Atomic line filter blocks the light

Acquisition of a laser beacon (which may be located on a satellite, aircraft, or earth station) to initiate a free-space communications link requires rejection of very high background light. To accomplish this, the Lasercom transceiver uses an extremely narrow-band atomic line filter that relies on crossed polarizers and the Faraday effect in an atomic vapor to block out background light with a rejection ratio better than 10-5.

Incoming unpolarized light (beacon signal and noise) passes through the first polarizer, and light at the beacon wavelength (852 nm) is rotated 90° by the cesium vapor in a cell with a magnetic field located between the two polarizers. The beacon signal then passes through the second polarizer to the detector. Light at other wavelengths is not rotated and is consequently blocked by the second polarizer.

The laser output of the system beacon is stabilized to match the atomic line filter. Diode lasers operating at the 852-nm wavelength of the cesium atomic line filter are used. Optical feedback through a filter in the laser cavity locks the beacon wavelength at the filter transmission peak.