Imaging Atomic Line Filter For Satellite Tracking

Abstract

To reduce power requirements for a high data rate optical satellite communications network it is advantageous to broadcast a signal with a very small angular divergence. If the receiver also has a narrow field of view, background light (from the daylit earth, etc.) can be sufficiently blocked with conventional passive filters. A narrow communications transceiver field of view, however, requires an accurate mutual tracking capability. Initial acquisition and subsequent tracking will require a tracking receiver with a much larger field of view than what is necessary for communications. To minimize weight and absolute pointing requirements on the telescope steering mechanism, this field of view should be as large as possible subject to the constraint of keeping collected background light levels below an acceptable level, and maintaining a tracking accuracy better than the communications laser divergence angle. Background light can be minimized and tracking accuracy can be maintained by using a narrow bandwidth, wide field of view imaging atomic line filter which matches the wavelength of a semiconductor diode laser. The operational principles of such an active image preserving atomic line filter are explained, especially as related to satellite tracking for a communications network. Preliminary results showing image preservation at a spatial resolution of 0.5 mm by a cesium atomic line filter at 852 nm with an acceptance bandwidth of 0.002 nm are presented. Expected limiting resolution, conversion efficiency, time response and laser power requirements for image preserving alkali atomic line filters are also discussed.