Real-time holographic vibrometry with a ferroelectric liquid-crystal spatial light modulator

A fast ferroelectric liquid-crystal spatial light modulator, originally developed for optical computing, has found a new application in vibrometry. A new scheme of vibration-synchronized double-exposure holographic interferometry is proposed that makes full use of the speed of the ferroelectric liquid-crystal spatial light modulator. Preliminary experiments were performed that demonstrate virtually continuous real-time vibrometric data acquisition.     

Real-time optical aberration correction with a ferroelectric liquid-crystal spatial light modulator

Real-time correction of an optically aberrated wave front by use of a 10 x 10 ferroelectric liquid-crystal spatial light modulator as the correction device and a point-diffraction interferometer as the wave-front sensor is demonstrated. This type of interferometer requires no reference arm and so can be used, in theory, in an astronomical adaptive-optics system. We discuss some of the unusual features of the point-diffraction interferometer for wave-front sensing.     

Holographic switch with a ferroelectric liquid-crystal spatial light modulator for a large-scale switch

A holographic switch with a ferroelectric liquid-crystal spatial light modulator is proposed for large switching systems such as those used in subscriber networks. Preliminary experiments have achieved a one-input, 48-output switch. The relationship between the power of the control-light source and the number of outputs is calculated; the results agree well with the experiment. The calculation suggests that 10384-output switching can be obtained with a 25-mW control-light source. It should therefore be possible to control a large-scale switch with low-power control-light sources.     

Real-time binary phase holograms on a reflective ferroelectric liquid-crystal spatial light modulator

A ferroelectric liquid-crystal spatial light modulator with an active silicon backplane is used to implement reconfigurable reflective phase holograms. Optical results are presented for an optimized computergenerated Fourier hologram.     

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.     

Real-time phase-difference amplification with a liquid-crystal spatial light modulator

We describe a simple system for achieving real-time phase-difference amplification of interferograms. We arrange the interferogram such that it contains high-spatial-frequency carrier fringes and project it onto the write side of an optically addressed phase-only spatial light modulator. The resultant phase pattern on the modulator is read out by two readout beams, and diffraction by the carrier fringes provides the spatial heterodyning that is necessary for achieving phase-difference amplification. We present results that demonstrate real-time phase-difference amplification by as much as a factor of 10.     

Two-dimensional polarization encoding with a phase-only liquid-crystal spatial light modulator

We show how to two dimensionally encode the polarization state of an incident light beam using a parallel-aligned liquid-crystal spatial light modulator (LCSLM). Each pixel of the LCSLM acts as a voltage-controlled wave plate and can be programmed over a 2pi phase range at a wavelength of 514.5 nm. Techniques are reviewed for either rotating the major axis of elliptically polarized light or for converting an input linearly polarized beam into an arbitrary elliptically polarized beam. Experimental results are demonstrated in which we generate various two-dimensional spatial patterns of polarized light. Several potential applications are suggested. We also report an unexpected edge-enhancement effect that might be useful in image processing applications.     

Measurement of the spatial phase modulation of a ferroelectric liquid-crystal modulator

Ferroelectric liquid crystal is used as a dynamic light-wave phase modulator. A device that performs phase-only modulation is investigated, especially with regard to the boundary effect caused by the inevitable gap between controlling electrodes. The dependence of cell retardation and polarizer alignment is discussed. Two phase-retrieving algorithms are used to determine the complex lightwave front after traversing the device. In operation it was found that the electrode gap region was partly controlled.     

Aberration production using a high-resolution liquid-crystal spatial light modulator

Phase-only liquid-crystal spatial light modulators provide a powerful means of wavefront control. With high resolution and diffractive (modulo 2pi) operation, they can accurately represent large-dynamic-range phase maps. As a result, they provide an excellent means of producing electrically controllable, dynamic, and repeatable aberrations. However, proper calibration is critical to achieving accurate phase maps. Several calibration methods from previous literature were considered. With simplicity and accuracy in mind, we selected one method for each type of necessary calibration. We augmented one of the selected methods with a new step that improves its accuracy. After calibrating our spatial light modulator with our preferred methods, we evaluated its ability to produce aberrations in the laboratory. We studied Zernike polynomial aberrations using interferometry and Fourier-transform-plane images, and atmospheric aberrations using a Shack-Hartmann wavefront sensor. These measurements show the closest agreement with theoretical expectations that we have seen to date.     

Phase calibration and applications of a liquid-crystal spatial light modulator

A simple phase-characterization method for spatial light modulators is proposed. The low-cost method permits high-precision measurement and provides data for the setting of the spatial-light-modulator operating point in the phase-modulation mode. The dynamic phase response is used to perform efficient kinoform recording. In order to record the kinoform, we modify the global iterative coding to compute phase holograms. Finally, modified phase-phase correlation is introduced. The phase-phase correlator permits sharper correlation peaks, better energy transmission, and higher discrimination than an amplitude-phase correlation. Optical experimental results are presented.     

Programmable color liquid-crystal television spatial light modulator: transmittance properties and application to speckle-correlation displacement measurement

Drive electronics developed for a color liquid-crystal television (LCTV) display enable data to be written onto individual pixels. Display transmittance characteristics obtained with the new and the original TV drive electronics are compared. The enhanced performance obtained through this development has some potential for spatial light modulator applications in color, optical information processing based on the low-cost LCTV. As an example, we describe a novel, to our knowledge, speckle metrology technique used to display fringes and to output correlation peaks resulting from in-plane object displacement. This requires only a single LC display to encode, simultaneously in three pixel colors, speckle and fringe patterns for real-time measurements. Relative merits of this technique, including displacement range and temporal resolution, are discussed.     

Real-time velocity measurement by the use of a speckle-pattern correlation system that incorporates a ferroelectric liquid-crystal spatial light modulator

We describe a technique for noncontact velocity measurement by using double-exposure speckle-pattern techniques with optical signal processing. The two speckle patterns are recorded on a ferroelectric liquid-crystal (FLC) spatial light modulator (SLM), which is a bistable optically addressed SLM, and the composite pattern is then analyzed by an optical system similar to a joint transform correlator, in which another FLC-SLM and a position-sensitive detector are used. We show that the performance of the system can be significantly improved by adjusting the time between exposures using a real-time feedback system that is based on the position of the correlation spot in the output plane.     

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.     

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.     

Wave-front correction and production of Zernike modes with a liquid-crystal spatial light modulator

I describe a nematic liquid-crystal spatial light modulator that can be used as a high-precision wave-front control device. I present results showing the open-loop correction of wave-front aberrations and demonstrate wave-front shaping by the production and quantification of the first 15 significant Zernike terms.     

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.

     

Polarization properties of a nematic liquid-crystal spatial light modulator for phase modulation

The polarization properties of a nematic zero-twist liquid-crystal (NLC) spatial light modulator (SLM) were studied. A large ratio between the liquid-crystal (LC) layer thickness and the pixel pitch combined with spatial variations in the applied electric field causes fringing fields between pixels. Depending on the LC alignment, the electric field components within the LC layer can result in a twist deformation. The produced inhomogeneous optical anisotropy affects the polarization of light propagating through the device. We experimentally examined polarization effects in different diffraction orders for both binary and blazed phase gratings. Simulations of the LC deformation together with finite-difference time-domain simulations for the optical propagation were used to calculate the corresponding far-field intensities. It was demonstrated how rigorous simulations of the NLC SLM properties can be used to understand the polarization features of different diffraction orders.     

Application of a liquid-crystal spatial light modulator for brightness adaptation in microscopic topometry

Three-dimensional optical topometry of technical surfaces becomes increasingly important for the control of industrial processes. However, the local reflectance of the surface of the investigated sample often varies within a wide range, making accurate measurements by fringe projection difficult. We demonstrate the use of a liquid-crystal spatial light modulator as the fringe-generating element in a standard stereo microscope. With this device the brightness of the projected patterns can be adapted pixelwise. This technique leads to a significant improvement of the results of our measurements with a phase-shifting algorithm.     

Design and characterization of a liquid-crystal spatial light modulator for a polarization-insensitive optical space switch

We report preliminary results concerning a free-space optical switch between single-mode fibers with a ferroelectric liquid-crystal (FLC) spatial light modulator (SLM). In particular, we show experimentally that such a device can operate in a polarization-insensitive manner. The influence of the geometrical and physical features of the FLC SLM on the overall performance of the optical fiber switch are also discussed.     

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)].