LC SLM Research Articles

Implementation Features of Invariant Optical Correlator Based on Amplitude LC SLM

Mathematical simulations of invariant optical-digital correlator operation are performed with the LC SLM used for the input images display and for display of the correlation filter holograms. Different phase dependences on amplitude are considered, particularly the measured dependence of the LC SLM HoloEye LC 2002. A method for optimization of correlation filters to eliminate the object recognition error due to the presence of additional phase modulation is proposed.

Wavefront reconstruction method based on wavelet fractal interpolation for coherent free space optical communication

Existing wavefront reconstruction methods are usually low in resolution, restricted by structure characteristics of the Shack Hartmann wavefront sensor (SH WFS) and the deformable mirror (DM) in the adaptive optics (AO) system, thus, resulting in weak homodyne detection efficiency for free space optical (FSO) communication. In order to solve this problem, we firstly validate the feasibility of liquid crystal spatial light modulator (LC SLM) using in an AO system. Then, wavefront reconstruction method based on wavelet fractal interpolation is proposed after self-similarity analysis of wavefront distortion caused by atmospheric turbulence. Fast wavelet decomposition is operated to multiresolution analyze the wavefront phase spectrum, during which soft threshold denoising is carried out. The resolution of estimated wavefront phase is then improved by fractal interpolation. Finally, fast wavelet reconstruction is taken to recover wavefront phase. Simulation results reflect the superiority of our method in homodyne detection. Compared with minimum variance estimation (MVE) method based on interpolation techniques, the proposed method could obtain superior homodyne detection efficiency with lower operation complexity. Our research findings have theoretical significance in the design of coherent FSO communication system.

Visible light high-resolution imaging system for large aperture telescope by liquid crystal adaptive optics with phase diversity technique

There are more than eight large aperture telescopes (larger than eight meters) equipped with adaptive optics system in the world until now. Due to the limitations such as the difficulties of increasing actuator number of deformable mirror, most of them work in the infrared waveband. A novel two-step high-resolution optical imaging approach is proposed by applying phase diversity (PD) technique to the open-loop liquid crystal adaptive optics system (LC AOS) for visible light high-resolution adaptive imaging. Considering the traditional PD is not suitable for LC AOS, the novel PD strategy is proposed which can reduce the wavefront estimating error caused by non-modulated light generated by liquid crystal spatial light modulator (LC SLM) and make the residual distortions after open-loop correction to be smaller. Moreover, the LC SLM can introduce any aberration which realizes the free selection of phase diversity. The estimating errors are greatly reduced in both simulations and experiments. The resolution of the reconstructed image is greatly improved on both subjective visual effect and the highest discernible space resolution. Such technique can be widely used in large aperture telescopes for astronomical observations such as terrestrial planets, quasars and also can be used in other applications related to wavefront correction.

Application of optoelectronic micro-displays for holographic binary data recorder based on computer generated fourier holograms

Present article highlights the researches provided during the development of holographic memory system based on application of computational methods to encode binary data pages as amplitude computer generated Fourier holograms (CGFH). Using electro-optical micro-display and projection optics CGFH can be recorded onto photosensitive medium. The type of display that is used in data recorder determines the specificities of optical scheme architecture with its own limitations and advantages. There are three projection schemes of binary data recorder discussed in the paper. A linear scheme based on transparency-type liquid crystal spatial light modulator (LC SLM), a scheme based on reflection-type liquid crystal on silicon spatial light modulator (LCoS SLM) which uses a beamsplitting cube, and the most compact and simple scheme based on self-emitting OLED-display. The results of experimental implementation of all the three projection schemes for CGFH of binary data pages record onto the holographic carrier and consequent optical reconstruction and analysis of the encoded data are presented.

Measurement of characteristics and phase modulation accuracy increase of LC SLM "HoloEye PLUTO VIS"

Phase liquid crystal spatial light modulators (LC SLM) are actively integrated in various optical systems for dynamic diffractive optical elements imaging. To achieve the best performance, high stability and linearity of phase modulation is required. This article presents results of measurement of characteristics and phase modulation accuracy increase of state of the art LC SLM with HD resolution "HoloEye PLUTO VIS".

High-Resolution Correction of Arbitrary Wavefront Aberration Using Liquid Crystal Spatial Light Modulator

Wavefront correction plays significant role in some fields like astronomical observation, laser processing and medical imaging, etc. Liquid crystal spatial light modulator ( LC SLM) is an ideal device for high-resolution wavefront correction because of its low cost, low consumption, large number of pixels and independent programming control of each unit. It is researched experimentally that LC SLM is used as a wavefront correction device and corrects arbitrary wavefront aberration. Wavefront correction is performed based on phase conjugation and periodic phase modulation with modulo-2π. The experimental results show that the PV value of the irregular wavefront aberration is 1.56λ, RMS value is 0.25 and Strehl ratio is 0.08 before correction, but the PV value of the residual aberration is reduced to 0.26λ, RMS value is 0.02 and Strehl ratio is increased to 0.97 which is approximated diffraction limit after correction. It is proved to be feasible and effective that LC SLM is used to the high-precision and high-resolution wavefront correction.

Experimental validation of closed-loop adaptive optics based on a self-referencing interferometer wavefront sensor and a liquid-crystal spatial light modulator

A closed-loop adaptive optics system based on a self-referencing interferometer wavefront sensor (SRI WFS) using phase-shifting point-diffraction technique and an electrically addressed phase-only liquid-crystal spatial light modulator (LC SLM) is built and validated experimentally. The wavefront aberration of incoming beam is directly measured by analyzing four frames phase-shifted interferograms captured by a single CCD camera in two camera shots, and then by loading the conjugate function onto the SLM the wavefront aberration is corrected. The proposed scheme does not rely on any complicated control algorithm or wavefront-reconstruction algorithm and can achieve high-resolution and high-accuracy correction. Closed-loop correction results of single order Zernike aberrations and a Kolmogorov turbulence phase screen show that considerable improvements in the Strehl ratio (of greater than 0.94) is achieved.

Diffractive characteristics of the liquid crystal spatial light modulator

The liquid crystal spatial light modulator (LC SLM) is very suitable for wavefront correction and optical testing and can produce a wavefront with large phase change and high accuracy. The LC SLM is composed of thousands of pixels and the pixel size and shape have effects on the diffractive characteristics of the LC SLM. This paper investigates the pixel effect on the phase of the wavefront with the scalar diffractive theory. The results show that the maximum optical path difference modulation is 41 μm to produce the paraboloid wavefront with the peak to valley accuracy better than λ/10. Effects of the mismatch between the pixel and the period, and black matrix on the diffraction efficiency of the LC SLM are also analysed with the Fresnel phase lens model. The ability of the LC SLM is discussed for optical testing and wavefront correction based on the calculated results. It shows that the LC SLM can be used as a wavefront corrector and a compensator.

Investigation of optical testing with a phase-only liquid crystal spatial light modulator

We illustrate that the phase-only liquid crystal spatial light modulator (LC SLM) can be used for optical testing. The large phase change and the phase modulation precision are discussed. The computer generated holograms (CGH) method is used to acquire the significant phase modulation. And the phase modulating characteristics of the LC SLM are measured. It shows the phase modulation depth is more than 2pi and the modulation precision is down to 1/14lambda (PV) and 1/100lambda (rms) (lambda=632.8nm). In order to verify this method, the former surface of a convex lens is tested by ZYGO interferometer. The parallel straight fringes are obtained. It is shown that PV is 1/3lambda and rms is 1/20lambda after compensated by the LC SLM.

Self-induced nonlinear Zernike filter realized with optically addressed liquid crystal spatial light modulator

Visualization of phase objects introducing small distortions to a plane wave front (such as turbulence of air) can be realized with the well-known Zernike filtering. In this work we discuss and present the use of an optically addressed liquid crystal spatial light modulator (OA LC SLM) performing self-induced nonlinear Zernike-type filtering. The OA LC SLM is made of a planar nematic liquid crystal layer sandwiched between a photoconductor (PVK:TNF 100-nm-thick polymer) and a polyimide orienting layer. The advantage of such a nonlinear Zernike filter over a conventional one is that no precise optical adjustment is necessary and the filtering is relatively easy to control by tuning the externally applied field to the modulator and the incoming light intensity.

Study of electron beam with modulated energy distribution for optical klystron FELs

The development of an intense and coherent light source is expected in the X-ray region, because of its many applications in this region. But it is difficult to obtain such a short wavelength with ordinary FEL using the resonator and conventional laser. SASE FEL without the resonator is a promising scheme due to the improvement of accelerator technology. The high power FEL in the 2—4 nm range has already been proposed at SLAC [1,2]. The quasi-stable optical klystron [3,4] can reduce the wiggler length significantly for a soft x-ray SASE FEL. According to the numerical study the gain curve of the quasi-stable optical klystron has multiple gain peaks with narrow width and high gain. Fig. 1a shows the gain curve of the quasi-stable operation of the optical klystron and modulated energy distribution of the electron beam coupling to the positive gain peaks. Fig. 1b represents the gain curve of the uniform wiggler. When the energy spread of an electron beam is larger than a gain peak, the FEL gain becomes very low due to the negative gain of the gain function. But if the distribution of electron energy is modulated and its modulation peaks couple to the positive peaks of the optical klystron gain function, we can obtain the high FEL gain although the total energy spread of the electron beam is large. With the latest technology of laser pulse shaping, it is possible to produce femtosecond pulses [5,6]. This pulse shaping apparatus called liquid crystal spatial light modulator (LC SLM). Two LC SLMs are combined between a pair of polarizes. When the LC modulator is operated by applying a voltage, the liquid crystals rotate towards the direction of propagation of light and produce the modified pulses. With this apparatus, the user can specify the waveform with arbitrary profiles. When the modulated laser that is obtained in this way irradiates the photocathode, the modulated electrons are extracted with the same temporal profile, because the response time of metal photocathode is of femtosecond order. This is a very important property of our concept. This is the advanced operation of quasi-stable optical klystron with energymodulated electron beam. The electrons extracted with temporal modulation are accelerated with an RF gun. Then these electrons are accelerated in different RF phases.

Spherical aberration correction using a liquid-crystal spatial-light modulator in off-axis electron holography

One can correct spherical aberration in a transmission electron microscope by using a newly developed aberration-correction method involving off-axis electron holography. In this method, a liquid-crystal spatial-light modulator (LC SLM) is employed during the holographic reconstruction step to compensate for spherical aberration. Application of this method to high-resolution off-axis electron holograms of fine gold particles is presented. The phase distribution of the corrected object wave is visualized by the Zernike phase-contrast method carried out with the same LC SLM.