This PDF file contains the editorial “It's Your Turn” for OE Vol. 38 Issue 07

This PDF file contains the editorial “Guest Editorial: Special Section on Acousto-optic Devices and Optical Information Processing: Research and Developments” for OE Vol. 38 Issue 07

The near field of ultrasonically diffracted light is examined for a wide range of Raman?Nath (v) and Klein?Cook (Q) parameter values up to the region of total light rediffraction phenomenon occurrence to the zeroeth diffraction order. The behavior of the secondary interference plane position is studied, both theoretically and experimentally, for various Q and v values. In addition, the relationship is shown between diffracted light intensity modulation minima localization shift and rediffraction phenomenon. A comparison is made of the results of the general Fresnel region theory, including the influence of complex light amplitudes over the diffracted light intensity distribution, with those obtained from Cook's simplified near-field description. The same, limited validity range of the latter theory is revealed. For all the experimental conditions considered, quite good agreement is observed between experimental data obtained and the appropriate theoretical predictions.

Light diffraction by two adjacent ultrasonic waves having the same frequencies is examined experimentally beyond the Raman?Nath diffraction regime. Measurements were taken of the 0 and ±1 order diffracted light intensities for a few (settled) Raman - Nath parameter values, as a function of the phase shift between the sound beams. The experimental results obtained are discussed and compared with relevant, generalized, theoretically numerical predictions, as well as with the ones obtained by means of the 0 and ±1 order approximation only. In both cases, however, within all the experimental conditions considered, a quite good quantitative agreement between the experimental data and the appropriate theoretical predictions is observed.

A transfer function formalism developed earlier for the propagation of profiled optical beams through acousto-optic Bragg cells is revisited and applied to a thick holographic grating. The results based on the holographic coupled wave model and the acousto-optic multiple scattering model are shown to be compatible, and equivalent parameters such as the Q and grating strength are defined for the two systems. Results for a Gaussian spatial profile are numerically computed and compared. For the holographic grating, a profiled beam may be interpreted as an angular misalignment or Bragg-angle mismatch problem. The case of Bragg-wavelength mismatch is also investigated for the case of a polychromatic READ beam with a uniform and a Gaussian amplitude spectrum. The resulting spatial amplitude distribution of the scattered order at the grating output is plotted as a function of the departure from the correct Bragg direction.

It is commonly known that the spatial profiles of the diffracted light beams during Bragg acousto-optic interaction are distorted due to the Bragg angle selection mechanism. All the conventional studies on this effect use the paraxial approximation. But this approximation should be amended when the incident angle of the light is large enough that the diffracted light waves do not propagate closely along the optic axis of the acousto-optic diffraction system. By using a spatial Fourier transform approach, we rigorously study the light beam profile deformation effect of the diffracted light during the Bragg acousto-optic interaction beyond the paraxial approximation. Starting from the wave equation, a set of coupled-wave equations is derived to depict the acousto-optic effect. Analytic solutions that describe the profiles of the diffracted light are derived from the acousto-optic coupled-wave equations. From the solutions, differences can be found between the ones with and without paraxial approximation.

We use Mason's equivalent circuit formalism for evaluation of the parameters of a piezoelectrical transducer. An influence of the intermediate bonding layer to the acoustical impedance matching of the transducer and acousto-optical medium is analyzed. The phenomenological method of electrical circuit synthesis for broadband transducer matching using the Smith chart is discussed. The experimental results of the electrical matching of the shear wave LiNbO3 transducer bonded on an acousto-optical medium from TeO2 are presented.

Significant advances have been made in both integrated acousto-optic (AO) and magneto-optic (MO) Bragg cell modulators in recent years. Presented are similarities in the basic interaction geometry and the characteristics associated with AO and MO Bragg diffraction and a comparison of the resulting AO and MO Bragg cell modulators in terms of the substrate material, center carrier frequency, bandwidth and their electronic tunability, modulation and switching speed, diffraction efficiency, fabrication and integration technologies, and applications. While the technology toward realization of monolithic integrated AO device modules in indium phosphide (InP)-based waveguide substrate is considerably ahead of its MO counterpart in yittrium iron garnet/gadolinium garnet (YIG/GGG)-based waveguide substrate, the basic MO Bragg cell modulators have demonstrated a number of unique and important advantages over their AO counterparts. Further research and development toward ultimate realization of integrated AO and MO device modules is also discussed.

A new acousto-optic tunable filter (AOTF) that has a wide spectral tuning range, high resolution, no sidelobe, and requires low rf power was designed and developed. This filter fabricated from TeO2 is based on a novel collinear configuration in which the acoustic wave is collinear with the optical beam. However, different from the traditional collinear AOTF, the diffracted beam emerging from this filter is spatially separated from the transmitted beam. As a consequence, this collinear beam AOTF (CB-AOTF) has the combined advantages of the collinear AOTF and the noncollinear AOTF. Specifically, it requires only 350 mW rf power, which is about 14 times lower than the driving power required by a noncollinear AOTF with the same aperture. Even at this nominal rf power, the filter can diffract light for a spectral range of at least 600 nm with a relatively constant diffraction efficiency of about 70% for the entire tuning region. This diffraction efficiency is about 43% higher than the corresponding noncollinear AOTF. More importantly, the CB-AOTF has no sidelobe. Results of the comparison study of the CB-AOTF, noncollinear AOTF, and integrated AOTF clearly demonstrate that this CB- AOTF has the combined advantages of the other two devices and hence is particularly suited for applications in spectroscopy as well as telecommunication.

The experimental investigation of the problem of collinear acousto-optic tunable filters driven by variable acoustic pulses is carried out. It is shown that bandwidth and shape of transmission function can be altered in this case by varying the shape of a driving pulse. Experimental acousto-optic tunable filter transmission curves and their side wings are presented. It is shown that the utilization of pulses with a smooth temporal envelope results in a decrease of sidelobe level in the acousto-optic tunable filter transmission curves. It is found that the diffracted light can be generated continuously if the interpulse distance is equal to the crystal length. The transmission bandwidth in this case is determined by the length of each acoustic pulse. The impact of the phase difference between acoustic pulses on the diffraction efficiency and on the diffracted light form is investigated.

An acousto-optic system with optoelectronic feedback providing laser beam direction stabilization is studied. Different modifications of the system based on isotropic and anisotropic diffraction of light are considered. Influence of acousto-optic selectivity on the operating angular range and the stabilization coefficient is analyzed in detail. Results of experimental investigation of the system based on a paratellurite crystal cell are presented as well.

We present a simple acousto-optic-based system useful for acquiring time-averaged probability density functions of arbitrary signals. A heuristic analysis is presented as well as experimental results that are shown to agree with theory.

We propose a novel technique for a new division-of-amplitude photopolarimeter capable of complete measurement of the state of polarization of light, as determined by the four Stokes parameters. This photopolarimeter uses an acousto-optic cell in the Raman-Nath regime as a beam splitter to divide the incident beam into four or more components that are used to determine the four Stokes parameters of the incident beam. The acousto-optic photopolarimeter uses no movable parts and thus rapid and simultaneous measurements of the four Stokes parameters are obtained. The instrument matrix of this instrument is nonsingular, hence the state of polarization is completely determined. The acousto-optic photopolarimeter utilizes the useful dynamic grating property that functions as a temporary beam splitter. Power distribution and angle deviations can be easily controlled by varying the driving acoustooptic intensity and frequency. When properly enhanced, this dynamic property can be very useful for several in-line analysis applications.

We present a reconfigurable optical implementation of an acousto-optic algebra processor, based on a calomel (single crystal mercurous chloride) matrix-vector multiplier. Two successful applications are demonstrated: neural network training and curve detection. In the first, a perceptron learns two-input Boolean functions. in the second, a connec- tionist model of the Hough transform is generalized to handle arbitrary curves. Experiments for line detection and circle detection are performed, and the Hough transform's throughput is solely limited by the update latency of the acousto-optic unit.

The solid-state imaging subsystem (SSI) on the National Aeronautics and Space Administration's (NASA's) Galileo Jupiter orbiter spacecraft has successfully completed its 2-yr primary mission exploring the Jovian system. The sSi has remained in remarkably stable calibration during the 8-yr flight, and the quality of the returned images is exceptional. Absolute spectral radiometric calibration has been determined to 4 to 6% across its eight spectral filters. Software and calibration files are available to enable radiometric, geometric, modulation transfer function (MTF), and scattered light image calibration. The charge-coupled device (CCD) detector endured the harsh radiation environment at Jupiter without significant damage and exhibited transient image noise effects at about the expected levels. A lossy integer cosine transform (ICT) data compressor proved essential to achieving the SSI science objectives given the low data transmission rate available from Jupiter due to a communication antenna failure. The ICT compressor does introduce certain artifacts in the images that must be controlled to acceptable levels by judicious choice of compression control parameter settings. The SSI team's expertise in using the compressor improved throughout the orbital operations phase and, coupled with a strategy using multiple playback passes of the spacecraft tape recorder, resulted in the successful return of 1645 unique images of Jupiter and its satellites.

We present a method for measuring the modulation transfer function (MTF) of a charge-coupled device (CCD) using frequency- variable sine grating patterns. Compared with other methods, the advantages of our method include a target with continuously variable spatial frequencies, simple data processing and the straightforward projection of the sine pattern to the measured CCD array without an optical system. Measured MTF results for the typical linear and area CCDs within the range of zero to the cutoff spatial frequency are shown.

A compact joint transform correlator is designed and constructed to test the properties of a practical application and is applied to fingerprint recognition. The system performance is evaluated on fingerprint recognition accuracy under thermal conditions ranging from 0 to 40°C and is found to provide a high level of identification and rapid processing abilities under actual environmental conditions.

A new one-step joint transform correlator (JTC) is presented, in which the joint transform of the images to be correlated is filtered by a moving Ronchi ruling. An alternative JTC architecture without moving parts using two Ronchi rulings is also suggested. A lens produces the spatial integral of the product of the joint transform image and the transmission factor of the Ronchi ruling, and a single element detector is used to capture the integral. The amplitude of the light modulation on the detector gives the cross-correlation function of the input images at coordinates (0,0) without major processing. Therefore, in principle this technique significantly reduces the time required to evaluate the crosscorrelation function, compared to that for usual JTCs. Experimental validation of the proposed one-step architecture is presented.

Large broadband asynchronous transfer mode (ATM) switching nodes require novel hardware solutions that could benefit from the inclusion of optical interconnect technology, since electronic solutions are limited by pin out and by the capacitance/inductance of the interconnections. We propose, analyze and demonstrate a new three stage free space optical switch that utilizes vertical cavity surface emitting lasers (VCSELs) for the optical interconnections, a liquid crystal spatial light modulator (SLM) as a reconfigurable shutter and relatively simple optics for fan out and fan in. A custom complementary metal oxide semiconductor (CMOS) chip is required to introduce a time delay in the optical bit stream and to drive the VCSELs. Analysis shows that the switch should be scalable to 1024x 1024, which would require 2048 ~2 mW VCSELs.

We propose a novel optical probe for surface profile measurement of optical surfaces. This probe uses a laser beam to measure local slopes of a surface and obtains the surface profile through numerical integration of the measured slopes. Slope measurement is based on a new angle measurement method, namely, angle measurement based on the internal-reflection effect (AMIRE). The probe is a high-resolution (0.1-arcsec), compact (80x60x40 mm) device with a simple measurement principle. For surface profile measurement, the probe is installed on a coordinate measuring machine (CMM), which provides the necessary scanning motion. A commercial laser interferometer is used to measure the rotational error of the CMM in real time. The result is subtracted from the slope data collected by the probe for error compensation. Measurement results of a spherical mirror are presented that show a nanometer-order resolution in surface profile measurement.

The ratio of the sampling frequency to the optical bandpass limit of an incoherent diffraction-limited optical system is a fundamental design parameter for digital imaging systems. This ratio is denoted by ?FN/p, where ? is the mean wavelength, FN is the system f/number, and p is the detector sampling pitch. The value of ?FN/p for a remote sensing system can have a profound impact on the image quality and the utility of the acquired images. The interaction between ?FN/p and image quality is sensitive to the system design parameters such as modulation transfer function (MTF), signal-to-noise ratio (SNR), and ground sampled distance (GSD). Image simulations and analysis are presented that illustrate the changes in image quality as a function of ?FN/p. System design trades that may influence the determination of the optimal lFN/p for a remote sensing system are also discussed.

A new method in digital speckle pattern interferometry is introduced. It is a two-stage process. In the first stage, two states (unloaded and loaded states) of the master object are recorded in a holographic plate using the usual holographic arrangement. In the second stage, various phase stepping algorithms are applied, using the reconstructed wavefronts of the master object as a holographic optical element to produce reference wave fields in the comparison process. The method is investigated, and its viability is demonstrated by experimental results and computer simulations.

Precisely figured optics coated with high-reflectance multilayer films are an integral part of an extreme ultraviolet (EUV) projection lithography system. Since multilayer film stress deforms these precisely figured optics, it is important that this stress be characterized and that suitable methods are developed to negate the effects of the stress. However, these techniques must reduce or compensate for the effects of film stress without significantly reducing the EUV reflectivity, since the reflectivity has a strong impact on the throughput of an EUV lithography system. Different techniques for reducing multilayer stress are examined. The technique of varying the base pressure (impurity level) yielded a 10% decrease in stress with a 2% decrease in reflectance for our multilayers. A study of annealing during Mo/Si deposition is performed; a stress reduction of 70% is observed at 200°C, similar to what has been found for postdeposition annealing. However, the reflectance loss was 3.9% versus 1.3% for postdeposition annealing, indicating that if annealing is performed on our films it should be done after multilayer deposition. A decrease of approximately 9% in the bilayer period thickness was observed for annealing during deposition at temperatures near 120 to 140°C, much larger than the thickness changes observed during postdeposition annealing. A new athermal buffer-layer technique is developed to compensate for the effects of stress. Using this technique with amorphous silicon and Mo/Be buffer layers it is possible to obtain Mo/Be and Mo/Si multilayer films with a near zero net film stress and less than a 1% degradation in reflectivity. It is important that the multilayer coated optics are stable with time and that the stress reduction and compensation techniques do not degrade the stability. The temporal stability of 100-nm-thick Mo, Si, and Be single-layer films, Mo/Si and Mo/Be multilayer films, and films treated with various stress reduction/compensation techniques are investigated. Results are reported showing the reflectance peak wavelength to be stable to within 0.15% for both the multilayers and the stress-compensated multilayers. Therefore, stability is maintained when these buffer-layer stress compensation techniques are applied. Mo/Si multilayer films exhibit a ~10% short-term decrease in the stress over the first few months after deposition, but the stress stabilizes thereafter. The Mo/Si reflectance is observed to be stable to within 0.4% over a period of over 400 days. The Mo/Be multilayer film stress was stable to within about 2% over 300 days, and the reflectance was observed to decrease by about 1.5% over the same period. For Mo/Si on a Mo/Be buffer layer the stress changed from -28 to +3 MPa, and the reflectance decreased by approximately 0.4% for a period of over 300 days. For Mo/Be on an a-Si buffer layer, the stress changed from -23 to -3 MPa, and the reflectance decreased by —1.8% for a period of over 300 days; this drop in reflectance is believed to be due primarily to the Mo/Be and not the addition of the buffer layer.