Vassili Sergan

Wide-Angle Achromatic Prism Beam Steering for Infrared Countermeasure Applications

The design and analysis of achromatic doublet prisms for use in laser beam steering is presented. The geometric relationships describ- ing the maximum steering angle are given, as are discussions of first- and second-order dispersion reduction. Infrared (IR) material alterna- tives and optimum IR material characteristics for wide-angle achromatic prism beam steering are also investigated. Sixteen materials in 120 dif- ferent combinations have been examined to date. For midwave IR appli- cations it is shown that the minimum dispersion currently achievable across the full 2 to 5 mm spectrum is 1.7816 mrad at an average maxi- mum steering angle of 45 deg. This is accomplished using LiF/ZnS dou- blet prisms. Several issues related to the azimuth and elevation angles into which light is steered as a function of prism rotation angles are also presented.

Liquid crystal on silicon (LCOS) wavefront corrector and beam steerer

A spatial light modulator, which is capable of high-resolution wavefront compensation and high accuracy beam steering, has been demonstrated using a Liquid Crystal On Silicon (LCOS) microdisplay with 1024×768 XGA resolution. When the device is used as a wavefront corrector, about 18.7 waves (peak-to-valley at 632.8nm) of aberration in the optical system is corrected to a residual of 1/9 wave (peak to valley) or 1/30 wave rms. Measurement of the far field beam profile confirmed the strehl ratio improved from 0.006 with the wavefront correction off, to a strehl ratio of 0.83 after correction. An additional linear phase ramp was added to the correction phase ramp to simultaneously correct and steer the laser beam. We demonstrated we can steer the beam continuously in the range of ±4 mrad in X-Y plane, with a steering accuracy better than 10μrad, or about 1/10 the diffraction limited beam divergence. The quality of the steered beam remains very high during the steering as the ellipticity of beam is smaller than 1±0.04, focused beam waist is 1.3x the diffraction limited beam waist and strehl ratio remains higher than 0.66. The 1-D beam steering efficiency is 80% at the maximum steering angle of 4 mrad, which agrees very well with our Finite Difference Time Domain (FDTD) simulation result of diffraction efficiency 86% at maximum steering angle. These results suggest that an LCOS device can be used to achieve very high-resolution wavefront control at very high efficiency.

Surface Localized Polymer Aligned Liquid Crystal Lens

The surface localized polymer alignment (SLPA) method allows complete control of the polar pretilt angle as a function of position in liquid crystal devices. In this work, a liquid crystal (LC) cylindrical lens is fabricated by the SLPA method. The focal length of the LC lens is set by the polymerization conditions, and can be varied by a non-segmented electrode. The LC lens does not require a shaped substrate, or complicated electrode patterns, to achieve a desired parabolic phase profile. Therefore, both fabrication and driving process are relatively simple.

Spatial and Orientational Control of Liquid Crystal Alignment Using a Surface Localized Polymer Layer

We present an alignment method for the surface contacting liquid crystal (LC) director. This method allows complete control of the polar pretilt angle as a function of position in a liquid crystal device, and has the potential of controlling the azimuthal orientation of LC. Important considerations of this method are to form a thin layer of reactive monomers at the LC cell interior surface, and to control the deleterious effects of flow due to polymerization induced concentration gradients. To achieve these, the voltage and frequency of the applied electric field and the UV intensity during the polymerization process are significant.

The transition mechanism of the transient planar to planar director configuration change in cholesteric liquid crystal displays

In cholesteric liquid crystals with a positive dielectric anisotropy, the relaxation from the electric field-aligned director configuration to the stable zero field director configuration proceeds via a metastable transient planar director configuration which has a pitch distinct from the equilibrium state. The transition from the transient planar to the equilibrium zerofield state is shown here to occur via a Helfrich-type instability which continuously leads to an in-plane helical structure. The equilibrium planar state is then seen to grow continuously from the in-plane state, leaving behind walls whose length then spontaneously shrinks.

CHARACTERISTIC TIMES IN THE HOMEOTROPIC TO PLANAR TRANSITION IN CHOLESTERIC LIQUID CRYSTALS

The effect of surfaces on the relaxation from the homeotropic state to the planar state of cholesteric liquid crystals is investigated. By using an optical retro-reflection technique, the orientation and pitch of cholesteric helices as a function of time have been isolated for various surface treatments. It is found that the pitch relaxes to its minimum value in a matter of milliseconds, independent of surface treatment. Secondary pitch relaxations take place after this. The time dependence of the angular distribution of helical axes is also found to be complex, operating on several time scales.

In situ control of surface molecular order in liquid crystals using a localised polymer network and its application to electro-optical devices

A liquid crystal (LC) alignment technique has been developed that allows local control of the polar pretilt angle over the range of 0–90°. This was achieved through the formation of a polymer network localised in the vicinity of the LC cell substrates. The network was formed as a result of in situ UV-induced polymerisation of a photo-reactive monomer added at concentrations of 0.5–1%. Localisation of the polymer network at the LC–substrate boundary was achieved by the application of a high voltage before polymerisation. The resultant pretilt angle was determined by the voltage applied during the polymerisation and/or the duration of the voltage application before the polymerisation step. The desired pretilt angle could be set over a small area of the sample, which allows the fabrication of LC devices with spatially variable optical retardation. Using this method we fabricated a converging lens, a bi-prism, and a phase diffraction grating with resolution greater than 50 lines mm−1.

High-Performance Active Liquid Crystalline Shutters for Stereo Computer Graphics and Other 3-D Technologies

Stereoscopic computer displays create a 3-D image by alternating two separate images for each of the viewers eyes. Field-sequential viewing systems supply each eye with the appropriate image by blocking the wrong image for the wrong eye. In our work, we have developed a new mode of operation of a liquid crystal shutter that provides for highly effective blockage of undesired images when the screen is viewed in all viewing directions and eliminates color shifts associated with long turn-off times. The goal was achieved by using a pi-cell filled with low-rotational-viscosity and high-birefringence fluid and additional negative birefringence films with splay optic axis distribution. The shutter demonstrates a contrast ratio higher than 800:1 for head-on viewing and 10:1 in the viewing cone of about 45deg. The relaxation time of the shutter does not exceed 2 ms and is the same for all three primary colors