Phase correction of light modulators

Optical inspection of liquid crystal variable retarder inhomogeneities

Liquid crystal variable retarders (LCVRs) are starting to be widely used in optical systems because of their capacity to provide a controlled variable optical retardance between two orthogonal components of incident polarized light or to introduce a known phase shifting (PS) between coherent waves, both by means of an applied voltage. Typically, the retardance or PS introduced by an LCVR is not homogeneous across the aperture. On the one hand, the LCVR glass substrates present a global bend that causes an overall variation of the retardance or PS. On the other hand, in the manufacturing process of an LCVR, there sometimes appears a set of micro-air bubbles that causes local retardance or PS inhomogeneities. In this work, we present an interferometric technique based on a Mach-Zehnder interferometer that is insensitive to vibrations and capable of inspecting and characterizing the LCVR's retardance or PS inhomogeneities. The feasibility of the proposed method is demonstrated in the experimental results, where the LCVR retardance is measured with an error of about 0.2 rad. The thickness of possible micro-air bubbles is obtained with a resolution of about 50 nm.

Influence of flatness distortion on the output of a liquid-crystal-television-based joint transform correlator system

The use of liquid-crystal panels from a commercially available Sanyo video projector as spatial light modulators in a standard joint transform correlator system is investigated. It is found that the flatness distortion of the panels disturbs the output correlation signal in general. Since the reported solutions for the flatness corrections are either expensive (liquid gates) or suffer from low light efficiency (holographic techniques), we have investigated a possibility to minimize the influence of these distortions on the correlation output without flatness correction. First, we quantify optical flatness across the transparent panel area, and then we measure the effects of flatness distortion by changing the display location of the input objects and the resulting joint power spectrum. It is found that the correlation peak is 1 order of magnitude more sensitive to phase distortions of the input scene than to the same distortions of the joint power spectrum. Choosing the flattest location on the panel allows the utilization of the panels to be demonstrated through recognition of cuneiform inscription signs.

Object-binarized phase-only matched filtering with dual liquid-crystal spatial light modulators

A real-time optical processing system with dual liquid-crystal spatial light modulators is constructed and used as both an amplitude-input device and a multilevel phase-only filter. Fourier analysis is given to show the performance of light efficiency, signal-to-noise ratio, and discrimination capability with the binarization of gray objects. The ratio of the dc power spectrum to the power spectra for input objects is introduced to incorporate the power spectrum into discrimination-capability evaluation. A numerical calculation is performed for gray-level and binarized amplitude-phase correlations. Improvement of the performance criteria has been achieved for an amplitude in a binary mode to a phase correlator. The higher the threshold level of the binarized objects is, the better performance criteria are produced. The effect of illumination over an input object on the autocorrelation maximum is experimentally investigated. Experimental results are presented to support the calculations.

Astigmatic phase correction for the magneto-optic spatial light modulator

We report a simple low-cost technique for evaluating the phase distortion in a magneto-optic spatial light modulator. We find that the dominant distortion is caused by astigmatism and is easily compensated by encoding of the complex-conjugate pattern onto the device. Two experimental results are shown. First, the focused spot size from a Fresnel lens is sharpened when the aberrations are corrected. Second, we show that the pattern that generates a first-order Bessel-function nondiffracting beam does not work unless the aberrations are corrected.

Interaction of multiple distortions in spatial light modulators

Spatial light modulators are the key components in real-time optical image-processing systems. The phase and the intensity of their outputs will often depart from ideal behavior. An experimental method is described that permits the effects of multiple distortions, present simultaneously, to be modeled. A computer simulation of a bismuth silicon oxide-based correlator is presented, with spatial light modulators subject to three types of distortion, including phase and amplitude. The experimental method permits both the main effects of the distortions and their interactions to be predicted. Combining all the distortions simultaneously gives a more accurate assessment of the suitability of a spatial light modulator for a given optical processing task. Images of 256 × 256 pixels were used, and the simulation took 15 min. with a Sun SPARCstation 2.

Optical correlator using very-large-scale integrated circuit/ferroelectric-liquid-crystal electrically addressed spatial light modulators

The use of 2-kHz 64 × 64 very-large-scale integrated circuit/ferroelectric-liquid-crystal electrically addressed spatial light modulators as the input and filter planes of a VanderLugt-type optical correlator is discussed. Liquid-crystal layer thickness variations that are present in the devices are analyzed, and the effects on correlator performance are investigated through computer simulations. Experimental results from the very-large-scale-integrated/ferroelectric-liquid-crystal optical-correlator system are presented and are consistent with the level of performance predicted by the simulations.

Experiments on a multichannel holographic optical switch with the use of a liquid-crystal display

A new configuration of holographic switches is proposed and verified for multichannel optical switching. Experimental 1 x 64 and 2 x 32 switching is achieved by using real-time binary phase-only holograms generated by a twisted nematic liquid-crystal display. This holographic free-space switching is applicable to photonic switching systems and optical interconnections.

Effects and correction of magneto-optic spatial light modulator phase errors in an optical correlator

Here we study the optical phase errors introduced into an optical correlator by the input and filter plane magneto-optic spatial light modulators. We measure and characterize the magnitude of these phase errors, evaluate their effects on the correlation results, and present a means of correction by a design modification of the binary phase-only optical-filter function. The efficacy of the phase-correction technique is quantified and is found to restore the correlation characteristics to those obtained in the absence of errors, to a high degree. The phase errors of other correlator system elements are also discussed and treated in a similar fashion.

Optical laboratory morphological inspection processor

Morphological transformations are applied to industrial inspection problems. A real time optical architecture to implement morphological transformations such as erosion, opening, closing, and skeletonization is described and analyzed. The first real time optical laboratory results of erosion and opening are presented for locating string in tobacco.

Liquid crystal TV-based white light optical tracking novelty filter

A compact white light optical tracking novelty filter is demonstrated. Based on the use of two inexpensive liquid crystal televisions, a filtered and collimated white light source, digital delay, and video recorder, this portable white light device performs two major image comparison operations, a real time image subtraction and novelty tracking operations. Some preliminary experimental results are presented.

Liquid crystal television spatial light modulators

The spatial light modulation characteristics and capabilities of the liquid crystal television (LCTV) spatial light modulators (SLMs) are discussed. A comparison of Radio Shack, Epson, and Citizen LCTV SLMs is made.

Phase-only modulation using a twisted nematic liquid crystal television

We used a commercially available liquid crystal television display unit as a spatial optical phase-only modulator. To do this, we removed the integral polarizers from the unit and double passed the light under modulation through it. We found that it was possible to obtain continuous phase modulation from 0 to PI with essentially no change in the state of polarization of the output light and absorption changes of <2.5%. We wrote computer-generated phase-only holograms on the LCTV and reconstructed them optically.

Evolutionary development of advanced liquid crystal spatial light modulators

Taking into account recent developments and present trends in devices and component technologies, the future development of electrically addressed liquid crystal spatial light modulators is considered. In particular, the combination of single-crystal-silicon active backplane and chiral smectic C liquid crystal technologies is shown to be promising. The ultimate limitations of such technologies for producing faster devices of higher complexity and functionality are assessed, and an advanced device, presently under development, is described.

Free-space folded-path optical programmable logic array

A new method to realize a medium-scale, free-space optical programmable logic array is proposed. By using either a two-dimensional optical spatial light modulator or an array of one-dimensional spatial light modulators inside a lens-based multiple-beam-path cavity, an array of optical multiple-variable logic product terms is generated. This device, together with a programmable multiple-variable OR matrix, can be used to implement any Boolean combinatorial logic operations. For an optical binary combinatorial logic computation, the proposed method efficiently uses three-dimensional space and optical elements. Preliminary experimental results obtained using an inexpensive liquid-crystal television are included.

Modulation depth characteristics of a liquid crystal television spatial light modulator

The performance of a commercially available liquid crystal TV display was characterized in terms of its modulation depth. Measurements of screen transmittance and modulation depth, as a function of signal level, showed that the primary limitations of the device as a spatial light modulator were due to the nature of the video scan format and the display drive electronics. The resolution of the device, as measured by the modulation transfer function, is limited more by the physical pixel spacing than by pixel crosstalk. The optical flatness of the screen was characterized interferometrically, both with and without polarizers, to show the improvement in wavefront quality obtained by replacing the original polarizers.

Optical parallel logic gates using inexpensive liquid-crystal televisions

The optical implementation of parallel logic gates using inexpensive liquid-crystal televisions (LCTV's) is demonstrated, and basic experimental results are presented. While it is certainly unreasonable to expect general-purpose optical computers to be built from LCTV's, demonstration-level systems can be built, from which a great deal may be learned.

Real-time joint transform correlator with phase conjugation for distortion correction

A joint transform correlator has been demonstrated that incorporates phase conjugation in a BaTiO3 crystal to correct for distortions in a liquid-crystal television (LCTV) spatial light modulator. The optical arrangement permits real-time operation in spite of the relative slowness of the photorefractive effect. Correlation outputs displayed on a video monitor are illustrated for inputs to the LCTV from a video camera viewing a stationary scene. The sensitivity of the correlator to target rotation is also presented.

Real-time deformation invariant optical pattern recognition using coordinate transformations

The well-known scale and rotation invariant polar-logarithmic coordinate transformation is used to achieve in-plane distortion invariant pattern recognition. The coordinate transform is produced by a computergenerated hologram on a laser printer. Attention is given to weighting terms in the output and their effect on resolution and the number of input plane pixels removed near the origin. The optically produced coordinate transformed input pattern is interfaced to a correlator by a pocket liquid crystal TV to provide real-time processing. Experimental results are included.

Real-time computer-generated hologram by means of liquid-crystal television spatial light modulator

A novel use of liquid-crystal television (LCTV) is described. It is shown that, if the phase nonuniformity of the LCTV is corrected by a liquid gate, then a simple computer-generated hologram can be written and coherently reconstructed.