Scattering from Dielectric Mirrors

The improvement of dielectric mirrors during recent years has reduced their surface scattering considerably. Nevertheless, there are applications which are limited by these small amounts of scattered light. One of them is the laser gyroscope whose locking threshold depends on the light scattered back into the direction of incidence. Measurements have been performed recently to analyze this case.1 Another application is the simultaneous transmission of many laser beams in an optical waveguide for communication purposes.2 The focusers in such a guide will probably be front surface mirrors rather than lenses because, for the large apertures needed, lenses are apt to have imperfections in the bulk. Dielectric mirrors have fewer imperfections, but they still scatter some light into adjacent beams where it produces crosstalk. It was the purpose of our experiment to measure some representative mirror surfaces as a basis for later feasibility studies on multiple beam waveguides. Only the light in a narrow cone around the beam is collected by the next focuser and eventually contributes to the crosstalk. The experiment showed that, in this cone, the scattered light intensity decreases relatively fast with increasing angle. 511