This PDF file contains the editorial “Editorial: Welcome Mohan; Thank You Lorretta” for OE Vol. 34 Issue 05

Satellite jitter adversely affects the pointing, acquisition, and tracking (PAT) functions of an intersatellite laser communication system. Reliable aircraft-based testing of PAT systems requires that the detrimental effects of aircraft jitter be controlled and a realistic satellite jitter environment be emulated. A novel jitter rejection technique, the self-tuning feed-forward compensation scheme, is developed to minimize the effects of aircraft vibration on the PAT terminal. The self-tuning results in the implicit characterization of the mechanical jitter propagation path, thus facilitating the injection of prerecorded satellite jitter in the control circuitry of steering mirrors.

The design and development of decision systems capable of adaptively learning in the operational environment is presented. Innovative adaptive learning concepts and methodologies are offered that are designed for enhancing the performance of decision systems, such as automatic target recognition systems, wherein robustness of performance is a significant issue. The fundamental concept underlying this design is that of learning in partially exposed environments, wherein, at the start, the system is not necessarily aware of all the pattern classes that may be encountered in the future phase of operations. The decision system is based on a variant to the widely popular nearest-neighbor concept. Several stages of sophistication of the system design are presented. The potential problem of increase in computational loads is addressed in detail by exploring the benefits of employing the recently proposed concept of minimal consistent set. The effectiveness of the system design is experimentally illustrated using two data sets, the now classical IRIS data and some real-world TV image data.

In order to quantify the effect of oversampling in a pixel array, a new transfer function is defined. This transfer function is shown to provide a direct comparison between the practical performance of a pixel array with and without oversampling. It is also found to be useful for measuring the influence of different lattice structures on the array. Explicit formulas for the transfer function have also been derived for a few common types of pixel arrays. The same concepts and formulas also turned out to be useful in evaluating the performance of ordinary pixel arrays when the image is not bandlimited.

We derive an analytic formula for the transmittance of a dielectric amelia when the interference between successive internal reflections is only partially spatially coherent. This allows effects such as surface roughness and nonparallelism, which produce cumulative distortions in the phase front with each reflection, and which result in a loss of fringe contrast at high frequencies, to be accounted for quantitatively. The transmittance of a Si lamella, measured with a Fourier-transform interferometer over the range 20 to 1000 cm^-1 agrees with our formula to within the accuracy of the data, which is dominated by systematic instrumental effects.

The phenomenon of total internal reflection sets practical limits for blazing in the case of transmission gratings. Similar effects can be observed in the case of kinoform lenses. The effects manifest themselves in limiting the object distance, the image distance, or the semidiameter of the kinoform lens. These limits are set by the refractive index of the kinoform material and the imaging geometry. This paper presents the results of our investigations on various aspects of these geometric optical limits in the case of planar kinoform lenses for stigmatic imaging between any two prespecified points on the axis.

In space research electrostatic analyzers are frequently used in combination with time-of-flight spectrometers and/or secondary-electron multipliers. The purpose of such electrostatic analyzers is not only to determine the energy-to-charge ratios of particles but also to separate charged particles from EUV light. Since most particle detectors are extremely sensitive to EUV, substantial suppression factors are required in order to limit the undesired background counts caused by EUV radiation. For instance, in a typical application for the investigation of the solar-wind ion composition, a number ratio of EUV photons to minor ions of the order of i09 is encountered. Hence, the quality of an energy analyzer system crucially depends on the optical design and on the EUV reflection properties of the materials used. We present results from an investigation of EUV reflection properties of several materials that are frequently employed for the construction or for the surface treatment of ion-optical instruments in space research.

The noise-equivalent-temperature difference (NETD) is calculated in fixed-pattern-noise-limited mercury cadmium telluride (MCT) focal-plane arrays (FPA5) for both 3- to 5-μm and 8- to 14-μm terrestrial imaging applications, on the basis of a model for MCT in which a linear two-point compensation scheme is considered. The contributions to the NETD from the fixed-pattern noise of an array-in terms of gain, offset, and residual error-are calculated and taken in quadrature to obtain the NETD of the FPA. The effect of stability of the focal-plane temperature is included, but the contribution of readout is not considered. The largest contribution to the NETD is from the residual error in the compensation scheme. Decreasing the intrascene temperature difference and including a sharp-cutoff filter are shown to decrease the NETD. The composition has been varied so as to obtain the lowest NETD in each band: at x = 0.31 in the 3- to 5-μm band, and at x = 0.22 to 0.23 in the 8- to 14- μm band (where x is the composition in the alloy Hg_1-xCd_xTe).

Links between the fractal Hausdorff dimension, the Fourier transform of two-dimensional scenes, and image segmentation by texture are discussed. It is shown that the fractal Hausdorff dimension can be derived by integration of the intensity in the spatial frequency domain (i.e., the Fourier plane) over a set of bandlimited spatial filters. The differences between a computational and an optical approach to determine the Hausdorff dimension are shown, and advantages of each method are discussed. Possible future directions for research and improvements are mentioned. Simulated fractal scenes are considered as test images for both the computational and the optical approach.

The short-time Fourier transform (STFT) is a typical time-frequency 2-D analysis method for nonstationary signals. Its applications in engineering are limited because of its fixed window, which results in low time-frequency resolution. To surmount this disadvantage, a modified short-time Fourier transform (MSTFT) is proposed. The minimum entropy is adopted as a criterion to determine the width of the window, and variable-width windows to accommodate the signal's characteristics are used instead of constant-width windows. Since it is difficult to find the optimal window width, wavelet packet transformation is utilized to find an approximate solution for it. This method is also applied to multi-target imaging for inverse synthetic aperture radar, and satisfactory results are achieved.

We first briefly review a new 3-D imaging technique called optical scanning holography (OSH). We then discuss the technique's 3-D holographic magnification in the context of optical scanning and digital reconstruction. Finally, we demonstrate the 3-D imaging capability of OSH by holographically recording two planar objects at different depths and reconstructing the hologram digitally.

Films made of the halobacterial photochrome bacteriorhodopsin (BR) can be used in a number of holographic real-time applications. Their application as active material in a dual-axis joint-Fourier-transform (DAJFT) real-time correlator was shown recently. The BR films have a strong nonlinear intensity dependence on the light-induced absorption and refractive-index changes. Therefore the holographic diffraction efficiency also shows a nonlinear dependence on the writing intensity. We investigate the effect of this nonlinearity on the result of the correlation process in a bacteriorhodopsin-based DAJFT correlator. Numerical models supporting the experimental observations are presented. It was found that the BR film combines the holographic function for most objects with that of a spatial bandpass filter, whose center frequency is tuned by the writing intensity. This results in smaller peak widths and a suppression of the sidelobes. BR films allow the application of this nonlinear behavior in real time to the all-optical correlation process.

The output of an electronic holographic apparatus represents the magnitude and phase of the coherent light reflected from a three-dimensional object. The amplitude and phase can be fed into a spatial light modulator for three-dimensional object reconstruction, and the technique is thus named electronic holography. Resolution studies of recording and display devices for electronic holography are presented. The theoretical analysis for electronic holography is first carried out. Mathematically, the additive noise at the exposure of an electronic hologram is more serious than that at the reconstruction. Simulation is used to verify the theoretical results, and the resolution requirements for an electronic hologram are deduced. In our simulation, the point source is placed 500 mm in front of the recording device and the reference plane wave is tilted at angle π/4 in the -y direction with the normal to the recording plane. To discrimmate a two-point object with 0.17-mm separation, an 8-bit 256x256 charge injection device with a pixel size of 10 x 10 μm² is necessary. In addition, the additive noise from the ambient light during the exposure or reconstruction of an electronic hologram should be less than 1/100 of the signal amplitude for a 40-dB output SNR.

We describe a new precision displacement probe-the precision holographic optical probe (PHOP)-with the use of double holographic optical element (DHOE) technology. It is a novel noncontact method for the measurement and inspection of displacement and surface quality control. To improve the measurement resolution and linearity, the differential method for the PHOP is introduced. Infrared DHOEs are made by using a computer-generated hologram method. Mathematical models have been developed to analyze the performance of the probes. Computer simulations and experimental results have been obtained, showing sensitivity on the order of nanometers for these probes. The complexity and the size of the probes can be significantly reduced. They are both compact and inexpensive.

In dentistry, a defect of cementation on the margins of crowns accumulates bacterial plaque easily. This can result in recurrent caries and periodontal disease. In this paper holographic interferometry was applied to evaluate the effect of masticatory force on various complete crowns. Four complete molar crowns made from different materials (Au alloy, Pd-Ag alloy, Ni-Cr alloy, and porcelain fused to metal) were tested. The out-of-plane displacements of the crown specimens were measured by the method of multiple observations. The displacements measured range from 6 to 10 pm under normal load (25 N). However, the marginal openings of all four crowns were estimated to be less than 0.2 μm. In addition the defect of the crown was examined.

A method for developing an accurate temperature-dependent dispersion model is described. It combines a room-temperature Sellmeier dispersion model with temperature-dependent refractive index measurements at a few wavelengths to predict the refractive index over a wide temperature and wavelength region. The method is applied to nine well-characterized materials to give dispersion over a wide temperature and wavelength region.

Light waves propagating through a region of inhomogeneous index of refraction are aberrated. Turbulent air flows cause such inhomogeneities to arise from density fluctuations within the flow, which affect the index of refraction of air. Here we describe a technique for measuring the 3-D structure of the index-of-refraction distribution in a turbulent air flow using optical wavefront sensor measurements and tomographic signal reconstruction. Use of wavefront sensor measurements offers higherspeed measurement than previously used holographic methods, while providing high-fidelity reconstructions. We discuss Hartmann sensor measurements and wavefront reconstruction. Sampling requirements are also addressed, including the effects of a limited number of projections and wavefront subaperture size. Theory, simulation, and experimental results are presented.

We discuss the design of optical concentrators based on dielectric and hollow compound parabolic concentrators (CPCs), for use in free-space infrared communication receivers. In order to acheive a high signal-to-noise ratio in a direct-detection receiver, it is desirable to use an optical bandpass filter that passes the signal but attenuates ambient radiation. Placement of a planar bandpass filter at the entrance aperture of a CPC results in a receiver having a narrow passband and high gain, but a narrow acceptance angle. The addition of a second, angle-transforming CPC at the entrance aperture allows the receiver to achieve simultaneously a narrow passband and an acceptance angle approaching 90 deg. We have employed a Monte Carlo ray tracing method to calculate the optical gains of several optical concentrator designs. We find that the optical gains of single and double CPCs are, respectively, about 94% and 93% those of ideal optical concentrators, while addition of planar bandpass filters decreases these gains to about 88% and 86%, respectively. We compare the performance and size of CPC-based concentrators with those based on dielectric hemispheres fitted with hemispherical bandpass filters.

We describe three instrument effects that cause polarized infrared images of a blackbody source to exhibit intriguing polarization dependence and image nonuniformity. The origins of these problems are reflection of background radiation from the wire-grid polarizer and frictional heating of the polarizer mount. Our model shows that the two surfaces of a wire-grid polarizer act like a partial polarizer-analyzer pair for reflected radiance. These effects must be considered carefully in applications such as polarimetric remote sensing, which require calibration uncertainties less than 1%.

One important issue in phase-shifting interferometry is to eliminate phase-shift errors so as to enhance achievable measurement accuracy. We propose a new phase-measuring algorithm that can identify the actual amounts of phase shifts directly from interlerograms without prior calibration. This is named the A-bucket algorithm and has the advantage of being inherently free from any phase-shift error that is constant across an interferogram. The main emphasis of this paper is given to a practicable application of the A-bucket algorithm to prove that accurate measurement is allowed in portable mode even in the presence of vibration.

We have deposited crystalline diamond films on sintered Si₃N₄ substrates with a conventional hot-filament method and further finished the surfaces with a diamond lapping method. The diamond mirrors fabricated are proposed and demonstrated as among the most promising mirrors for vacuum ultraviolet rare-gas excimer lasers. We have obtained pulses with 15-mJ energy, 2.7-MW peak power, and 6-ns duration at 126 nm from an argon excimer laser, and with 70 mJ, 5.5 MW, and 5 ns at 146 nm from a krypton excimer laser without any surface damage. The latter values are the highest so far obtained.

A novel optical mode-switching device is proposed and theoretically demonstrated with a coupled triple-stripe semiconductor laser structure. The switching operation results from electrically controlling the laser lateral modes. It generates an effective far-field pattern switching useful for fiber optic systems or systems using small objective apertures.

We have made a Nd:YAG laser which operates in a cw mode, a pulsed mode, and a mixture of the two. This has been achieved by mixing the dc and the pulsed pumping currents. A thyristor-switched pulse power supply and a cw power supply are isolated from each other with high-current diodes. The output performance of the three modes has been compared by varying the reflectivities of the output mirrors. Two kinds of lamp, a specially designed flashlamp and an arclamp, were used in optimizing the output power. A peak power as high as 3.6 kW has been obtained in the mixed-mode operation with a Kr flashlamp at 2-atm pressure.

Precise optical wavelength measurements can be performed by use of a novel wavelength sensor. This low-cost sensor consists of a wavelength-sensitive photodiode and a high-precision log ratio amplifier. The center wavelength of monochromatic or narrowband light sources such as semiconductor lasers in the visible or near-infrared wavelength range can be determined. By simultaneously testing the properties of monomode laser diodes with a standard wavemeter, the feasibility and precision of the wavelength sensor are demonstrated. Wavelength variations less than 0.1 nm are detected clearly.

The NASA Goddard Space Flight Center's stratospheric ozone lidar has undergone several modifications and improvements since it participated in the Stratospheric Ozone Intercomparison (STOIC) campaign at Table Mountain, CA, in July 1989. Changes have been made in both the transmitter and receiver. The transmitter has been changed to include a XeF laser to generate the "off-line" wavelength; the detector has been changed to include mechanical choppers for each of the high-sensitivity channels, and the number of channels has been increased to six. In addition to the four elastic-scattering channels previously described, detectors at the N2 Raman-scattered wavelengths for each of the transmitted wavelengths have also been added. The vertical resolution of the acquisition has been improved from 300 to 150 m, with the capability to record data with 75-m resolution. The Raman channels permit the measurement of ozone in air parcels with heavy loadings of aerosols, and give additional information about the aerosols themselves. Data from recent campaigns are presented to illustrate these points.

A neural-network-based algorithm is proposed for the restoration of nuclear medicine images as required for antibody therapy. The method was designed to address the particular problem of restoration of planar and tomographic bremsstrahlung data acquired with a gamma camera. Restoration was achieved by minimizing the energy function of the Hopfield network using a maximum entropy constraint. The performance of the proposed algorithm was tested on simulated data and planar gamma camera images of pure p-emitting radionuclides used in radioimmunotherapy. The results were compared with those of previously reported restoration techniques based on neural networks or traditional filters. Qualitative and quantitative analysis of the data suggested that the neural network with the maximum entropy constraint has good overall restoration performance; it is stable and robust even in cases where the signal-to-noise ratio is poor and scattering effects are significant. This behavior is particularly important in imaging therapeutic doses of pure β emitters such as yttrium-90 in order to provide accurate in vivo estimates of the radiation dose to the target and/or the critical organs.

A step-frequency fault locator was constructed as a new diagnostic tool for integrated optics devices. The wavelength of the light was swept stepwise from 1.5 to 1.6 μm in steps less than 1 nm. The image of the target was processed using a neural network algorithm. Detailed comparisons between the results using the neural network algorithm and FFT algorithm have been presented. Obtained resolution was of the order of microns. The overall dimensions were reduced to 7 x l0 x 2 in. by replacing bulk optic elements with fiber-optic elements.

A behavioral model of a wavelength-division multiplexing communication network for transporting data at SONET/SDH OC-192 using VHDL is described. VHDL simulation results are given for the SONET frame generator, electrical-to-optical conversion, data transporter, and optical-to-electrical conversion. Relative bitdelay, due to wavelength dispersion, between adjacent bits is included in the model of a 10-km transmission link.

Symbolic substitution (SS) for digital optical computing initially used intensity coding with one pixel representing one bit. This required recognition of both the dark and the bright pixels in a pattern for complete recognition. To avoid this double recognition, dual-rail coding was introduced, where it is sufficient to recognize either the dark or the bright pixels alone for complete recognition. Subsequently, the modified signed-digit number system was proposed to fully exploit the parallelism of SS for arithmetic operations. This introduces a third symbol - 1 , and with dual-rail coding, this again necessitates recognition of both dark and bright pixels. We argue the usefulness of triple-rail coding to avoid this double recognition. We show that the nature of this one-of-three coding enables simplified recognition of a set of patterns. We present a new implementation of SS based on this coding. We also show that this one-of-many coding can be extended to high-radix arithmetic with equal efficiency. We present an evaluation scheme based on the number of optical hardware elements required by each implementation and use it to compare the different implementations of SS.

The bimodal optical computer (BOC) technique to reconstruct 2-D conductivity is presented. This technique does not require any presimplification or preprocessing for the matrix. The effects of three different factors on the accuracy of the solution are studied.

The classic Schmidt design has its aspheric corrector plate designed for and located at the center of curvature of its companion spherically concave primary mirror. So configured, it is well corrected for objects at infinity imaged at focus. Any closer object, however, is imaged with increasing spherical aberration as its distance from the corrector plate is diminished. Shifting the unchanged corrector plate toward its primary mirror increases the range of object distances for which the system retains excellent stigmatic correction. At the corrector plate's near-optimum location, this range may extend from a few of the primary mirror's radii in front of the corrector plate to infinity. Coma increases, but likely will remain tolerably small except for very fast systems.

We present a theoretical method that makes it possible to analyze 3-D integrated optical waveguides with arbitrary refractive-index profiles. With this method it is easy to obtain effective indices, propagation constants, and coupling/switching properties of planar and channel optical waveguides. This theoretical approach involves modeling the original optical waveguide by means of an equivalent optical waveguide whose eftective index is evaluated by applying a technique that we call the asymptotic eftective-index method. The numerical values show good convergence and accuracy for effective indices, propagation constants, and coupling/switching characteristics. Theoretical and experimental values are given.

A novel method of generating optical phase conjugation using an azo-doped liquid crystal valve is proposed for the first time. Its response time, diffraction efficiency, and threshold intensity are measured. The resolution of the device is discussed. An observation of diffraction rings caused by self-phase modulation when a low-power laser beam passes through the azo-doped liquid crystal valve is reported. Finally, these phenomena are shown to be results of the reorientation of liquid crystal molecules driven by the photoinduced isomerization, not merely an azo dye effect or a local heating effect.

Techniques for measuring time-varying biospeckle of botanical specimens are investigated. Experimental evidence on the probability density function is presented. Several applicable techniques, such as power spectral density and correlation, are used for measuring and analyzing the temporal speckle intensity variations. These techniques are shown to exhibit information about the shelf life and aging of some botanical specimens used in our study. We also present a new phenomena related to the spatial properties of time-varying speckles. Theoretical and experimental results on the size of a speckle in a speckle pattern and its relation to the temporal intensity variation are also detailed.

A way is proposed to make multilayer dielectric mirrors with unusual dependence of reflection phase on frequency, dΨ/dω > 0. Simultaneously, the reflection amplitude can be nearly unity. The method of calculation is based on the theory of reflecting interferometers. The resulting multilayer structure can be a starting point for further refinement by the usual optimization methods. The mirrors can be used in a dispersion-free Fabry-Perot interferometer, which is insensitive to frequency deviations of a light source.

Multilayer phase gratings have higher diffraction efficiency than amplitude gratings in the soft x-ray region. The authors give a detailed description of the fabrication, characterization, and performance of soft x-ray phase modulation multilayer laminar gratings. The experimental results are discussed and compared with theory.

We describe a spectrometer used to evaluate curved crystals, crystals originally intended for cosmic x-ray spectroscopy on the AXAF satellite. The most important feature of the AXAF instrument was the high-resolution spectra (λ/Δλ as high as 2000) that could be obtained from astronomical objects over the λ range 1.2 to 100 Å. The crystals are formed into narrow cylindrical facets and assembled into a pseudotoroidal surface. The spectral resolution is degraded by a number of effects, which we describe. The primary effect over which we have control is the figure of each cylindrical segment. We measure deviations from the nominal cylindrical shape, which we call Δθ_slope using a laboratory x-ray spectrometer. We present spectrometer data for several crystal samples, evaluated at different energies. We compare these results with similar tests performed at optical wavelengths. Our results indicate that the Δθ_slope error can be controlled, and that curved crystals with the desired resolving power can be fabricated.

This PDF contains the communication "Biomedical and potential clinical applications for pulsed lasers operating near 6.45 μm"

This PDF file contains the editorial “Book Rvw: Fundamentals of Multiaccess Optical Fiber Networks," by Denis J. G. Mestdagh for OE Vol. 34 Issue 05

This PDF file contains the editorial “Book Rvw: Design and Fabrication of Acousto-Optic Devices," edited by A. P. Goutzoulis and D. R. Pape for OE Vol. 34 Issue 05