Kristina M. Johnson

High-speed binary optically addressed spatial light modulator

We describe the structure and operating characteristics of a high‐speed optically addressed spatial light modulator (OASLM) with a hydrogenated amorphous silicon (a‐Si:H) photosensor and a ferroelectric liquid‐crystal modulator. The photosensor is a p‐i‐n photodiode, which switches the liquid crystal into one of two stable states. Under a write‐light intensity of 6 mW/cm2, the OASLM exhibits a response time of 155 μs, a contrast ratio of 20:1, and a resolution of 40 lp/mm. The writing sensitivity per pixel is 0.1 pJ.

Maximized photorefractive holographic storage

Recording angularly multiplexed holograms in photorefractive media requires an understanding of both the recording and erasure dynamics. In this paper a coupled‐wave analysis is used to describe both the recording and erasure processes. This analysis has been applied to the recording of multiplexed holograms, resulting in a procedure to record maximum diffraction efficiency holograms. Experimental results confirming the theory for both the dynamics of a single exposure and the efficiencies of multiple exposures are presented. Using an uncoupled recording wave analysis, an expression for the dependence of the diffraction efficiency on the number of exposures in the case of equal record/erase time constants is presented. Approximate results are presented for the case of unequal time constants. This serves to set an upper limit on the diffraction efficiencies for a given saturation index of refraction modulation.

Optical interconnection network using polarization-based ferroelectric liquid crystal gates

A polarization-based 4 x 4 optical interconnection network using surface stabilized ferroelectric liquid crystal (SSFLC) gates is demonstrated. The SSFLC gates are comprised of an SSFLC device sandwiched between two polarizing beam splitters. Optical crosstalk using these fast switching programmable devices can be limited to ~-20 dB/gate, which would allow 2-D interconnection networks to be fabricated with thirty-one input channels and 3-D interconnection networks with approximately 225 input channels.

High-speed continuously tunable liquid crystal filter for WDM networks

We describe a high-speed wavelength tunable liquid crystal filter which can be utilized as the tuning element at the receiving end of wavelength division multiaccess (WDMA) optical networks. The filter uses chiral smectic A electroclinic liquid crystals as the active cavity material in a Fabry-Perot etalon in order to obtain a microsecond switching speed. Using the commercially available BDH764E liquid crystal material, we demonstrate a tunable optical filter both in a bulk Fabry-Perot and a fiber Fabry-Perot (FFP) configuration. A bandwidth of about 0.7 mm and effective finesse of 70 were obtained in the FFP configuration. A FFP tuning range of 13 nm with a switching time of less than 10 /spl mu/s were measured at the operating wavelength of 1.55 /spl mu/m. A theoretical analysis of the expected filter performance in the FFP configuration is given. Diffraction in the Fabry-Perot cavity is identified as the dominant loss factor, resulting in reduced throughput and finesse broadening. It is calculated theoretically that an effective finesse of 130 and a throughput loss of 2.2 dB are achievable for a mirror finesse of 200 and a liquid crystal cavity thickness of 5 /spl mu/m. A short waveguide piece is assumed to be included in the cavity. Other expected loss sources for the filter in the FFP configuration have been calculated, showing negligible effect on the filter performance.

256 × 256 liquid-crystal-on-silicon spatial light modulator

A 256 × 256 pixel spatial light modulator (SLM) is designed and constructed by the use of liquid-crystalon-silicon technology. The device is a binary electrically addressed SLM with a measured zero-order contrast ratio of 70:1 and an imaged contrast ratio of 10:1. The pixel pitch is 21.6 µm, which gives an array size of 5.53 mm. The electronic load time is 43 µs, and the 10%-90% switching time of the liquid crystal is ~75-80 µs at room temperature, which implies a maximum frame rate of ~8.3 kHz. We discuss the design trade-offs that are intrinsic to this type of device and describe how the primary application for the device in an optical correlator influenced the final design.

Optical Computing And Image Processing With Ferroelectric Liquid Crystals

High-contrast, submicrosecond switching ferroelectric liquid crystal spatial light modulators have many applications to optical computing and image processing. In this paper we describe the use of these low-voltage, low-power, and bistable devices to perform a variety of functions including polarization- and intensity-based logic gates, input/output displays, optical crossbars, and spatial filtering masks.

An Approach to the Design of Ferroelectric Liquid Crystals with Large Second Order Electronic Nonlinear Optical Susceptibility

Abstract Ferroelectric liquid crystal thin films in the Clark-Lagerwall surface-stabilized geometry exhibit well known spontaneous polar orientation of functional groups. The symmetry of the system thus allows the existence of bulk electronic second order nonlinear hyperpolarizability x(2) within thc context of the simple dipolar model. For all FLC materials examined to date, however, the magnitude of x(2) is small, presumably since the particular functional group arrays oriented along the polar axis possess small molecular hyperpolarizdbility β. Using the Boulder Model for the molecular origins of the polar order occurring in FLC films, it is possible to design materials with functionalized aromatic rings oriented along the polar axis of the film. Since such functional arrays may show respectable values of β, it should be possible to obtain FIX films (both low molecular weight and polymeric) with useful magnitude of x(2) using this approach. Results of initial experiments aimed at design of FLC materials...

Motivations for using ferroelectric liquid crystal spatial light modulators in neurocomputing

Spatial light modulators can be used in neurocomputing as input and output display devices and storage media for the synaptic weights. We discuss the operating characteristics of a new class of spatial light modulator that utilize ferroelectric liquid crystals and their application to building optical neural network architectures.

Ferroelectric liquid-crystal tunable filter

A new discrete-channel tunable Lyot filter that uses ferroelectric liquid-crystal (FLC) smectic C* wave plates is described and demonstrated. The device is tuned by changing the retardation of each birefringent element. This is achieved by electrically rotating the FLC wave plates in each stage by 45 degrees , which effectively changes the design wavelength of the Lyot filter. The transmission characteristics of such a device are demonstrated in a three-stage filter and show good agreement with theory. Advantages of the FLC tunable filter over existing filter structures include low switching voltages (+/-10 V), rapid tunability (~100 kHz), potentially high transmission and wide field of view, and large entrance aperture.

High‐speed analog spatial light modulator using a hydrogenated amorphous silicon photosensor and an electroclinic liquid crystal

A high‐speed optically addressed spatial light modulator is described which uses the electroclinic effect in chiral smectic A liquid crystals with a p‐i‐n photodiode of hydrogenated amorphous silicon (a‐Si:H). The near microsecond response time is a function of the liquid‐crystal mixture and temperature. We present and analyze optical modulation measurements of a device which exhibits a response time of 40 μs at 29 °C and 4 μs at 50 °C. The optical response is continuous and linear with electric field and write‐light intensity, allowing for grey level applications.