Nonlinear optical correlators: exploiting phase-with-constrained- magnitude filter devices

There is a lot of interest in technology that controls the phase of light two dimensionally, such as laser waveform shaping and optical phase compensation. And a spatial light modulator is highly desired for practical use of these technologies. We have developed several types of spatial light modulators for years. The history of our research and development of spatial light modulators is shown here.

It is increasingly recognized that the ability for optical techniques to provide parallel processing may enable us to greatly enhance the speed capability of computing. To satisfy requirements of parallel optical processing many types of spatial light modulators (SLMs) are now being investigated. In this paper we discuss a SLM with internal memory which is electrically addressable and digitally operated.

Lack of mutual occlusion capability between computer-rendered and real objects is one of fundamental problems for most existing optical see-through head-mounted displays (OST-HMD). Without the proper occlusion management, the virtual view through an OST-HMD appears “ghost-like”, floating in the real world. To address this challenge, we have developed an innovative optical scheme that uniquely combines the eyepiece and see-through relay optics to achieve an occlusion-capable OST-HMD system with a very compelling form factor and high optical performances. The proposed display system was based on emerging freeform optical design technologies and was designed for highly efficient liquid crystal on silicon (LCoS) type spatial light modulator (SLM) and bright Organic LED (OLED) microdisplay. The proposed display technology was capable of working in both indoor and outdoor environments. Our current design offered a 1280x1024 color resolution based on 0.8 microdisplay and SLM. The MTF values for the majority of the fields at the cutoff frequency of 40lps/mm, which is determined by the pixel size of the microdisplay, are better than 15%. The design achieved a diagonal FOV of 40 degrees, 31.7 degrees horizontally and 25.6 degrees vertically, an exit pupil diameter of 8mm (non-vignetted), and an eye clearance of 18mm. The optics weights about 20 grams per eye. Our proposed occlusion capable OST-HMD system can easily find myriads of applications in various military and commercial sectors such as military training, gaming and entertainment.

The performance of Fourier transform optical processors, e.g. optical correlators, beam steering systems, associative memories, etc., depends intimately both on the physical characteristics of the particular spatial light modulator (SLM) and on the particular algorithms that map the signal into the available modulation range of the device. For the most general Fourier systems the information/signal is complex-valued. This is an essential requirement for multi- spot beam steering systems and composite pattern recognition filters. Since practical and/or affordable SLM's only represent a limited range of values in the complex plane (e.g. phase-only or quantized phase), numerous approaches have been proposed and demonstrated for representing, approximating, encoding or mapping complex values to the available SLM states. The best approach depends on the space bandwidth product of the signal, the number of SLM pixels, the computation time of the encoding algorithm, the time available for the application, and the quality of the optical processor, as measured by an application-specific performance metric. Based on the low pixel count and the high cost per pixel of most current SLM's we argue for encoding algorithms that map one signal value to one pixel value, as opposed to group-oriented encoding. This maximized the usable area of the frequency plane. We also recommend algorithms that maximize the fidelity over the entire frequency range as opposed to maximum diffraction efficiency/minimum mean squared error design. These ideas are illustrated with several simulated and experimental results for pseudorandom, minimum Euclidean distance, error diffusion and hybrid/blended encoding algorithms.

The behavior of a binary, spatial light modulator (SLM) in both the input and Fourier planes of a coherent optical correlator system is investigated using both a computer model and a working laboratory correlator. The computer model, based upon the fast Fourier transform, simulates the effect of the SLMs in the system and permits the examination of such parameters as input noise and shifts and their effect upon the theoretical output. The experimental correlator contains a SLM in the input plane operating in the binary amplitude mode and a SLM in the Fourier plane operating in the binary phase-only mode. The output of the system is compared to the model and analyzed. Significant performance results obtained with the computer model are also examined.

Most state-of-the-art optical see-through head-mounted display (OST-HMD) lacks mutual occlusion capability between computer-rendered and real objects so that the virtual view through an OST-HMD appears "ghost-like", floating in the real world. In this paper, we demonstrated a light-weight, compact OST-HMD with mutual occlusion capability by exploring a highly innovative optical approach based on emerging freeform optical design and fabrication technologies. Our approach enabled us to achieve an occlusion-capable OST-HMD system with a very compelling form factor and high optical performance. The proposed display technology is designed for highly efficient liquid crystal on silicon (LCoS) type spatial light modulator (SLM) and bright Organic LED (OLED) microdisplay, which is capable of working in both indoor and outdoor environments. Our current design offered a 1280×1024 color resolution with a field of view (FOV) of 40 degrees and lightweight optics about 30 grams per eye.

A new type of spatial light modulator, based on the surface plasmon resonance effect, is proposed and demonstrated. The theory and design of these devices is discussed, as are their special features and performance potential in coherent optical systems. The prototype devices use liquid crystal as the active material, and encouraging results have been obtained for speed (2 ms), contrast (100:1), and resolution (20 μm). Higher performance devices using other structures are proposed; these include multi-quantum-well diodes, Langmuir-Blodgett films, and reversible electroplating cells.

Hybrid optical implementation of discrete wavelet transforms: a tutorial

The use of a binary, magneto-optic spatial light modulator as an input device and as a spatial filter in a VanderLugt correlator is investigated. The statistics of the correlation that is obtained when the input image or the spatial filter is thresholded are estimated. Optical correlation using the magneto-optic device at the input and Fourier planes of a VanderLugt correlator is demonstrated experimentally.

A hybrid correlation system incorporates an optical correlator, spatial light modulators(SLM), digital cameras and a computer. Spatial light modulators and cameras are used to dynamically update the input and the spatial filter. The hybrid correlation system integrates the parallel processing capability of the optical correlator and the flexibility of the digital system. It can be used as a high-speed multi-function information processor. This paper focuses on the design and fabrication of a hybrid optical correlator and how it is used as an intelligent instrument. We address the issues of rigorous requirements for filter registration and matched filtering by proposing practical approaches. We include the analysis and use of intensity filters based on commercial SLMs for real-time pattern recognition. We present the engineering details of a specific hybrid optical correlator for applications to the real-time identification of an aircraft.

We present a general analytical treatment of optical correlation in correlators that use pixellated spatial light modulators with transmissive (or reflective) dead zones in both the input and filter planes. The active areas of the pixels modulate the light intensity while the dead zones transmit (or reflect) all of the light. Our model can predict the changes in the correlation peak and the signal-to-noise ratio with changes in dead zones, calculated in Part I [Appl. Opt. 32, 6527(1993)] from computer simulations. This model is also a general one: It applies to correlators in which one spatial light modulator contaions only opaque dead zones while the other contains only transmissive dead zones; it also applies to the case in which any one spatial light modulator contains both opaque and transmissive dead zones.

Diffractive optical element (DOE) design techniques can be applied to the generation of pattern recognition filters for optical correlators, in which case the spatial light modulator (SLM) in the filler plane is thought of as a real-time DOE. The designer must first select the desired complex pattern recognition filter and then apply DOE design techniques to map the fully complex function onto the limited dynamic range SLM. To illustrate the procedure, we apply the techniques to the problem of implementing a matched filter with limited dynamic range Fourier plane SLM’s. We use computer simulations to evaluate the effectiveness of the technique. We quantify the performance by measuring the mean squared error between the ideal matched output and output from quantized filters. Numerical simulations indicate that the DOE method yields a significant improvement over direct quantization. The improvement in mean squared error comes, however, at the expense of light efficiency. The DOE technique has been applied to design of matched filters with binary phase, ternary phase, and phase-only SLM’s for coherent as well as incoherent optical correlators. The additional phase freedom in the incoherent design results in improved performance, but incoherent systems are limited to correlation with real positive reference scenes. A proof-of-principle laboratory experiment has been constructed to complement the numerical simulations.

We have designed and constructed a small number of prototype spatial light modulators (SLMs) based on a pixel array of 512 x512. The digital CMOS backplane allows fast writing of binary data onto the FLC layer for optical readout. We anticipate the use of the design in coherent applications such as optical correlators and programmable holograms as well as incoherent applications such as miniature displays.

Engineering implementation of a hybrid optical correlator using SLMs

Preprocessing of the input scene can be used to improve the performance of an optical correlator. The result of the preprocessing algorithms can be bipolar real-valued images, with positive and negative values, for input to the optical correlator. We present different procedures for the implementation of the bipolar real-valued signal with spatial light modulators that work in either amplitude-only or phase-only regimes. We apply these techniques to the implementation of bipolar realvalued preprocessed input images in color pattern recognition.