Optically Addressed Spatial Light Modulator Research Articles

Compensation method for performance degradation of optically addressed spatial light modulator induced by CW laser

Abstract In this paper, we propose an effective method to compensate for the performance degradation of optically addressed spatial light modulators (OASLMs). The thermal deposition problem usually leads to the on-off ratio reduction of amplitude OASLM, so it is difficult to achieve better results in high-power laser systems. Through the analysis of the laser-induced temperature rise model and the liquid crystal layer voltage model, it is found that reducing the driving voltage of the liquid crystal light valve and increasing the driving current of the optical writing module can compensate for the decrease of on–off ratio caused by temperature rise. This is the result of effectively utilizing the photoconductive effect of Bi12SiO20 (BSO) crystal. The experimental results verify the feasibility of the proposed method and increase the laser withstand power of amplitude-only OASLM by about a factor of 2.5.

Reduction of response time in transmissive optically addressed spatial light modulator using solution-based ZnO NP/PEDOT:PSS heterojunction

An optically addressed spatial light modulator (OASLM) works by using the intensity of a ‘write’ light to modulate the phase of a ‘read’ light. OASLM using ZnO nanoparticles (NPs) as a photoconductor is important to holographic displays because of its record-high spatial resolution. However, its response time to the write light is long and the switch-off time (τoff) can take tens of seconds due to the trap states in the ZnO NP layer. This results in residual images and poses limitations to its application in dynamic holographic displays. In this work, a ZnO NP photoconductor was replaced by a solution-processed ZnO NP/PEDOT:PSS heterojunction photodiode and the τoff of OASLM was significantly reduced to about 0.6 s. Electrical and optical properties of the heterojunction were characterized and a barrier height of 0.604 eV was determined by CV measurement. The heterojunction-based OASLM was then fabricated and its light modulating performance was investigated by using a diffractive phase grating. Finally, impedance spectroscopy was used to analysis the device equivalent circuit and optimize the operation of the OASLM.

Large-size updatable optically addressed spatial light modulator (OASLM) based on ZnO nanoparticles for large-area holographic 3D displays

Optically addressed spatial light modulators (OASLMs) provide an appropriate solution for large-area and wide-viewing angle holographic displays because of the possibility of uploading holograms on it through tiling and with sub-micron diffraction feature sizes. A prototype with a large-size OASLM of 100 mm × 100 mm was fabricated using a solution-based deposition process for ZnO nanoparticles (NPs). ZnO NP-based OASLM is suitable for hologram tiling because of the extended charge carrier lifetime as a result of trap states in the ZnO NP layer annealed at a low temperature of 180°C, of which the activation energy was determined firstly by low-temperature measurements. Operating the ZnO NP OASLM in a DC driving mode was subsequently proposed, to utilize the extended charge carrier lifetime in the photosensor layer for temporary information storage. Finally, techniques to tile computer generated holograms (CGHs) spatially on a single OASLM were explored and evaluated, including the demonstration of simultaneous image replay from sequentially tiled two separate phase-only CGHs.

相位型光寻址空间光调制器稳定性分析

相位型光寻址空间光调制器稳定性分析

Graphene-based spatial light modulator operating at near infrared spectral range

In this paper, a near infrared operating spatial light modulator is demonstrated, assembled in a way to combine the excellent photoconductivity of Ru-doped Bi12SiO20 (BSO:Ru) crystal, strong birefringence of liquid crystal (LC) and exceptionally high transparency and conductivity of graphene. A photo-alignment method instead of mechanical rubbing is used to simplify the fabrication procedure and to prevent detachment of the graphene layer from the substrate. The proposed device operates at low driving voltage (competitive to a reference device using ITO electrodes) moreover requires much less near infrared intensity for the light modulation. It is assumed the effect is due to the photo-induced space charge exchange between BSO:Ru and graphene allowing charge redistribution and optical doping effect resulting in a modulation of graphene’s properties. The voltage-dependent transmittance and phase retardation show high contrast ratio with the response time of ∼100 ms at 1064 nm. In addition, by projecting a video image through the proposed structure the response of modulated pump light intensity is demonstrated which supports device ability to work as near infrared optically addressed spatial light modulator (OASLM). The obtained performances reveal great potentials of graphene-based electro-optic devices for near infrared applications.

光寻址空间光调制器研制进展及性能测试

The development of the optically addressed spatial light modulator (OASLM) was reported. Its structure and working principle was introduced in detail. Meanwhile, the photosensitive response curve, static wave-front distortion and temporal waveform distortion was investigated in detail in the process of experiment. Besides, OASLM was successfully demonstrated in the online experiment. The results show that OASLM not only has the ability to achieve the desired transmittance distribution, but also has no additional effect on the high-power laser system. Finally, the shaping effect of spatial filter in the laser system was analyzed. The result shows that the size of spatial filter was also an important factor which determined the shaping precision when this device was used in high power laser systems.

A high-resolution optically addressed spatial light modulator based on ZnO nanoparticles

AbstractAn optically addressed spatial light modulator (OASLM) can modulate the wavefront of a read light by displaying a phase pattern or a hologram configured by the intensity distribution of a write light. Using ZnO nanoparticles (NPs) as a novel photoconductor, a high-resolution OASLM was fabricated. A ZnO NP suspension was spin-coated on an indium tin oxide (ITO)-coated glass substrate and annealed to form a photosensitive layer. The device was characterized electrically and optically. The device was operated at low driving voltages in the transmission mode. Updatable recording of a diffraction grating up to 825 lp mm−1 with a diffraction efficiency (DE) of 0.05% and binary holograms with pixel sizes from 2 µm down to 0.72 µm were demonstrated using a 405 nm wavelength write laser and a 635 nm wavelength read laser.

Liquid crystal high-resolution optically addressed spatial light modulator using a nanodimensional chalcogenide photosensor

The fabrication and performance of an optically addressed spatial light modulator (OASLM) based on nematic liquid crystal and nanodimensional amorphous arsenic trisulfide (a-As2S3) chalcogenide glassy films are described. The photoconductive a-As2S3 layers are used both as photoalignment material and as a photosensor. The use of the OASLM as a color converter is demonstrated in the transmission mode. The phase retardation dynamic range is over 3π. Diffraction efficiency measurements show a high resolution (150 lp/mm at 50% MTF). A wide variety of materials from the chalcogenide glass (ChG) family are useful for simple fabrication of high-resolution OASLMs depending on the desired wavelength.

Chemically deposited PbS thin film photo-conducting layers for optically addressed spatial light modulators

Lead sulfide semiconducting thin films were chemically deposited on indium tin oxide coated glass plates for use as photoreceptor layers in conjugation with optically addressed spatial light modulators (OASLMs).

Integration of chalcogenide glassy films and liquid crystals for photoalignment and optically addressed modulators

AbstractThe present paper reports on our recent achievements including the photoalignment achieved of liquid crystals (LCs) using amorphous chalcogenide semiconductors (AChS) glassy films and the creation of optically addressed spatial light modulators (OASLMs). The photoalignment in this case is based on the phenomenon of photoinduced anisotropy, essentially found to be efficient in the nano‐dimentional (ND, <50 nm) AChS films while the OASLM action is based on their photosensitivity.

OPTICALLY ADDRESSED SPATIAL LIGHT MODULATORS FOR 3D DISPLAY

An optically addressed spatial light modulator (OASLM) records the image on a write beam and transfers it to a read beam. Some example application areas are: image transduction; optical correlation; adaptive optics; and optical neural networks. Current interest in OASLMs has been generated by the work of Qinetiq on 3D display. This work is based on Active tiling, where an image can be recorded in one part of the device and is memorised, whilst the remainder of the device is updated with images. This paper will explain this system and survey the technological alternatives for this application.

Multiplexed two-photon microscopy of dynamic biological samples with shaped broadband pulses

Coherent control can be used to selectively enhance or cancel concurrent multiphoton processes, and has been suggested as a means to achieve nonlinear microscopy of multiple signals. Here we report multiplexed two-photon imaging in vivo with fast pixel rates and micrometer resolution. We control broadband laser pulses with a shaping scheme combining diffraction on an optically-addressed spatial light modulator and a scanning mirror allowing to switch between programmable shapes at kiloHertz rates. Using coherent control of the two-photon excited fluorescence, it was possible to perform selective microscopy of GFP and endogenous fluorescence in developing Drosophila embryos. This study establishes that broadband pulse shaping is a viable means for achieving multiplexed nonlinear imaging of biological tissues.

A spatial light phase modulator with an effective resolution of 4 mega-pixels

We report the design, construction and characterization of a 4 mega-pixel, optically-addressed, spatial light modulator (OSLM). The intensity distribution corresponding to a kinoform is displayed across two wide-screen liquid crystal on silicon displays, the images of which are combined and relayed to the address face of a 40 mm aperture OSLM. This spatially varying intensity profile is converted into a phase hologram on the readout side of the OSLM. When illuminated at 532 nm we measure a first-order diffraction efficiency of ≈50% at 400 line pairs and ≈20% at 900 line pairs. We show that aberration associated with the non-flatness of the device can be corrected within software by modification of the hologram.

PROJECTION OF HOLOGRAMS FROM PHOTOREFRACTIVE OASLMs

Liquid crystals doped with fullerenes and carbon nanotubes (CNTs) act as good optical nonlinear materials. We have used these materials to build optically-addressed spatial light modulators (OASLMs). The devices comprise a single layer of doped liquid crystal acting as an active layer. Undoped LC devices with surfaces coated with fullerenes are also studied. Such OASLMs allow recording of phase holograms, and we record by imaging pre-calculated pre-recorded holograms. Writing is performed at normal incidence and reading at 45° oblique incidence. Both transmission and reflection modes of operation are used. Experimental results as well as comparison with commercially available OASLMs are presented.

Optimization of gray-scale performance in pixellated-metal-mirror FLC-OASLM by equivalent circuit model

An equivalent circuit model of a pixellated-metal-mirror ferroelectric liquid crystal (FLC) optically addressed spatial light modulator (OASLM) is proposed. Using both structure and FLC material parameters of a real device and real material as the simulation parameters, the model is firstly confirmed by good agreement of simulation results and the reported experimental ones, and then utilized to optimize gray-scale performance of the OASLM. The model is developed from an improved FLC equivalent circuit, and has the ability to describe the voltage dropped across the modulating layer and to predict how optical outputs vary over time with the input drive voltage and control image. Simulation results indicate that gray-scale performance of the OASLM is highly dependent on write pulse width. More than 10 gray scales are observed when write pulse width is 100μs and the number reduces to four when it reaches 500μs. Other parameters of drive voltage can be set to adjust the region of write light intensity over which gray scales are best produced, and write pulses are found to be primary at high write light intensities, whereas erase pulses are dominant at low write light intensities. Furthermore, although gray-scale performance is weakly dependent on erase-light intensity, the erase light is necessary to ensure a proper erasure of the device and at least 1mW/cm2 is required in this study.

OPTICAL NONLINEARITY IN AZO-ANTHRAQUINONE DOPED LIQUID CRYSTALS

The optical nonlinearity in liquid crystals doped with 2,5 azo-substituted anthraquinone (ASAQ) dye is studied. Dynamic holographic grating formation is observed under conditions of low power laser light and no external fields. This new dye is an interesting alternative to methyl red (MR) dopant for liquid crystals (LCs) as materials for single-layer optically addressed spatial light modulators (OASLMs). The thickness and concentration dependence are explored in ASAQ-doped LCs. Stability and suitability for device applications are researched. In MR-doped LC systems the performance of the devices deteriorates quickly, while ASAQ devices are remarkably stable.

Oblique-Incidence Characteristics of a Parallel-Aligned Nematic-Liquid-Crystal Spatial Light Modulator

We have developed optically-addressed and electrically-addressed liquid crystal spatial phase-only light modulators having no pixelized structures. We obtained a large depth of phase-only modulation and high diffraction efficiency based on the electro-optical characteristics of a parallel-aligned nematic liquid crystal. These spatial light modulators (SLM) are of the reflection type, so there would be a loss of power in the readout light from the half mirror, which was set up so as to separate the incident and reflected lights. To optimize the characteristics of a reflection type spatial phase-only light modulator, we have proposed an oblique incident optical readout setup. We have examined the effect of conditions such as the polarization direction and the incidence angle of the readout light, and the orientation of liquid crystal molecules in the SLM. High diffraction efficiency close to the theoretical maximum value was obtained by adjusting the above conditions. The simulation analysis can well explain the experimental results of phase modulation.

Non-scanning fluorescence confocal microscopy using speckle illumination and optical data processing

We present an improved method of speckle-illuminated non-scanning fluorescence confocal microscopy that uses an optically addressed spatial light modulator (OASLM) to perform real-time image processing. Results are presented from an experimental system in which a miniature LCD in combination with an analogue CCD camera operates as the OASLM. The ability of this method to produce processed images possessing confocal characteristics and to remove additive detector noise is demonstrated with both computer simulation and experimental results.

COMPARISON BETWEEN PIXELATED-METAL-MIRRORED AND NON-MIRRORED FERROELECTRIC LIQUID CRYSTAL OASLM DEVICES

The resolution and switching characteristics of two Bistable Optically Addressed Spatial Light Modulators (OASLMs) one with and one without a pixelated metal mirror (PMM) are compared. The two devices use amorphous hydrogenated silicon (a-Si:H) Schottky diode structures as their photosensitive layer, Indium Tin Oxide (ITO) layers for electrodes, Nylon-6 alignment layer and a ferroelectric Liquid Crystal (LC) material (ZLI-5014-100) as their modulating layer. Both devices exhibit bistability when driven by alternating monopolar pulses. The pixelated metal mirror OASLM has both higher resolution and better optical efficiency than the non-mirrored device.

Spatial sampling in pixelated-metal-mirror ferroelectric-liquid-crystal optically addressed spatial-light-modulator devices

We examine the spatial resolution of pixelated-metal-mirror (PMM) ferroelectric-liquid-crystal (FLC) optically addressed spatial light modulators (OASLMs). To characterize the resolution of OASLM devices, their diffraction efficiency is measured as a function of the spatial frequency of the write image. The results show that aliasing is produced by the PMM devices, which correspond qualitatively with the predictions of sampling theory.