Plane-wave theory of a Michelson laser coupler with a dielectric slab beam splitter

The plane-wave theory for the transmittance and absorbtance of a perfectly aligned Michelson coupler with a dielectric slab beam splitter is presented. It is shown that the transmittance and absorbtance vary sinusoidally and in quadrature. As a result of this quadrature relationship, the maximum transmittance occurs at a setting of the translatable coupler mirror at which the absorbtance is not at an extremum, and so the curve of output power as a function of coupler setting is asymmetrical with respect to the setting yielding maximum transmittance. Experimental measurements of the output power of a far-infrared HCN laser as a function of the coupler setting confirm this asymmetry, which seems to have been overlooked or ignored in previous studies.