Fraunhofer Filters To Reduce Solar Background For Optical Communications

Abstract

A wavelength that lies within a Fraunhofer line (a spectral interval of reduced solar emission) can carry optical communications in the atmosphere and space with reduced interference from direct or reflected background sunlight. Suitable Fraunhofer lines are located within the tuning range of good candidate laser sources. The laser should be tunable dynamically to track Doppler shifts in the sunlight incident on any solar system body that may appear in the background as viewed by the receiver, or in earthlight for ground-based daylight reception. The Fraunhofer filter used with a direct-detection receiver should be tuned to match the Doppler shifts of the source. The required tuning is calculated here for various situations. A typical Saturn-to-Earth data link can reduce its source power requirement from 8.8 W to 2 W of laser output by employing a Fraunhofer filter instead of a conventional multilayer dielectric filter.