It has been widely observed that when a multimode gas laser is employed in photomixing experiments, the heterodyne signal amplitude depends on the optical path length difference between the paths traversed by the two light beams. In this paper, a simple phenomenological model of multimode gas laser radiation is used to calculate the dependence of the heterodyne signal amplitude on optical path length difference for various numbers of oscillating axial modes in a multimode gas laser. The results have important implications for the design of optical heterodyne receivers.
J. W. Foreman, Jr., "Optical Path-Length Difference Effects in Photomixing with Multimode Gas Laser Radiation," Appl. Opt. 6, 821-826 (1967)