The right color for satellite or aircraft laser communications through seawater to a submarine, depends upon the laser beam’s penetration of seawater and having a very narrow band filter/receiver on the submarine so as to have a high signal to noise ratio.
The transmitter, according to Figure 1, should best transmit a blue light with an approximate wave length of 4590 Angstroms. This sort of beam, as shown, is least absorbed by seawater and should arrive at a deep submarine with sufficient intensity to convey to the submarine’s receiver a considerable amount of information in a very short period of time.
The two main transmitters which produce about 4600 A0 light beams are the Xe/CI Raman gas laser which produces a blue beam of 4593 A0 , and the Nd:YAG doubled solid state laser which produces a blue-green beam at 5320 A0
The receivers on submarines are either the CsABF (Cesium atomic resonance filter) which accepts only a narrow band (10 milli A ) of blue, centered at 4593 A , or the tuned quartz filter/receiver for the 5320 A beam which accepts a beam about 1 A wide — about 100 times wider than the Cesium filter/receiver. However, such actors as atmospheric scattering, variations in seawater, etc., make the blue Raman gas-laser at least 30 times better than the solid-state blue green laser and about 100 times better on the average.
The present satellite laser communication program is consequently baselined for the Raman gas laser transmitter and the Cesium filter for the submarine receiver. This allows for transmission of a blue laser beam at the general area where a submarine might be and the sub can pick up the transmitted communications in real time.
It is desirable to have different colors for the up and down links, and significantly, only one kind of laser beam receiver is needed per submarine.