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-μm transition of H
2
O is shown to increase from 0.24 percent m
^-1
to 0.43 percent m
^-1
with the addition of helium to 0.25 tort H
2
O at a discharge current of 1.8 A.