Liquid crystal (LC) wavefront correctors with modal addressing are described. Three different approaches are considered. The first one is based on a continuous thin-film resistive layer. This layer is used for forming of the local voltage profile that controls the phase distribution across the corrector’s aperture. The second approach is a modification of the first one, where the continuous resistive coating is replaced by a network of discrete resistors. It is based on silicon technology. The third approach makes use of distributed electric field in thick dielectric layers for forming of the modal response of an actuator. Technologies, methods of control and experimental results are discussed for each case.
In this paper we review progress towards making a liquid crystal spatial light modulator (LC-SLM) which has all the desired specifications required for (astronomical) adaptive optics (AO). Our work at Durham is currently focused on developing modal LCs, as they have some key advantages over conventional LC-SLMs. A modal LC-SLM is a device whose optical properties more closely resemble a deformable facesheet mirror than a conventional LC-SLM which are pixelated. Therefore they have a Gaussian, rather than a piston-only, influence function.
Static and dynamic numerical models of electro-optical characteristics of nematics are presented. A numerical model is used for the optimization of phase delay distribution in modal LC lenses. The dynamic control mode for modal LC lenses is simulated. The results are in a good agreement with experiment.
A novel method to control LC phase modulators is presented. Various modifications of modal LC correctors are described in detail, including adaptive spherical and cylindrical lenses and a 37-element wavefront corrector. Physical mechanisms are explained with the emphasis on their practical implications.
A comparative analysis of different electrical control methods of liquid crystal (LC) modulators is presented, and the dual- frequency control method is considered in detail. Theoretical evaluations of the speed and LC heating using dual-frequency control are reported. Methods to optimize the control voltage parameters are described. Next, it is shown how, using specific physical LC properties, it is possible to create a LC wavefront corrector that can be controlled modally. Modal wavefront correctors for the control of both low and high order aberrations are described. We describe some novel operating configurations of modal LC lenses. Finally, we make some brief comparisons of nematic with ferroelectric LCs for adaptive optics.
Detailed analysis of electric voltage distribution and electric currents in adaptive liquid crystal (LC) based lenses is presented. Approximation of constant LC layer impedance is shown to be useful for physical insight. More generally, a computer simulation is developed, taking into account the voltage dependencies of the LC capacitance and conductance. Computer simulated phase shift distributions are in good agreement with experimental profiles produced via Zigo interferometer measurements. Also addressed is the influence of the lens's electrical parameters on its dynamics.
The correction of low-order aberrations is important in many adaptive optics applications. Modal cylindrical adaptive lenses can be used to correct several low order aberrations. Furthermore, the same technology can be used for creating arrays of controllable lenses. The most significant feature of these cylindrical lenses is a modal control system based on nematic liquid crystals. Modal control allows the precise control of the spatial phase distribution in order to achieve an aberration-free lens. This has been investigated both by computer simulation and experiment. We found that the introduction of a 180-degree phase shift between the second or higher order harmonics and no phase shift between the first harmonic components of the control voltages improves the optical performance of the device. These extra harmonics eliminate the strong dependence of the liquid crystal orientation on the impedance of the device. This is especially important for devices with small apertures. It also was found that modal cylindrical lens controlled by two-harmonic voltages can produce a slit-like beam whose transverse structure has the shape of a pulse which remains unchanged over a long range in the direction of propagation. We investigated a device with two-crossed 1D control electrodes and produced a lens with controllable focus and astigmatism.
Model adaptive liquid crystal lenses are described with the emphasis on their performance. Process of the lens calibration is described and numerical and experimental calibrations are carried out. Imaging by the modal adaptive lens is implemented, as well as the focusing in the feedback system based on the adaptive LC lens.
A new type of phase liquid crystal modulators with distributed electrical parameters is proposed. These modulators make it feasible to use minimal number of electrodes to from a desirable phase shape. We devised methods of control and investigated the modal liquid crystal cylindrical and spherical lenses, and the multi-element wavefront corrector with controlled influence as well. The focusing by means of the adaptive lenses is presented. We demonstrate two approaches in optimal control. The first consists in computing the optimal control voltage through liquid crystal and lens parameters. The second consists in feedback use.
Different control methods of phase delay in the nematic liquid crystal layer are analyzed. The experimental optimization results of dual-frequency control voltage parameters are discussed and the adaptive system for hunting of these parameters is represented.
We used a combined approach, investigating capacitance, resistance, and electrooptic reply of nematic liquid crystals within the same experimental framework, which enabled us to plot the phase delay as a function of electric parameters. Phase delay vs. capacitance curves are almost linear, but the slope is frequency dependent for the liquid crystal with the low-frequency dispersion of the dielectric anisotropy. To interpret the results, equivalent electric parameters definition, consistent with the AC current measurement techniques, is presented and phenomenological theory of the RC-parameters is developed. When completed by the Ericksen-Leslie equation for the liquid crystal molecules realignment in the external electric field, the theory is in good agreement with the experiment.
A novel approach to the liquid crystal modulators design is suggested under which the liquid crystal is treated as a distribution capacitor. To control the capacitor, we introduced a distribution high resistance control electrode. We devised methods of control and investigated modal liquid crystal modulators that can be used as adaptive cylindrical and spherical lenses. Analytical derivations, computer and experimental results are presented and discussed.
The sensor based on the unharmonic grating which operates as a set of phase compensators of a wave front curvature is investigated. Applications of the sensor for a real-time measurement semiconductor laser output beam astigmatism via a pumping current and for a lens testing are presented.
It is considered the sensor which can measure a surface profile shape and a waviness as the space function. The optical head registering the local surface titles is applied in the sensor. The sensor is the adaptive action system. The device is classified as a coordinate-measurement tool.
A new type of wave front sensor is proposed. The possibility of the measuring sensitivity control is shown. The results of the numerical simulation and experiments of the wave front aberrations measurements are presented.
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