed two kinds of PA filters, fixed-order and adaptive-order PA filters, are tested on images corrupted by both fixed-valued and random-valued impulsive noise. Major improvements are obtained in comparison with other state-of-the-art algorithms.

An adaptive approach to the recovery of the least-squares (LS) solution employing regularization and conjugate gradient descent is developed. This is then implemented on image data corrupted by deterministic blur and additive Gaussian noise. The approach is useful where large data vectors are being processed with the need to conserve space and obtain reliable estimates rapidly.

Image registration algorithms based on piecewise linear interpolators (spatiotemporal gradients or block matching) are analyzed to determine subpixel registration accuracy. Results reveal not only random errors due to image noise but also systematic bias present even if the images are noise free. If the displacement between the images is small, bias is small. However, if the displacement between the images is larger than about 1/25 pixel, the bias component of the registration error exceeds the random component for most imaging conditions. Bias also depends on image content: it is generally larger for images with higher spatial frequency content than for images with lower spatial frequency content. We have developed a reduced-bias registration algorithm that takes advantage of the nearly linear relation between image displacement and bias that results for small displacements. The new algorithm is direct (noniterative), increases computational costs by approximately a factor of 4, and reduces the bias by approximately a factor of 4. This improvement is large compared to improvement obtained with averaging. For our applications, in which imaging noise is typically 50 dB smaller than the signal, registration errors using the new algorithm are smaller than 1/50 pixel.

A system for measuring nanometer motions of microscopic structures is demonstrated. Stop-action images of a target are obtained with a light microscope, CCD camera, and stroboscopic illuminator. Motions are determined directly from measured images using algorithms from computer vision. The accuracy of motion measurements using the system is assessed using a moving target with calibrated displacements. Accuracy is determined for specimens viewed under our most optimal conditions as well as for a number of suboptimal conditions that illustrate important degradation mechanisms. Measured errors are compared to predictions based on computer simulations of theoretical models. Results show that the most important hardware factors include substrate vibrations and camera imperfections. Measurement errors for the most optimal hardware conditions are primarily due to systematic bias in the computer vision algorithms. For our most optimal conditions, the system can resolve motions as small as nanometers. Thus, errors in motion measurements are small compared to both the wavelength of the light used to obtain the images and the pixel spacing of the video microscope.

All-optical multilayer perceptrons differ in various ways from the ideal neural network model. Examples are the use of nonideal activation functions, which are truncated, asymmetric, and have a nonstandard gain; restriction of the network parameters to non-negative values, and the limited accuracy of the weights. A backpropagation-based learning rule is presented that compensates for these nonidealities and enables the implementation of all-optical multilayer perceptrons where learning occurs under computer control. The good performance of this learning rule, even when using a small number of weight levels, is illustrated by a series of computer simulations incorporating the nonidealities.

We present a portable, versatile optical pattern recognition system that can process 256x256 pixel images at high speed. The system is fully controlled by Windows-based software and can easily be switched between VanderLugt and joint transform operation. It combines the advantages of optics and electronics to form an advanced hybrid system where pre- and postprocessing algorithms can be varied depending on the application. Simulation and experimental results show that edge enhancement combined with binarization gives good results when applied to images where the illumination is varying.

We describe a joint transform correlator (JTC) that uses multiple input images encoded in the spatial domain for multiwavelet feature extraction. We extend the theory of a JTC to multiple inputs, which enables various combinations of cross-correlations between input images to be performed. Furthermore, we provide experimental results for four inputs with an optically addressed spatial light modulator in the Fourier plane. In addition, it is possible that the space-bandwidth product for multiwavelet feature extraction can be made the same as for a two-input JTC.

We have designed an optical backplane with free-space optical interconnections using tunable beam deflector arrays and a mirror for terabit per second class bookshelf-assembled asynchronous transfer mode (ATM) switches. Optical beam transmitter arrays and optical beam receiver arrays are placed on the edges of a printed circuit board. Optical beams are deflected individually by the tunable deflector arrays, emitted obliquely and downward, and reflected at the mirror placed on the backplane, passing through some printed circuit boards, and reach the targeted receivers on the edges of printed circuit board. As a preliminary experiment, we show that board-to-board free-space arbitrary optical interconnections are possible using a tunable beam deflector array and 1-mm-pitch 2 x2 optical beam array provided with a vertical cavity surface emitting laser diode, we also show that these optical interconnections are made very stable by attaching reinforcing frames to the printed circuit board. Furthermore, we show that more than 1000 channel interconnections per printed circuit board are possible using a 1-mm-pitch optical beam array. As the speed per channel may be more than 1 Gbit/s, the throughput of the interconnection per board reaches almost the terabit/s level. Using this optical interconnection system is used, ATM switches with a huge capacity of 1.0 to 10 Tbit/s will be possible.

Basic concepts and theory are developed for the effect of optical radiation on the dc characteristics of a generalized model of an ion-implanted GaAs metal-semiconductor field effect transistor (MESFET). The optical radiation is allowed to fall on a transparent or semitransparent Schottky gate and the spacing between the gate source and gate drain. The continuity equations are solved for the excess carriers in the depletion region below the Schottky gate in the channel region and in the depletion region at the junction of the active layer and the substrate. The current-voltage (/- V) characteristics show a remarkable enhancement in the drain source current compared to opaque gate optically controlled field effect transistor (OPFET) (a special case of the generalized dc model). This highlights the importance of two photovoltages developed across the Schottky junction and the channel-substrate junction. The transconductance, the channel conductance, and the photovoltages are also observed to be strongly affected by the flux densities.

We study nonuniform transmission filters to improve resolution and/or depth of focus (DOF) in lithography. To understand the behavior of these filters for periodic structures we study the changes produced by the defocus in the image of a five-bar test. We analyze the behavior of the system with different degrees of coherence. For partially coherent illumination, we use the apparent transfer function (ATF), which is defined by the contrast in the image of a cosine grating object of the corresponding spatial frequencies. We also compare the ATF for incoherent illumination with the optical transfer function (OTF).

A simple method of testing deep aspheric surfaces is presented. The apparatus consists of a Twyman-Green interferometer and a liquid compensatory container. Two lenses, one with spherical surfaces and the other with a spherical surface and an aspheric surface, were tested by using this method. The device is very simple and easy to assemble.

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

The wavelet transform is a useful transformation for signal processing and compression, however, when it is calculated digitally, it requires a high amount of computation effort. Optics has solved partially this problem by implementing the hybrid wavelet transform in which the set of scaled wavelet mother functions is discrete but the shift is continuous. We use white light illumination to obtain a 2-D, fully continuous, on-axis wavelet transformer. Experimental demonstrations are given.