John J. Pouch

Modeling and Design of an Optimized Liquid-Crystal Optical Phased Array

In this paper, the physics that determines the performance limits of a diffractive optical element based on a liquid-crystal (LC) optical phased array (OPA) is investigated by numerical modeling. The influence of the fringing electric fields, the LC material properties, and the voltage optimization process is discussed. General design issues related to the LC OPA configuration, the diffraction angle, and the diffraction efficiency are discussed. A design for a wide-angle LC OPA is proposed for high-efficiency laser beam steering. This work provides fundamental understanding for a light beam deflected by a diffractive liquid-crystal device.

Performance evaluation of a liquid-crystal-on-silicon spatial light modulator

D optical phased array antennas formed by a liquid crystal on silicon (LCOS) spatial light modulator are described for free-space laser communication and high-resolution wavefront control. The device consists of an 2-D array of 10243768 phase modulator elements, each with controlled voltage, and can induce a phase shift from 0 to 2p for wavelengths up to the near IR. When the device is used as a wavefront corrector, 18.7 waves peak-valley (at 632.8 nm) of aberration in the op- tical system is corrected to a residual of 1/9 wave peak-valley, or 1/30 wave rms. The Strehl ratio improved from 0.006 to 0.83 after correction. An additional linear phase ramp was added to the correction phase ramp to simultaneously correct and steer the laser beam. Continuous steering over 64 mrad in the X-Y plane with a steering accuracy higher than 10 mrad has been obtained. The 1-D beam-steering efficiency is 80% at the maximum steering angle of 4 mrad. These results suggest that an LCOS device can be used to achieve very high-resolution wavefront control at very high efficiency.

Plasma-deposited amorphous hydrogenated carbon films and their tribological properties

Recent work on the properties of diamondlike carbon films and their dependence on preparation conditions are reviewed. The results of the study indicate that plasma deposition enables one to deposit a variety of amorphous hydrogenated carbon (a-C:H ) films exhibiting more diamondlike behavior to more graphitic behavior. The plasma-deposited a-C:H can be effectively used as hard, wear-resistant, and protective lubricating films on ceramic materials such as Si(sub 3)N(sub 4) under a variety of environmental conditions such as moist air, dry nitrogrn, and vacuum.

Liquid crystal on silicon (LCOS) wavefront corrector and beam steerer

A spatial light modulator, which is capable of high-resolution wavefront compensation and high accuracy beam steering, has been demonstrated using a Liquid Crystal On Silicon (LCOS) microdisplay with 1024×768 XGA resolution. When the device is used as a wavefront corrector, about 18.7 waves (peak-to-valley at 632.8nm) of aberration in the optical system is corrected to a residual of 1/9 wave (peak to valley) or 1/30 wave rms. Measurement of the far field beam profile confirmed the strehl ratio improved from 0.006 with the wavefront correction off, to a strehl ratio of 0.83 after correction. An additional linear phase ramp was added to the correction phase ramp to simultaneously correct and steer the laser beam. We demonstrated we can steer the beam continuously in the range of ±4 mrad in X-Y plane, with a steering accuracy better than 10μrad, or about 1/10 the diffraction limited beam divergence. The quality of the steered beam remains very high during the steering as the ellipticity of beam is smaller than 1±0.04, focused beam waist is 1.3x the diffraction limited beam waist and strehl ratio remains higher than 0.66. The 1-D beam steering efficiency is 80% at the maximum steering angle of 4 mrad, which agrees very well with our Finite Difference Time Domain (FDTD) simulation result of diffraction efficiency 86% at maximum steering angle. These results suggest that an LCOS device can be used to achieve very high-resolution wavefront control at very high efficiency.

Finite-difference time-domain simulation of a liquid-crystal optical phased array

Accurate modeling of a high-resolution, liquid-crystal-based, optical phased array (OPA) is demonstrated. The modeling method is extendable to cases where the array element size is close to the wavelength of light. This is accomplished through calculating an equilibrium liquid-crystal (LC) director field that takes into account the fringing electric fields in LC OPAs with small array elements and by calculating the light transmission with a finite-difference time-domain method that has been extended for use in birefringent materials. The diffraction efficiency for a test device is calculated and compared with the simulation.

Optical and interfacial electronic properties of diamond-like carbon films

Abstract We have been preparing hard semitransparent carbon films on oriented polished crystal wafers of silicon, indium phosphide and gallium arsenide, as well as on KBr and quartz. Properties of the films were determined using IR and visible absorption spectroscopy, ellipsometry, conductance-capacitance spectroscopy and α particle-proton recoil spectroscopy. Preparation techniques include r.f. plasma decomposition of methane (and other hydrocarbons), ion beam sputtering and dual-ion-beam sputter deposition. Optical energy band gaps as large as 2.7 eV and extinction coefficients lower than 0.1 at long wavelengths are found. Electronic state densities at the interface with silicon as low as 10 10 states eV -1 cm -2 were found.

Optical dispersion relations for 'diamondlike' carbon films

Ellipsometric measurements on plasma deposited “diamondlike” amorphous carbon (a-C:H) films were taken in the visible, (E = 1.75 to 3.5 eV). The films were deposited on Si and their properties were varied using high temperature (up to 750 °C) anneals. The real (n) and imaginary (k) parts of the complex index of refraction N were obtained simultaneously. Following the theory of Forouhi and Bloomer (Phys. Rev. B34, 7018 (1986)), a least squares fit was used to find the dispersion relations n(E) and k(E). Reasonably good fits were obtained, showing that the theory can be used for a-C:H films. Morever, the value of the energy gap Eg obtained in this way was compared to the Eg value using conventional Tauc plots and reasonably good agreement was obtained.

Thin‐film hermeticity: A quantitative analysis of diamondlike carbon using variable angle spectroscopic ellipsometry

The purpose of this paper is twofold. First, we report on the successful application of variable angle spectroscopic ellipsometry to quantitative thin‐film hermeticity evaluation. Secondly, it is shown that under a variety of film preparations and moisture introduction conditions water penetrates only a very thin diamondlike carbon (DLC) top surface‐roughness region. Thus DLC is an excellent candidate for use as protective coatings in adverse chemical and aqueous environments.

Modeling and performance limits of a large aperture high-resolution wavefront control system based on a liquid crystal spatial light modulator

The aberration introduced by the primary optical element of a lightweight large aperture telescope can be corrected with a diffractive optical element called the liquid crystal spatial light modulator. Such aberration is usually very large, which makes the design and modeling of such a system difficult. A method to analyze the system is introduced, and the performance limitation of the system is studied through extensive modeling. An experimental system is demonstrated to validate the analysis. The connection between the modeling data and the experimental data is given.

Spatial resolution limitation of liquid crystal spatial light modulator

The effect of fringing electric fields in a liquid crystal (LC) Optical Phased Array (OPA), also referred to as a spatial light modulator (SLM), is a governing factor that determines the diffraction efficiency (DE) of the LC OPA for high resolution spatial phase modulation. In this article, the fringing field effect in a high resolution LC OPA is studied by accurate modeling the DE of the LC blazed gratings by LC director simulation and Finite Difference Time Domain (FDTD) simulation. Influence factors that contribute significantly to the DE are discussed. Such results provide fundamental understanding for high resolution LC devices.