We report on a novel approach to the realization of nematic liquid-crystal (LC) phase correctors to form spherical and cylindrical wave fronts. A LC cell with a distributed reactive electrical impedance was driven by an ac voltage applied to the cell boundary to yield the desired spatial distribution of the refractive index. The two-dimensional function of the phase delay introduced into the light beam depends on the frequency of the ac control voltage, the geometry of the boundary electrode surrounding the LC cell, and the electrical parameters of the cell. We realized a cylindrical adaptive lens with a clear aperture of 15 mm 3 4 mm and a spherical adaptive lens with circular aperture of 6.5 mm. Both devices are capable of focusing collimated light in the range`. . . 0.5 m. © 1998 Optical Society of America OCIS codes: 010.1080, 160.3710, 220.3620.Liquid-crystal (LC) phase modulators have a great potential for use as adaptive optics 1 because of their light-transmitting operation, simple control, reliability, low power consumption, and low control voltage. In many applications the correction of low-order aberrations such as of defocus and astigmatism is of primary importance. The adaptive lenses described in Refs. 2 -4 and references therein are controlled by arrays of individual electrodes, approximating the wave front by step functions by means of the zonal correction principle. Good approximation to a continuous wavefront prof ile in these modulators can be achieved only with a large number of discrete control electrodes.We suggest a novel approach to forming a smooth continuous distribution of the refractive index in a nematic LC layer over the entire aperture of LC cell, realizing a modal control principle. 5The spatial modulation of the wave front is def ined by the geometry of the contacts located at the periphery of the modulator aperture, by the frequency spectrum of the control voltage, and by the electric characteristics of the LC cell. Ultimately we need a single circular contact to control a spherical adaptive lens and two linear equidistant contacts for a cylindrical lens.Let us consider the LC cell configuration shown in Fig. 1. The LC layer is sandwiched between two transparent plate electrodes deposited upon glass substrates. The distributed resistance of the control electrode is much greater than that of the ground electrode. A control voltage is applied to the contacts deposited at the periphery of the highly resistive electrode. The initial LC layer alignment is determined by the alignment coating, and its thickness is set by dielectric spacers.When an ac control voltage is applied to the peripheral contacts (Fig. 1) the active impedance of the highresistance control electrode and the reactive impedance of the capacitor formed by the LC layer sandwiched between the control and ground electrodes form a distributed voltage divider, resulting in a nearly parabolic distribution of the ac voltage over the LC layer. A simplified equivalent circuit corresponding to the situation described is shown...

Liquid crystal modal lenses are switchable lenses with a continuous phase variation across the lens. A critical issue for such lenses is the minimization of phase aberrations. In this paper we present results of a simulation of control signals that have a range of harmonics. Experimental results using optimal sinusoidal and rectangular voltages are presented. A lack of uniqueness in the specification of the control voltage parameters is explained. The influence of a variable duty cycle of the control voltage on an adaptive lens is investigated. Finally we present experimental results showing a liquid crystal lens varying its focal length.

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We describe two electro-optical systems for adaptive focusing of linearly polarized light. The aperture size is 5 mm and the focal length can be varied from 1 to 4 m for wavelengths from 0.663 to 0.85 μm. The first is a commercially available system including a PC compatible control unit and software, and an adaptive liquid crystal lens. The other is an experimental system consisting of a self-contained unit with an autonomous power supply and an adaptive lens. The safe operating limit in the visible region is 10 W/cm2 with a transmission of 70% without antireflection coating. The switching speed of focus variation from 2 to 1 m and from 1 to 2 m is 780 and 860 ms, respectively. The operating principles of the spherical adaptive lenses and their control units are described. Phase aberrations of the lenses were measured by a Zygo phase shifting interferometer, and the results are presented.

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Abstract:Results are presented of the properties of a liquid crystal wavefront corrector for adaptive optics. The device is controlled using modal addressing in which case the device behaves more like a continuous facesheet deformable mirror than a segmented one. Furthermore, the width and shape of the influence functions are electrically controllable. We describe the construction of the device, the optical properties, and we show experimental results of low order aberration generation.