Synchronous traveling wave electro-optic light pipe modulator

Abstract

There is disclosed a synchronous traveling wave electro-optic modulator for ultra-wideband infrared laser communications systems which provides optical waveguiding of the laser beam through an interaction distance which is large relative to the diffraction limit of the laser beam and which affords essentially perfect radio frequency and optical velocity synchronism at a relatively high electrical impedance level by appropriate choice of the design parameters of the device. The device comprises an array of very slender rectangular rods of electro-optic material such as cadmium telluride laid end to end in juxtaposition to form a light pipe for an infrared laser beam. The length to cross-section ratio of the light pipe is many times greater than that which is determined by the diffraction limit and consequently the voltage and power required to achieve a given depth of modulation is substantially reduced. The modulating signal is applied as a synchronous or nearly synchronous traveling wave across two parallel strip electrodes which form a TEM transmission line exhibiting a purely resistive impedance, with matched termination, and a theoretically infinite base bandwith. An appropriate dielectric environment is provided by surrounding members enclosed in electrical shielding. A structural variable or parameter which can be adjusted to achieve a desired signal velocity for a given dielectric environment is w/d, the width of the strip electrodes relative to their separation by the thickness of the electro-optic material.