Secure two-way submarine communication system

An arrangement for secure two-way tactical laser communications between a submarine submerged in an ocean and an airborne platform. During an initital acquisition mode, the airborne transceiver sends a downlink pulse-modulated blue-green laser beam to the ocean surface and below using a predetermined IFF code to identify the transceiver to the submarine. In the preferred embodiment the transmit optics spread the beam out into an elongated elliptically-shaped pattern to maximize coverage of the search area. When the downlink beam energy is within range of the submarine, an optical receiver on the submarine detects the beam, filters out the background light with a very narrow-band filter, and converts the light pulses to equivalent electrical pulse. A signal processor in the submarine receiver decodes the electrical pulses and verifies the IFF code to prevent the submarine from responding to a laser beam from an unfriendly source. If the IFF is verified, the submarine laser transceiver transmits a pulse-modulated uplink beam response at the same wavelength, but timed so that the light pulses are time interleaved with the downlink pulses. The uplink beam power is carefully controlled to the minimum power level required by the airborne receiver to recover the uplink beam. As soon as the airborne receiver verifies the uplink IFF code, communications being over the laser link for the duration of time that the airborne platform receiver is within range of the uplink beam. In one embodiment of this invention, both receivers employ a Cesium filled atomic resonance filter (ARF) to separate the blue-green beam from any background light.