GB868275A - Improvements in or relating to light modulators - Google Patents

Abstract

868,275. Light modulating devices. STANDARD TELEPHONES & CABLES Ltd. Sept. 18, 1959, No. 31864/59. Class 40(3) [Also in Group XXXVI] A light-modulating device, which makes use of the variation of the reflection coefficient of the surface of a semi-conductor body with the number of free charge carriers contained therein, comprises an assembly of semi-conductor material arranged to present a surface made up of regions 6, 7, 8 of N-type material and regions 2, 3, 4 of P-type material, means being provided to apply between ohmic contacts 10 and 11 a reverse bias sufficient to cause an appreciable proportion of the surface to be occupied by depletion regions and an alternating signal whereby the depletion regions may be varied to cause a corresponding variation of visible or infra-red light reflected from the surface. Maximum depth of modulation is produced if the whole of the surface is covered by depletion regions. To approach this ideal, strips 2-4 and 6-8 are each made 1 thousandth of an inch wide. As an alternative to the arrangement illustrated, where strips 6-8 are joined by a common strip 5 bearing a single ohmic connection 11, a separate ohmic connection may be made to each of strips 6-8 and the connections joined in parallel. Reference is made (without giving details) to the use of circular strips. An increase in the depth of modulation may be achieved by increasing the number of surface reflections. A multiple reflection modulator based on two reflecting surfaces is described with reference to Fig. 2 (not shown).