NITROGEN GAIN  AND  OPTICS

            Nitrogen lasers operate without mirrors, a type of laser called super radiant. The gain medium exhibits such high gain that spontaneous emission from nitrogen molecules at the end of the laser are amplified by the same group of nitrogen molecules in the tube, producing a usable output pulse in a relatively short path length. Gain for a nitrogen laser is on the order of 40 to 50 dB/m or more, depending on the specific laser. Even at the lower figure of 40 dB/m, light is amplified by a factor of 10,000 for every meter of travel through the laser tube! This is an enormously high gain for a laser which also serves to prohibit tuning of the laser through any of the 61 lines in the gain band, since they will lase without any cavity mirrors, and hence a wavelength selector is ineffective.

        Although not required for laser action, a single high-gain reflector is frequently installed in a nitrogen laser tube as well as a 4% reflective (one uncoated surface of a window) output coupler. Logically, one expects to double output power in this manner (by adding the rear and forward beams), but the actual increase is over threefold, since light reflected into the laser channel does not simply pass through the tube but is further amplified along the way. Use of a cavity such as this also decreases beam divergence by at least 50%. Optics must be designed for UV, so the coating of the high reflector must reflect UV (aluminum is frequently used), and windows on the laser tube must be made of quartz or some other material that is transparent to UV radiation.