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

Surface-stabilized ferroelectric liquid-crystal diffraction gratings with micrometer-scale pitches

The first-order diffraction efficiency eta1 of surface-stabilized ferroelectric liquid-crystal (SSFLC) phase gratings is calculated for device thicknesses in the range d = 1 to 5 microm and for pitches p of 5 to 20 microm assuming incident light at 633 nm. The peak value of eta1 as a function of d has negligible dependence on the incoming polarization when p = 20 microm. For smaller pitch values the peak value of eta1 decreases and becomes increasingly dependent on the orientation of the incoming polarization owing to the influence of the domain walls that occur between the SSFLC pixels.

Calculation of the efficiency of polarization-insensitive surface-stabilized ferroelectric liquid-crystal diffraction gratings

A rigorous analysis is presented of the diffraction efficiency of a polarization-insensitive surface-stabilized ferroelectric liquid-crystal (SSFLC) phase grating, taking full account of the internal structure of the ferroelectric liquid-crystal layer. When no field is applied, the twisted director profile in the relaxed state gives an optimum diffraction efficiency for a device thickness between the half-wave-plate and the full-wave-plate conditions. The influence of the magnitude of the spontaneous polarization and applied ac fields are investigated, and it is shown that the diffraction efficiency of a binary SSFLC phase grating can be strongly enhanced with the technique of ac stabilization.

Optically computing the hit-miss transform for an automated cervical smear screening system

The hit-miss transform serves as a region-of-interest locator for cells from cervical smear images that show abnormal changes, which are indicative of malignancy, in their nuclei. An optical implementation of the hit-miss transform algorithm uses an analog spatial light modulator for gray-scale modulation at the filter plane of a 4f optical correlator. Gray-scale modulation at the filter plane improves correlator performance in comparison with a binary phase-only filter (BPOF) by reduction of the edge enhancement of kernels used in morphological detection of cancerous cervical cells. The hit-miss transform with a gray-scale amplitude and binary phase optical filter (GABPOF) for the hit filter and a BPOF for the miss filter shows a 47% reduction in total error versus the use of only BPOF filters to locate abnormal cells.

Analysis of spatial light modulator contrast ratios and optical correlation

We have performed a general analysis of optical correlators with spatal light modulators (SLM's) whose primary defect is a finite contrast ratio (CR). Our mathematical analysis identifies three noise terms that appear in addition to the correlation term. The filter SLM contains either a phase-only filter (POF) or a binary-phase-only filter (BPOF). Insertion of a dc block at the center of the filter SLM decreases the noise background in the correlator plane; this dc block is larger than that required for the same level of performance in a correlator whose SLM's have transmissive (or reflective) dead zones. With a noise-free input and the dc block, our computer simulations that show the peak intensity falling off as the CR decreases are in quantitative agreement with the correlation term of the mathematical model. For a cluttered, disjoint noise input this agreement is only qualitative, and at low CR's the dc block is definitely required for the BPOF correlator if the secondary peaks in the output are to be brought below the correlation peak.

Scale-invariant optical correlators using ferroelectric liquid-crystal spatial light modulators

New experimental results for scale-invariant implementations of the binary phase-only matched filter and the nonlinear joint transform correlator using ferroelectric liquid-crystal spatial light modulators are presented. We provide a comparative study of both architectures for real-time road-sign recognition. Signal-to-peak-noise ratios in excess of 5 dB over a scale range of 1.0 to 2.0 are achieved under realistic conditions of clutter.

Fractional pixel translations of Fresnel lenses written onto spatial light modulators

The focal point of a lens written onto a spatial light modulator can be translated laterally by displacement of the central location of the encoded lens function. We show that the beam can be translated by a fraction of a pixel, in contrast to the expected single-pixel limitation. This increases the positional sensitivity obtainable with this technique. Experimental results are presented.

Scale-invariant binary phase-only matched filter using a ferroelectric-liquid-crystal spatial light modulator

We describe the implementation of a scale-invariant binary phase-only matched filter using a 128 × 128 ferroelectric spatial light modulator. A 5-dB discrimination was obtained between a 10-image training set of highly correlated characters (E's and F's) over a scale range of 1.0 to 1.5.