This PDF file contains the editorial “Editorial: Editorial Responsibilities” for OE Vol. 31 Issue 10

This PDF file contains the editorial “Guest Editorial: Special Section on Acousto-Optics” for OE Vol. 31 Issue 10

A survey is given of research in theoretical acousto-optics carned out at the universities of Gent (since 1943) and Kortrijk (since 1972). Methods and results are discussed that are used in solving light diffraction problems caused by single (progressive and standing), superposed, parallel, antiparallel, and crossing ultrasonic beams. The intensity of the diffracted light has been calculated for both the near field and the far field of the ultrasonic beams.

Polish achievements in acousto-optics are reviewed, including optical holographic interferometric methods for ultrasonic fields and vibrating transducer examination, ultrasonic light diffraction phenomena and their modulation problems, material examination of anisotropic liquids and solids, and construction of acousto-optical devices. Topics reflect research being conducted in university centers in Gdansk, Gliwice, Poznan, and Warsaw. The studies are described and illustrated by means of arbitrarily chosen examples. Over one hundred references are cited.

Acousto-optic devices are used for the control of laser radiation in different branches of science and technology, and for real-time information processing. Optical systems with two-dimensional architectures make it possible to realize space-multichannel information processing systems. We provide a historical excursus, a presentation of the contemporary state of research, and examples of experimental studies carried out at the Leningrad Institute of Aviation Instrument Making. Some realizations of acousto-optic systems for spectral, correlation, and filtering analysis of electrical signals are discussed. The parameters of several tunable acousto-optic filters are presented. Some problems connected with the manufacturing of acousto-optic elementary base and potentially feasible parameters of acousto-optic systems are discussed.

The results of computer simulation experiments concerning strong interaction with strongly curved wave fronts of sound are presented. Such interaction is found to occur locally, at successive points in the interaction region. This is in agreement with the eikonal theory of acousto-optics and its associated diffracted ray diagrams. Based on the latter, measures are presented for the separation between interaction points, their relative independence, and the effective interaction length. These measures are shown to accurately describe the simulation results. Diffraction efficiency for the various orders shows close correspondence with the eikonal theory of uncoupled local interactions. Following the discussion of theory and simulation, implications for wideband acousto-optic devices are considered.

First results on the investigation of close to collinear coustooptical interaction in paratellurite single crystals are presented. Anisotropic Bragg diffraction of light on ultrasound in TeO₂ with a length of interaction of up to several centimeters is examined. A peculiarity of the interaction is optical beam propagation in the crystal collinearly with group velocity of ultrasound. It is shown and verified experimentally that the interaction is characterized by an extremely high selectivity of scattering. Some applications of the examined regime of diffraction are discussed.

A straightforward Fourier-transform approach is employed to investigate acousto-optic interaction between an input optical beam with arbitrary profile and contrapropagating cw sound in the Bragg regime. The process can be analyzed in terms of the simultaneous scattering of light by the two sound waves in the interaction region. Analytic expressions for the equivalent transfer functions are obtained and the scattered light profiles are plotted.

We solve the 3-D Bragg diffraction problem for a plane light wave, incident on a rectangular acoustic column, in the near zone of acousto-optic (AO) diffraction for the case of weak interaction. We show that the orientation of the boundaries of the acoustic column with respect to the incident light wave vector influences the direction of the diffracted light. It appears that the wave vector of the diffracted light is not necessarily coplanar with the wave vector of the incident light and the central wave vector of the sound column. The angle of deviation of the wave vector for the diffracted light is calculated. Conditions for applying the standard solution of the 2-D AO problem are studied. The problem is solved using the 4-D spectral representation of wave fields method.

New results achieved with recently grown Hg₂Cl₂ and PbBr₂ crystals are described. With an optimized crystal growth technique Hg₂Cl₂ crystals were grown that show a significantly reduced acoustic attenuation compared to prior crystals, from 13.4 to 8 dB/μs-GHz². These crystals allow the development of Hg₂Cl₂ Bragg cells with time-bandwidth product figures in the 5100 to 6900 range, frequency operation as high as that for Te02, and resolution about 25% higher than TeO₂ for similar crystal lengths. PbBr₂ crystals were also grown that exhibit a large figure of merit (M₂ 550) with an attenuation coefficient of 12 dB/μs-GHz². This material may be the choice for infrared devices where large diffraction efficienciesare needed.

The spatial resolution limit of noncollinear acousto-optic tunable filters used for imaging with incoherent light is derived based on phase-matching requirements. It is shown that the noncollinear filter can achieve near diffraction limited imaging. This conclusion was verified experimentally.

In the Fresnel zone of laser light diffracted by a sound wave, the light intensity is modulated in both time and space, and the pattern of this modulation contains all the information about the time waveform of the diffracting sound wave. A method is proposed to recompose the sound wave from the optical signal, recorded during one ultrasonic pulse period in a single point in the Fresnel region of the diffracted light. The technique can be considered as a real-time reconstruction and is applicable to both continuous and pulsed ultrasonic waves for physical parameters within the range of Raman-Nath-type diffraction.

Moiré techniques can be a powerful tool to determine surface shape or deviation of a shape in progress from a final or desired shape. The presence of the high-contrast viewing grating and the distorted grating in the final image plane makes the moiré pattern hard to see. Moving grating techniques have been developed to improve the visibility of the moiré pattern, but at the expense of complex moving parts. Several variable resolution projection moiré techniques have been developed that either move the grating or eliminate its presence electronically, and have neither mechanical moving parts nor any physical gratings. One system uses an acousto-optics cell to generate, project, and move the gratings, while the moiré is viewed through a second synchronized A-O cell. The second system uses an interferometer to generate and project variable spacing gratings that are made to move across the target and across a reference surface by an A-O beam deflector. Video processing of the reference image generates the transmissive filter that produces the moiré pattern. A third system removes the grating presence electronically but retains high-contrast moiré contours. Noise reduction is shown in a series of moiré images of targets.

The relationships between lower and higher order cumulants of deterministic and random continuous signals are presented. This theory is used to develop time- and frequency-domain algorithms for the estimation of correlations (spectra) from triple correlations (bispectra) using acousto-optic processors. Cumulants are of interest because they are insensitive to a wide class of additive noises, including Gaussian noise of unknown covariance. Thus, noise-insensitive correlation (spectrum) estimates can be derived from higher order correlations (polyspectra). The potential for noise insensitivity is examined through the variance of power spectrum estimates based on conventional (second-order) and bispectrum statistics. A proof-of-principle experiment was carried out using an acousto-optic four-product processor to estimate the autocorrelation of a wideband periodic signal. The experimental data are compared with a simulation to validate the results.

Multichannel Bragg cells are important components in many two-dimensional optical information processing systems. Multichannel Bragg cell design principles are discussed for both multichannel deflectors and modulators. Particular emphasis is placed on minimization of acoustic and electrical crosstalk and thermal effects through the use of acoustically anisotropic materials, rf stripline techniques, and high thermal conductivity materials. The use of a self-collimating shear mode in gallium phosphide (GaP) is found to substantially reduce acoustic crosstalk from that found in the commonly used tellurium dioxide cells. The use of stripline transmission lines substantially reduces electrical crosstalk over that obtained using the more conventional microstrip techniques. The performance is described of three different GaP multichannel Bragg cell deflectors and a GaP multichannel Bragg cell modulator designed using the principles Bragg cell deflectors and a GaP multichannel Bragg cell modulator designed using the principles outlined in this paper. Optical processing systems using multichannel Bragg cells for phased array antenna signal processing, multichannel rf spectrum analysis, and digital optical computing are discussed.

The past two decades have witnessed an impressive re-emergence of incoherent optical processing techniques. Instrumental in this evolution was the inclusion of a number of techniques, such as acousto-optic frequency shifting and phase-sensitive detection, that made it possible to design practical systems. This paper gives an overview of recently developed incoherent processing techniques. Methods for performing bipolar incoherent spatial filtering using an acousto-optic approach are discussed more specifically and a number of applications are reviewed.

Solutions to several problems in relief grating holographic fabrication are presented. (1) The requirements for high-spectral selectivity and for a low stray light level are contradictory, but this problem can be solved by a compromise in the recording arrangement. (2) A real-time groove-depth control is proposed that ensures the fast and effective determination of the accurate exposing and developing conditions. (3) The necessity of baking the photoresistive grating before its metalization is clearly demonstrated by experiment.

The performance of conventional restoration algorithms is highly degraded by the presence of outliers in the nominal noise process. A novel restoration approach that combines the properties of regularized and robust estimation schemes is introduced. Moreover, we address the potential of robust stabilizing functionals in preserving the detailed structure. Initially, the least-squares part of the regularized optimization scheme is modified according to the notion of M-estimation. An iterative algorithm is introduced for the derivation of the corresponding nonlinear estimate. The global convergence of this algorithm is rigorously studied. The robust regularized criterion provides nonlinear estimates, which do not suffer from artifacts due to the presence of outliers. In addition, the presence of the stabilizing functional enables the efficient suppression of such outliers. For the evaluation of the regularization parameter, several optimality measures are considered. Moreover, an adaptive structure for this parameter, which leads to high-quality estimates without significantly affecting the computational complexity or the global convergence of the algorithm, is introduced. To further account for the effective preservation of the detailed structure, the concept of M-estimators is embedded in both composite functionals involved in the regularized criterion. It is demonstrated that the corresponding error-penalizing strategy allows the reconstruction of sharp edges, while preserving the robust characteristics of the restoration algorithm. The capabilities of the robust regularized algorithms are demonstrated through restoration examples.

Beamsplitter cubes are widely used in white light interferometers to ensure that each interferometer path is balanced with equal amounts of glass. However, commercially available beamsplitter cubes can have alignment errors and size deviations that introduce path differences into the interferometer. A technique for constructing a beamsplitter cube that has extremely well balanced paths is described. Using a tunable dye laser with rhodimine 6G, we have readily measured path differences less than 0.5 μm. By monitoring the beamsplitter path difference with the dye laser, we can adjust the path difference during assembly, thereby sliding the prisms into proper alignment on a layer of UV-cured cement.

A system for reconstructing 3-D vascular structures from two orthogonally projected images is presented. The formidable problem of matching segments between two views is solved using knowledge of the epipolar constraint and the similarity of segment geometry and connectivity. The knowledge is represented in a rule-based system, which also controls the operation of several computational algorithms for tracking segments in each image, representing 2-D segments with directed graphs, and reconstructing 3-D segments from matching 2-D segment pairs. Uncertain reasoning governs the interaction between segmentation and matching; it also provides a framework for resolving the matching ambiguities in an iterative way. The system was implemented in the C Ianguage and the C Language Integrated Production System (CLIPS) expert system shell. Using video images of a tree model, the standard deviation of reconstructed centerlines was estimated to be 0.8 mm (1.7 mm) when the view direction was parallel (perpendicular) to the epipolar plane. Feasibility of clinical use was shown using x-ray angiograms of a human chest phantom. The correspondence of vessel segments between two views was accurate. Computational time for the entire reconstruction process was under 30 s on a workstation. A fully automated system for two-view reconstruction that does not require the a priori knowledge of vascular anatomy is demonstrated.

A novel, two-layer VLSI realizable, modular and expandable neural network multiassociative memory model is proposed. When compared to the Hopfield model, it requires 0(N) connections for N-bit long vectors; yields improved error correction and storage capabilities and a faster convergence rate, avoids the storage of false memories, and possesses analogies to biological neural networks.

The exact field solution of a step-index profile fiber was used to determine the injection efficiency of a thin-film distribution of polarized sources located in the cladding of an optical fiber. Previous results for random source orientation were confirmed. The behavior of the power efficiency, P_eff, of a polarized distribution of sources was found to be similar to the behavior of a fiber with sources with random orientation. However, for sources polarized in either the x or y direction, P_eff was found to be more efficient.

Polymer waveguide technology was exploited to build a photonic switch with enormous bandwidth capabilities at extremely low cost per path. The properties of polymer waveguides allowed the use of a low-cost multielement spatial light modulator in a guided wave application. A 6 x 6 ferroelectric liquid crystal array was used as a switching element to block or transmit light signals between input and output fibers. Visible laser diodes, plastic fibers, PIN detectors, and our proprietary polymeric mixing rod couplers were used to construct the switch. The polymeric fibers, couplers, and switching element allowed eliminating the need to time-division multiplex photonic channels through the switch because the optical paths were no longer costly. This, in turn, eliminated the electronic buffering and synchronization requirements of the switch. This is critical, because signals that can be buffered and synchronized can be switched electronically using GaAs cross-point arrays. Although data-transparent wavelength conversion is acceptable at the inputs and outputs of a photonic switch, buffering and synchronization are not acceptable for some applications. Our low cost polymer waveguide approach also eliminates the need for costly multi-GHz lasers when they are not needed on a per channel basis.

Invariant pattern recognition systems are based on spatial filters that are in most cases computer-generated holograms. Logarithmic harmonics were suggested to be used in implementations that perform projection invariant pattern recognition. A novel approach for optical generation of logarithmic harmonic matched filters of a universal nature, usable for arbitrary input patterns, is presented, followed by some initial experimental results.

The feasibility of polychromatic object recognition by multichannel correlation is experimentally demonstrated. Objects whose shape changes with the wavelength of the illumination beam are used. Separate high-pass matched filters, each one recorded with a different wavelength, are successively employed to perform the recognition of a polychromatic signal without ambiguity. Results of numerical simulation are also presented.

We examine how different imaging sensor point-spread functions (PSFs) affect the precision to which small-template first-derivative operators can measure step-edge orientation. Three PSF models are tested under different numbers of image quantization levels. For each model, the precisions produced by several first-derivative operators are computed using different error measures. Optimal estimates derived using the theory of locales are also provided as a basis for comparison. It is shown that the size of the PSF significantly affects the precision to which orientation can be found. Precision is highest when the PSF has a spread of 1.0 to 1.4 pixels and is dependent on the PSF shape. A method to produce optimal first-derivative templates for specific PSFs is also described.

This PDF file contains the editorial “Book Rvw: Highly Coherent Semiconductor Lasers," by Motoichi Ohtsu for OE Vol. 31 Issue 10

This PDF file contains the editorial “Book Rvw: Optical Scanning," edited by Gerald F. Marshall for OE Vol. 31 Issue 10