Transmitter for coherent light communication system

Properties of a direct current excited helium-neon gas laser, focusing or collimation of the light beam and a sensitive wide band modulator are discussed. From the theoretical study of Fabry-Perot resonators, it becomes apparent that the gas laser for communication purpose must oscillate only in TEMoo mode. The effect of iris on output power and brightness of a laser, and relation between spot size and output power is also examined. As to collimation, the formulae for calculation of beam spread are introduced and a Galileian telescope is shown to be most convenient. For the part of modulator, comparison of three typical versions of Pockel´s cells are made. The author would like to prefer cooled KDP modulator among them, because this type has moderate bandwidths comparable with that of existing systems and needs neither expensive nor complecated bulky circuitry.