Wavefront control with a spatial light modulator containing dual-frequency liquid crystal

Nearly every new large-scale laser system application at LLNL has requirements for beam control which exceed the current level of available technology. For applications such as inertial confinement fusion, laser isotope separation, laser machining, and laser the ability to transport significant power to a target while maintaining good beam quality is critical. There are many ways that laser wavefront quality can be degraded. Thermal effects due to the interaction of high-power laser or pump light with the internal optical components or with the ambient gas are common causes of wavefront degradation. For many years, adaptive optics based on thing deformable glass mirrors with piezoelectric or electrostrictive actuators have be used to remove the low-order wavefront errors from high-power laser systems. These adaptive optics systems have successfully improved laser beam quality, but have also generally revealed additional high-spatial-frequency errors, both because the low-order errors have been reduced and because deformable mirrors have often introduced some high-spatial-frequency components due to manufacturing errors. Many current and emerging laser applications fall into the high-resolution category where there is an increased need for the correction of high spatial frequency aberrations which requires correctors with thousands of degrees of freedom. The largest Deformable Mirrors currently availablemore » have less than one thousand degrees of freedom at a cost of approximately $1M. A deformable mirror capable of meeting these high spatial resolution requirements would be cost prohibitive. Therefore a new approach using a different wavefront control technology is needed. One new wavefront control approach is the use of liquid-crystal (LC) spatial light modulator (SLM) technology for the controlling the phase of linearly polarized light. Current LC SLM technology provides high-spatial-resolution wavefront control, with hundreds of thousands of degrees of freedom, more than two orders of magnitude greater than the best Deformable Mirrors currently made. Even with the increased spatial resolution, the cost of these devices is nearly two orders of magnitude less than the cost of the largest deformable mirror.« less

Recent advances in the shaping of ultrafast optical waveforms using liquid crystal (LC) spatial light modulators (SLM) are presented. Two LC SLMs are used in a novel arrangement to produce programmable waveforms with specified time-dependent amplitude and temporal phase profiles with the greatest fidelity and complexity to date. The apparatus is also used to demonstrate the generation of an ultrafast waveform with a programmable time-dependent polarization profile. A general theoretical result that describes the space-time electric field profile of waveforms shaped by the spectral filtering of spatially separated frequency components is also presented. The main result is that diffraction gives rise to a translational spatial shift in the electric field profile that varies linearly with time along the shaped waveform.

Synthesis and reconstruction of computer generated holograms by a double pass technique on a twisted nematic-based liquid crystal spatial light modulator

Holographic stereograms are an important way of generating 3-D images from photographs of real objects or of computer-generated 3-D objects. We have made holographic stereograms using several commercially available liquid crystal spatial light modulators, which have been modified in some cases, to produce transmission masks for a video image. The spatial light modulator is used to form the image in producing holographic stereograms. The image source has been either a video camera (or VCR) or the video signal from a computer graphics terminal. The use of the liquid crystal spatial light modulator eliminates the photographic intermediary in the process of forming the holographic stereograms. This technique speeds the process of forming holographic stereograms, increases automation of the process, and reduces the time required in creating the hologram. The major limitations of this technique are the contrast and resolution of the spatial light modulators which are commercially available and of low cost. Since the spatial light modulator technology is rapidly advancing, better performance in both of these areas is expected. Therefore, we believe that this technique offers an attractive means of producing holographic stereograms, especially from computer graphics, in situations where a short cycle time is important.

At present time methods of optical encryption are actively developed. The majority of existing methods of optical encryption use not only light intensity distribution, easily registered with photosensors, but also its phase distribution which require application of complex holographic schemes in conjunction with spatially coherent monochromatic illumination. This leads to complex optical schemes and low decryption quality. To eliminate these disadvantages it is possible to implement optical encryption using spatially incoherent monochromatic illumination which requires registration of light intensity distribution only. Encryption is accomplished by means of optical convolution of image of scene to be encrypted and encryption diffractive optical element (DOE) point spread function (PSF) which serves as encryption key. Encryption process is described as follows. Scene is illuminated with spatially-incoherent monochromatic light. In the absence of encryption DOE lens forms image of scene in photosensor plane. DOE serves as encryption element, its PSF - encryption key. Light passing through DOE forms convolution of object image and DOE PSF. Registered by photosensor convolution is encrypted image. Decryption was conducted numerically on computer by means of inverse filtration with regularization. Kinoforms were used as encryption DOE because they have single diffraction order. Two liquid crystal (LC) spatial light modulators (SLM) were used to implement dynamic digital information input and dynamic encryption key change. As input scene amplitude LC SLM HoloEye LC2002 with 800×600 pixels 32×32 μm² and 256 gray levels was used. To image synthesized encryption kinoforms phase LC SLM HoloEye PLUTO VIS with 1920×1080 pixels 8×8 μm² and 256 phase levels was used. Set of test images was successfully optically encrypted and then numerically decrypted. Encrypted images contents are hidden. Decrypted images despite quite high noise levels are positively recognizable. Results of optical encryption and numerical decryption are presented.

Liquid crystal spatial light modulators are emerging as a potential replacement to traditional optical beam steering methods. The performance of these devices for optical communication systems in the radiation environment for geostationary orbits (GEO) are of interest for applications in the next generation of satellites. As an initial investigation to the study presented, several liquid crystals were irradiated to total dose levels consistent with expected GEO environments. While prior irradiation work has been done on spatial light modulators none is known to include a first look at a liquid crystal and CMOS backplane. Parameters of retardation, contrast ratio and primary power current were monitored at incremental stages during the test and are presented.

To correct wave front distortion, a phased only Liquid Crystal Spatial Light Modulator (LCSLM) is used. LCSLM can modulate the phase of the incident beam by changing the electric field applied to the transparent electrodes of a liquid crystal cell. The voltage change causes the refractive index of the liquid crystal change, so a phase shift produces. The distortion of a wave front can be measured by using a wave front interferometer. Since each pixel of SLM may be addressed and controlled independently, so proper voltage is supplied to each pixel to compensate the phase corresponding to the distortion. Before wave front correction, the LCSLM is tested and corrected by employing a Twyman-Green interferometer, which shows the relationship between the applied voltage and the phase value of each pixel. Experiment result shows that after error compensation of LCSLM, the wave front distortion correction accuracy reaches 0.06 λ.

Phase-only liquid crystal spatial light modulator (SLM) has been widely used for accurate phase-manipulation in holographic display, optical tweezers, lithography, etc. Due to the nonlinear optical response of liquid crystal and manufacturing defects, the grayscale-phase response could be different for every single SLM device. The calibration for phase modulation is needed for improving the performance of the SLM involved system. The principles of phase-only SLM are reviewed. The main phase calibration methods for SLM are discussed and compared.

Liquid crystal (LC) based spatial light modulators (SLM) offer a low cost alternative to the expensive deformable mirrors used in most adaptive optics systems. Real time correction of simulated atmospheric turbulence is demonstrated. The corrective element is a 10 by 10 pixel ferroelectric liquid crystal (FLC) SLM. An improvement in the Strehl ratio from 23% to 33% with a frame rate of 6Hz was demonstrated. A nematic liquid crystal (NLC) SLM was used to generate realistic atmospheric turbulence.

Resolution improvement of surface plasmon-enhanced, liquid crystal spatial light modulator: Simulation studies

The performance characteristics of three unique optically-addressed spatial light modulator (SLM) structures have been experimentally investigated. The measured parameters include maximum resolution, visibility, imaging response time, and write light sensitivity. The modulators investigated include two relatively new technologies, the ferroelectric liquid crystal (FLC) SLM from the University of Colorado-Boulder and the amorphous silicon photoconductive twisted nematic liquid crystal SLM from GEC-Marconi Research, and a well established industry benchmark, the Hughes Liquid Crystal Light Valve (LCLV). A comparison of these devices in terms of the above performance parameters is discussed in this communication.

In space optical communication, owing to the influence of atmospheric turbulence, optical beams lose focus and become phase-distorted, which reduces the communication quality. Considering the polarization dependence of liquid crystal spatial light modulators and the dispersion effect of liquid crystal materials, the energy utilization rate of liquid crystal adaptive optics systems is low. In this study, a dual liquid crystal spatial light modulator adaptive optics system based on the GS algorithm is used to correct the wavefront distortion of a signal beam under different atmospheric turbulence intensities, and the Strehl ratio (SR) is used as the evaluation index. The simulation results show that the SR of the corrected system can be increased from 0.23, 0.41, and 0.72 to 0.77, 0.89, and 0.95, respectively. The corrected beam spot was more concentrated and the light intensity at the center of the beam spot was stronger. The experimental results show that, after the distortion wavefront is corrected by the dual liquid crystal spatial light modulator, the average gray value of the 10 × 10 pixels in the center of the spot increases from 159.3, 113.1, and 58.4 to 253.4, 247.7, and 198.3, respectively.

A simulation platform is set up to study the ship-borne laser communications. A liquid crystal spatial light modulator is used as a non-mechanical beam steering control device to simulate beam motions of moving platforms. Because of its light weight, low power and compact form, the liquid crystal spatial light modulator is superior to its mechanical counterparts. The theory of beam steering is introduced firstly. And then the performance of the simulation platform is researched and evaluated. The steering angles range from -2.89° to 2.89° with the control precision of ~ 0.02°. The simulated sinusoidal frequency of the spatial light modulator can reach 4 Hz in maximum. Finally, the beam motions of the ship platforms with different natural rolling periods are simulated based on the platform. The correlation coefficients between the theoretical and the simulated motion curves are greater than 0.9805. Results show that it is feasible to realize the beam motions simulation of specific moving platforms using the liquid crystal spatial light modulator.

ABSTRACTLiquid crystals offer an unique versatile platform for optical applications and light manipulation. Indeed, besides the well-known realization of liquid crystal displays, liquid crystal-based technologies and, particularly, spatial light modulators (SLM) allow achieving efficient control of the phase and amplitude of optical beams at various wavelengths and optical powers. Thanks to their transparency in a wide range of the electromagnetic spectrum, optical selectivity and high birefringence, liquid crystals offer the possibility of implementing optical functions, as spectral filters, light shaping, delay lines and phase shifter, in a wide range of optical wavelengths and input powers depending on the particular configuration and materials considered. In this review, we present an overview of our achievements on liquid crystal light valves, optically addressed SLM that combine liquid crystals with a photosensitive material in order to optimize the electro-optical response of the medium as a whole. We show the ability of light valves in behaving as nonlinear optical media where optical wave mixing can be performed with various interaction schemes. Different applications, such as slow light, interferometry, optical sensing and dynamic holography, are highlighted.

It is proposed in this paper to use phase only liquid crystal spatial light modulators to realize non-mechanical lateral and axial confocal microscopic laser scanning. With a phase only liquid crystal spatial light modulator used as a scanner to realize laser beam deflection, a confocal microscopic lateral beam scanning system is designed. A zoom illuminating lens is formed by incorporating a liquid crystal spatial light modulator along the confocal illumination light path, and thus the focus of the objective lens is axially shifted to realize the axial non-mechanical scanning. The theoretical analyses indicate that phase only liquid crystal spatial light modulators can be used to realize non-mechanical 3D confocal microscopic laser scanning.