The measurement of far-infrared laser gain and loss using a Michelson coupler

It is demonstrated that small values of small-signal gain and total internal losses in far-infrared lasers can, in principle, be determined by measuring the optimum and threshold output coupling coefficients with a Michelson coupler. Experimental results show that although accurate determination of the small-signal gain can be made, only a good approximation of the internal losses is obtained by this method. The gain of the 220 micron transition of H2O is shown to increase from 0.24%/m to 0.43%/m with the addition of helium to 0.25 torr H2O at a discharge current of 1.8 A.