This PDF file contains the editorial “‘‘How do you explain curiosity?’’” for OE Vol. 38 Issue 09

Atmospheric turbulence distorts the wavefront of the incoming light from an astronomical object and so limits the ability of a telescope to form perfect images. Adaptive optics is a combination of technologies that enable the correction of the wavefront distortion in real time. Conventional adaptive optics operate like auxiliary instruments and use additional relay optics, which reduce total throughput and introduce extra IR emissivity and polarization. Adaptive secondary mirrors avoid additional optical surfaces by providing the optical correction at an existing telescope surface (the secondary mirror). Previous studies have demonstrated the optical efficacy and mechanical feasibility of performing the adaptive correction in this way. A technique demonstrator is being developed to explore features and capabilities applicable to a large adaptive secondary mirror and to explore manufacturing, assembly/ disassembly, calibration, and measurement techniques. The paper describes the design of the demonstrator and its predicted performance.

A new method to overcome some limitations in the simulation of the propagation of waves originating from a point source through a very long path in a turbulent medium is presented. Existing propagation simulation algorithms suffer from either windowing or lack of resolution when applied to long paths. If Cartesian coordinates are used, the limited size of the numerical mesh eventually leads to windowing errors. Casting the classical split-step Fourier algorithm in a spherically diverging coordinate system allows one to get around this problem. In this way an angular mesh matching the source and the propagation algorithm to the problem geometry is used. But for long-path propagation, this spherical divergent mesh causes a loss of resolution that can become a serious problem in the evaluation of the field statistical moments. The method discussed in this paper overcomes both the windowing effect associated with Cartesian coordinates and the loss of resolution accompanying spherical coordinates by using a spherical-coordinate algorithm and performing repeated interpolations of the numerically propagated field before the mesh grows too large to sample the field accurately. Each time an interpolation is done, the angular window is decreased to maintain the matrix size.

The photoluminescent spectra of CaO and MgO oxides doped with rare-earth ions are studied. Photoluminescence stimulated by IR light at room temperature is observed for the first time. Optically stimulated luminescence in Ca(Mg)O:Eu,Sm for different concentrations of dopants after exposure to UV radiation (?=250 nm) and radiation with ?=488 nm are investigated. A physical model to explain luminescence and stimulated luminescence is proposed. Materials based on alkalineearth oxides activated by rare-earth ions are suitable for multiple rewriting of information.

We consider the problem of segmenting multitextured images using multiple Gabor filters. In particular, we present a mathematical framework for a multichannel texture-segmentation system consisting of a parallel bank of filter channels, a vector classifier stage, and a postprocessing stage. The framework establishes mathematical relationships between the predicted texture-segmentation error, the frequency spectra of constituent textures, and the parameters of the filter channels. The framework also enables the systematic formulation of filter-design procedures and provides predicted vector output statistics that are useful for classifier design. We focus on the mathematical framework and provide experimental results that confirm the utility of the framework in the design of a complete image-segmentation system. The results demonstrate effective segmentation using a straightforward classifier and fewer than half the number of filters needed in previously proposed approaches.

Hollow glass waveguides with losses as low as 0.2 dB/m at the 10.6 µm are an attractive fiber delivery system for a broad range of IR wavelengths. These guides are made by depositing, using liquid- phase chemistry methods, silver and silver iodide films on the inside of silica tubing. The wet-chemistry techniques used in the process reduce somewhat the mechanical properties of the hollow-silica guides compared to the uncoated tubing used to make the guides. In particular, the mean failure stress is reduced from 8.4 GPa for uncoated, as-drawn, 530-µm bore silica tubing to 6.12 GPa for the Ag/AgI coated, 530-µm bore waveguide. In addition, the Weibull modulus decreases from 96.9 to 17. This means that the bending radius of the hollow guides, which is inversely proportional to the mean failure stress, increases from about 1 to about 1.5 cm. The reason for the decrease in strength for the processed tubing is the slow reaction of the aqueous coating solutions with the silica surface. Nevertheless, the bending radii for the guides is still quite small and the fatigue behavior of the guides, as measured by the crack growth parameter n, is essentially the same for processed or uncoated tubing.

To maintain its mechanical strength, the glass fiber of optical fibers is coated by polymeric materials during the fabrication process. However, when the thermally induced shear stress at the interface of the glass fiber and primary coating is larger than its adhesive stress, the polymeric coatings will be delaminated from the glass fiber, and the optical fiber will lose its mechanical strength. The design of tightly jacketed double-coated optical fibers to minimize the thermally induced delamination of polymeric coatings is investigated. To minimize the coating’s delamination, the thermally induced shear stress at the interface of the glass fiber and primary coating should be reduced. The method to minimize such a shear stress is to select suitable polymeric coatings as follows. The thickness and Poisson’s ratio of the primary coating should be increased. On the other hand, Young’s modulus of the primary coating, the thickness, Young’s modulus and thermal expansion coefficient of the secondary coating, and Young’s modulus and thermal expansion coefficient of the jacket should be decreased. Finally, the optimal design of commercialized tightly jacketed double-coated optical fibers to minimize the thermally induced coating’s delamination is also discussed.

The multiresolution wavelet expansion is used as a simplifying mechanism for the parametric analysis of complicated highly correlated random fields. A previously developed approximation method is applied to simulated statistically self-similar random fields for further evaluation. This approach can be considered as a simplifying method for random variable transformations for some important applications. The approach overcomes many of the difficulties associated with predicting the output field probability distribution function resulting from passing a non-Gaussian random process through a linear network. Here, the multiresolution wavelet expansion can be considered as a linear network. The ideas are illustrated with three related simulated noise fields: a white noise input field distributed proportional to a zero order hyperbolic Bessel function and two 1/f noise processes resulting from filtering the white noise process. The fields are analyzed with an orthogonal multiresolution wavelet expansion. The expansion components are studied with parametric analysis, where the probability models are all derived from one family of functions. In addition, the study illustrates some interesting nonintuitive statistical properties of the filtered fields.

A new method of image decomposition is described. It uses a depth-of-contrast approach in which the image is factored into an ordered set of component images, the order being from coarse to fine in degree of contrast. The decomposition technique is unique in that it is completely independent of any geometrical structure, yet its application shows a clear trend from coarse to fine spatial frequency with the degree of contrast. For images with spatial structure and noise, the first factors contain more of the geometrical structure and the higher-level factors contain more of the noise. The factorization is expressed as an infinite product rather than as an infinite sum, and its convergence is shown. A conservative lower bound is derived that ensures that any positive definite real image can be fully factored.

A new seven-sample algorithm is derived, based on the Sur- rel six-sample algorithm, by using the averaging technique. The sensitivities of phase-shifting algorithms to phase-shift error and harmonics of nonsinusoidal waveforms are analyzed by the Fourier description method. Plots of the root-mean-square phase error, produced by computer simulation, are presented in order to compare the new algorithm with the other four well-known phase-shifting algorithms. It is shown that the new algorithm is the least sensitive to linear phase-shift error and to quadratic nonlinear phase-shift error with linear compensation, when the fringe signal contains the second-harmonic distortion. A 3-D fringe projection phase-shifting profilometer was constructed using a white light source and projecting a quasisinusoidal grating with some second- harmonic distortion. Experiments were carried out to compare the five phase-shifting algorithms when different phase-shift errors exist, and the shape of a 3-D object was measured using the new seven-sample algorithm.

The range-Doppler imaging algorithm is a basic imaging method for the inverse synthetic aperture radar, which is based on the uniform rotation of a target in a fixed plane. To a maneuvering aircraft, the rotational velocity and the axis of a maneuvering target often vary with time. The traditional imaging method will blur the image of the target, and the target cannot be recognized. The available imaging algorithm for maneuvering targets can deal with the targets only with loworder Doppler variations of scatterers. The raw radar data show that the Doppler variations of scatterers are sometimes complicated for maneuvering targets. In this paper, an imaging algorithm for maneuvering targets based on adaptive chirplet decomposition is proposed. Without much more calculation, the novel algorithm can be used to deal with maneuvering targets with complex Doppler variations, as is verified with raw radar data.

The measurement of thicknesses of micromachined structures consisting of etched glass bonded to c-Si and multilayer structures of both etched c-Si and etched glass is presented. The peaks within the recorded interferograms are identified by inspecting the equation for the reflection coefficient and the corresponding equation for the reflectivity. The amplitudes of the various peaks resulting from the 3-D structure underneath the spot of the beam are evaluated to be able to perform an unambiguous choice of the correct peak when performing automatic process monitoring. Furthermore, the peaks in the interferogram may be a strong function of wafer position (depending on device layout). In the latter case, it is demonstrated that information obtained directly from the interferogram can be used in wafer imaging. The dielectric dispersion functions for the glass and the c-Si wafer under investigation are evaluated from the reflectance measurements. An anomalous shift is observed in the interferogram peak corresponding to the glass thickness. The shift is found to be due to the strong dispersion in the refractive index of glass resulting from the SiOSi asymmetric stretching frequency around 1100 cm-1. Both simulations of the interferogram, and analysis provide a 5% correction for the thickness of the glass under investigation.

We describe a new method for measuring spatially nonuniform phase changes of a light beam from the spatial nonuniformity of the phase changes, which is obtained by detecting the relative reflectivity change of a self-pumped phase-conjugate mirror. The method is designed to decrease the accumulation of calculating errors. The usefulness of these methods is verified through computer simulations and measurements of the phase changes caused by spatially nonuniform deformation of a semitransparent sheet of silicone gum.

The concept of a three-dimensionally interconnected optical backplane for a high-performance system containing multichip module boards, operating at 850 nm, is introduced. The backplane reported here employs 2-D vertical-cavity surface-emitting lasers (VCSELs) and photodetector arrays as transceivers. By integrating 250-^m-pitch 2-D VCSELs, microlenses, and photodetector arrays into our backplane design, we have demonstrated a multi-bus-line optical backplane and experimentally realized this architecture with 2-D VCSEL and detector arrays while using the third dimension as the signal-propagating direction. Such an approach greatly increases the aggregate bandwidth of the backplane. Packaging issues such as misalignment, cross talk, and signal-to- noise ratio are studied. Eye diagrams up to 1.5 GHz were obtained with clear eyes, and the frequency response of a single bus line shows a bandwidth of 2.5 THz.

The observation of self-generating high-order diffraction associated with a conical ring in LiNbO3 on which two extraordinarily polarized beams are incident is reported. We propose that Bragg- mismatching diffraction, Bragg degeneracy, and multiple scattering through two-wave mixing could be responsible for the high-order diffraction and the conical ring.

A high-speed shape measurement system is developed based on the method of projected fringes. Sinusoidal fringes are projected onto the object using a commercial spatial light modulator, and the resulting fringe patterns are acquired by CCD camera at a rate of 30 s-1. The images are analyzed in real time using a pipeline image processor. The combination of phase shifting and temporal phase unwrapping enables discontinuous objects to be profiled as easily as continuous ones. An optimized sequence of fringe pitches is used in which the spatial frequency is reduced exponentially from the maximum value. A total time of 0.87 s is required from the start of the measurement process to the final display of a surface profile consisting of 250,000 coordinates. An accuracy of one part in 2000 was achieved with a maximum fringe density of 16 fringes across the field of view.

A novel method of estimating the remaining thickness of a defective plate having the form of localized thinning using digital shearography, based on a simple relationship between the curvature of the deflected defect and the defect geometry is presented.

An electronic speckle shearing interferometry method for the full field measurement of the slope of an object is presented. The object under study is placed on a rotating platform and illuminated by an expanded laser beam. Its image is recorded by a shearing CCD camera and stored directly into a computer. By recording two images before and after giving the object a small angle of rotation, the resulting correlation fringe pattern that represents the object surface slope can be obtained. The theory of the new method and some experimental results are presented.

The evolution of the spatial profile of a thermal lens induced by a low-power laser beam in an absorbing dye solution is directly visualized in real time using low-coherence light interferometry. Both spatial characteristics and response times of the lens are measured. Comparisons between experimental results and thermal diffusion theory are presented. The resolution of the measured refractive index in the thermal lens profile is better than 10-6.

The concept of sculptured thin films (STFs) is exploited to propose and theoretically examine a polarization-discriminatory handedness inverter of circularly polarized light. Suggestions for improved performance of the proposed two-layer, dielectric device as well as several production-related issues are discussed.

When pulsed lasers are applied in subtraction video speckle interferometry, the interline-transfer CCD camera used to acquire the primary interferograms to be subtracted can be operated in different modes. The features of each operation mode are discussed. It is shown that the visibility of the correlation fringes produced by subtraction depends on the operation mode in which the interline-transfer camera is used. By subtracting in different ways two primary interferograms recorded for an in-plane configuration using a cw laser, it is shown that results favor the use of CCD cameras operated in the noninterlaced field and noninterlaced frame modes.