Liquid crystal modulators with improved laser damage resistance

Abstract

Experimental results have been given on laser-damage resistance of the liquid crystal modulator with longitudinal operating electrical field. It was shown that laser-damage resistance of the modulator is limited by the ITO transparent electrodes and equals 2.5 -2.9 J/cm2 at 1.06 nm, 'c 15 ns. To improve this parameter we proposed an liquid crystal structure controlled by a transverse electric field in which the ITO electrodes are removed out of the zone intense laser radiation. The main characteristics of this mode liquid crystal modulator are discussed. Laser damage resistance of Liquid Crystal Modulators with longitudinal operating electrical field Liquid Crystal Modulators (LCM) are well known and used in a wide range of optical systems. Usually the LCM consists of a number of thin film layers sandwiched between two glass substrates. Transparent electrodes are deposited on the inner surfaces ofthe glass plates. Initial orientation of the liquid crystal (LC) molecules is determined by alignment layers deposited on transparent electrodes. The thickness of the LC layer is defined by a dielectric spacer. Electro-optical properties of LC layer are controlled by a longitudinal electric field. Light to be modulated passes through all layers of the LCM. Main characteristics of the LCM are: -maximum modulation frequency; -speed of response; -contrast ratio; -transmission in dark state; -transmission in light state; -laser damage resistance. The last parameter is extremely important in applications of the LCM involving high power laser systems.We tested laser damage resistance of twisted nematic LCM and its elements: glass substrates, ITO transparent electrodes, alignment and LC layers. We conducted laser interaction experiments using focused output beam a single-mode single frequency Q-switched YAG:Nd laser. Output energy was varied from 15 to 40 mJ, pulse duration was 15 ns, repetition rate -3 Hz. Special feature of our testing method was use of a probe beam of auxiliary He-Ne laser to visualize damages (Fig. 1). Shlieren picture of tested spot was SPIE Vol. 3682 • 0277-786X1981$1O.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx