Resolution and response-time dependence of ferroelectric liquid-crystal optically addressed spatial light modulators on grating profiles

We introduce a model to analyze the time response of an optically addressed spatial light modulator (OASLM) when a two-dimensional image is written on it. Comparison with experimental results is performed by use of gratings as input images. For a given spatial frequency, we show that the response time of the device depends on the grating profile. The effect of the time-constant mechanism is demonstrated theoretically and experimentally for a sinusoidal-wave and a square-wave intensity profile. An alternative explanation for the resolution limitation of the OASLM that is related to the time-constant mechanism and a new method for measuring the resolution of the device are proposed.     

Resolution in optically addressed spatial light modulators based on dye-doped liquid crystals

Dye-doped nematic liquid crystals (LCs) are studied as materials for single-layer optically addressed spatial light modulators. The dopant is 2,5-azo-substituted anthraquinone (ASAQ) dye. The resolution in the ASAQ-doped LC systems does not depend on the device thickness (in the 5-125 microm range). The efficiency increases with the increase of the thickness and begins to saturate in devices thicker than 40 microm. The limiting resolution in the thick devices is 400 line pairs per millimeter. The limitations of performance (efficiency and resolution) in the studied systems are discussed.     

Phase-aberration correction with dual liquid-crystal spatial light modulators

A phase-aberration-correction system that uses high-resolution, twisted nematic liquid-crystal spatial light modulators in a Mach-Zehnder interferometer is presented. A correction algorithm is described and experimentally verified by use initially of one liquid-crystal panel. Phase aberrations are successfully removed by a single liquid-crystal panel, but unacceptably high amplitude variation is introduced into the wave front because of the phase-amplitude coupling of the spatial light modulator. A second panel is used to remove the amplitude modulation. The modified optical system with a multiplicative architecture is described, and results are presented that show the correction of phase aberrations with an amplitude variation of less than 10%.     

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.     

Complex amplitude modulation by use of liquid-crystal spatial light modulators

We propose to study the conditions for implementation of complex amplitude modulation on standard liquid-crystal spatial light modulators when illuminated by polarized light. The spatial light modulators are used in a conventional configuration, i.e., the voltage is applied parallel to the wave-front propagation direction. The most commonly used liquid-crystal materials are compared and their usefulness in some general applications discussed. Their specificities with respect to different modulation types and application requirements are briefly described. Typical characteristics such as response time, modulation range, and wavelength insensitivity are also discussed. Finally, as an illustration, a first attempt at nomenclature is proposed for the case of a linearly polarized light.     

Inherent apodization of lenses encoded on liquid-crystal spatial light modulators

Focusing diffractive optical elements encoded in liquid-crystal spatial light modulators yields an inherent equivalent apodization of the focused spot as a result of the pixelated nature of these devices and the finite extent of each pixel. We present a theoretical explanation for and experimental evidence of this effect. We demonstrate an experimental procedure for measuring the apodization and a method to compensate for this effect.     

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.     

Dynamic holographic interconnects that use ferroelectric liquid-crystal spatial light modulators

Dynamic interconnect holograms are designed by the use of a simulated annealing algorithm and written to a 128 × 128 pixel ferroelectric spatial light modulator that is used in a binary-phase mode. Dynamic holograms are used to implement a 2 × 2 crossbar with single-mode fiber inputs and outputs, which function with as high as 27 dB of isolation between output ports. The principle is extended to two-dimensional interconnection holograms, and arbitrary fan-out to as high as 64 points is demonstrated with good performance.

Images of interconnection holograms are transferred from the spatial light modulator to an optically addressed spatial light modulator that is used in a binary-phase mode. The addition of a fixed array generator computer-generated hologram permits replication of the hologram image, thus creating a larger hologram with a high space-bandwidth product on the optically addressed spatial light modulator.

Results of a preliminary experiment are presented.

     

Modulation transfer function measurement method for electrically addressed spatial light modulators

The modulation transfer function (MTF), when used with amplitude modulation (m(A)) data, is a vital coherent optical performance measure for a spatial light modulator (SLM). A new image plane amplitude MTF (MTF(A)) measurement method is presented for electrically addressed SLMs. It involves digital analysis of the output image of a square-wave pattern written onto the SLM. Modulation-level effects are also addressed. Optical laboratory results are presented for two liquid-crystal SLMs. The need to consider amplitude rather than intensity modulation (when coherent optical processing applications are considered) is noted in terms of SLM biasing.     

电寻址空间光调制器制作灰度掩模技术的研究

介绍了制作微光学元件灰度掩模的两种方案,它们分别使用TFT-LCD(薄膜晶体管液晶显示器)和DMD(数字微镜器件)两种电寻址空间光调制器,采用并行直写和实时掩模技术,提高了灰度掩模制作的速度和灵活性。刷新率的变化和黑栅衍射效应将导致衍射效率下降,使得曝光深度误差增加。前者可延长曝光时间来消除,后者可通过高填充因子和放大滤波电路予以有效抑制。与LCD掩模相比,用DMD掩模制作的闪耀光栅的衍射效率提高了10%以上。     

Programmable multiple-level phase modulation that uses ferroelectric liquid-crystal spatial light modulators

We present a novel method of producing arbitrarily valued binary phase-only modulation from a commercially available ferroelectric liquid-crystal spatial light modulator that is used in conjunction with simple polarization components. By cascading of such stages, modulators with four and eight equally spaced phase levels are constructed with 128 × 128 pixels. Near-diffraction-limited performance, when stopped down to 64 × 64 pixels, is reported in producing simple diffraction patterns and when used to generate asymmetric spot arrays in the Fourier plane of a lens.     

Beam steering experiment with two cascaded ferroelectric liquid-crystal spatial light modulators

The design, construction, and evaluation of a laser beam steerer that uses two binary ferroelectric liquid-crystal (FLC) spatial light modulators (SLMs) operated in conjunction are presented. The system is characterized by having few components and is in principle lossless. Experimentally, a throughput of approximately 20% was achieved. The simple system design was achieved because of the high tilt angle FLC material used in the SLMs, which were specifically designed and manufactured for this study. By coherently imaging the first SLM onto the second SLM, pixel by pixel, we obtained an effective four-level phase structure with a phase step of 90 degrees. An appropriate alignment procedure is presented. The beam steering performance of the system is reported and analyzed.     

Hadamard transforms of images by use of inexpensive liquid-crystal spatial light modulators

Incoherent light imaging by Hadamard transformations by use of an inexpensive spatial light modulator from a liquid-crystal display television is demonstrated. The transforms are implemented by means of S matrices. Good-fidelity 31 x 33 pixel images are obtained. The image distortions caused by the limitations of the spatial light modulator are discussed.     

Fringe visibility improvement using an asynchronous image-subtracting optically addressed spatial light modulator

We demonstrate the application of an asynchronous image-subtraction optically addressed spatial light modulator to particle image velocimetry fringe processing. The device comprises an amorphous silicon p-i-n-i-p photosensor and a ferroelelectric liquid-crystal light-modulating layer. The images to be subtracted are encoded on two separate wavelengths. The operation of the device is described, and characterization shows a frame rate of 100 Hz, a resolution of 3 line pairs/mm, and a write-light sensitivity of ≈1 mW/cm(2) at a wavelength of 514 nm. The device is read by the use of light with a 633-nm wavelength whereas the subtraction light is at a wavelength of 670 nm. Using this device to subtract a nonuniform pedestal from the optically computed power spectral density function (the Young's fringe pattern), we find we can improve the signal-to-clutter ratio of peaks in the image-transmittance autocorrelation function of particle image velocimetry transparencies. The device also permits processing of very low-visibility fringe patterns, generated from doubly exposed images, in which one image has half the transmittance of the other. These could not be processed with a nonsubtracting, binary, liquid-crystal optically addressed spatial light modulator.     

4 x 1204 holographic switching with an optically addressed spatial light modulator

We extended the scale of a previously reported holographic switch [Appl. Opt. 34, 8137 (1995)] in which each input is assigned to one of the control light sources to control a large-scale switch with low power. The average losses of the four inputs were 28, 33, 35, and 36 dB. The average cross-talk-to-signal ratio of one of the inputs was -16 dB.     

Measurements on ferroelectric liquid-crystal spatial light modulators: contrast ratio and speed

The contrast ratio and the speed of a 16 × 16 electrically addressed spatial light modulator, composed of a ferroelectric liquid-crystal layer on top of a VLSI silicon backplane, are measured with different methods but consistent results. The results are presented and compared with recently reported results on a similar spatial light modulator [Appl. Opt. 33, 2775 (1994)].     

Open-loop control of liquid-crystal spatial light modulators for vertical atmospheric turbulence wavefront correction

We present an open-loop adaptive optics (AO) system based on two liquid-crystal spatial light modulators (LCSLMs) that profit from high precision wavefront generation and good repeatability. A wide optical bandwidth of 300 nm is designed for the system, and a new open-loop optical layout is invented to conveniently switch between the open and closed loop. The corresponding control algorithm is introduced with a loop frequency (the reciprocal of the total time delay of a correction loop) of 103 Hz. The system was mounted onto a 2.16 m telescope for vertical atmospheric turbulence correction. The full width at half-maximum of the image of the star α Boo reached 0.636 arc sec after the open-loop correction, while it was 2.12 arc sec before the correction. The result indicates that the open-loop AO system based on LCSLMs potentially has the ability to be used for general astronomical applications.     

Estimation of insertion loss of a holographic switch with an optically addressed spatial light modulator

We investigate the cause of the insertion loss of our holographic switch by determining the relation between the diffraction efficiency and the hologram patterns generated by the control beams. According to the calculation the theoretical insertion loss is 7.4 dB with loss distribution of 0.03 dB under the conditions of a previous experiment. We find that incomplete storage of the interference pattern on the optically addressed spatial light modulator is the strongest factor determining the insertion loss.     

Particle image velocimetry analysis using an optically addressed spatial light modulator: effects of nonlinear transfer function

Optical processors for generating a two-dimensional squared autocorrelation function have been presented for postprocessing particle image velocimetry photographs of fluid flows. The incoherent-tocoherent conversion can be performed by an optically addressed spatial light modulator. The transfer function of these devices is far from linear and will influence the performance of the optical processor.

Two different transfer functions, characterizing the two main types of commercial optically addressed spatial light modulators as an analog and a binary transfer function, have been simulated digitally.

Results of numerical simulations on the influence of introducing these nonlinear transfer functions to the correlation function for particle image velocimetry analysis are presented.

     

Spiking voltages for faster switching of nematic liquid-crystal light modulators

Electrical address circuits developed for driving fast-switching ferroelectric liquid-crystal spatial light modulators (SLM) can be programmed to increase the speed of much slower responding nematic liquid-crystal SLMs. Using an addressing circuit that can switch as fast as 0.164 ms, voltages are programmed for values of phase that exceed the desired phase, and when the phase reaches the desired value, the voltage is switched to the required steady-state voltage. For a SLM that has a phase range of 3.5pi and that is programmed over a 2pi range, switching speed is reduced from 400 ms to between 71 and 77 ms. The speedup algorithm is applied to each pixel of the SLM together with a digital correction for a spatially nonuniform phase.