Resonant Modes in a Maser Interferometer

Schawlow and Townes 1 have proposed infrared and optical masers using Fabry-Perot interferometers as resonators. Very recently, Mai453 454 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , MARCH 1 9 ( i l man 2 and Collins et al.3 have demonstrated experimentally the feasibility of stimulated optical radiation in ruby. In these experiments two parallel faces of the ruby sample were polished and silvered so as to form an interferometer. The radiation due to stimulated emission resonates in the interferometer and emerges from a partially silvered face as a coherent beam of light. In a maser using an interferometer for a resonator, a wave leaving one mirror and traveling toward the other will be amplified as it travels through the active medium. At the same time it will lose some power due to scattering by inhomogeneities in the medium. When the wave arrives at the second mirror some power will be lost in reflection due to the finite conductivity of the mirror and some power will be lost by radiation around the edges of the mirror. For oscillation to occur, the total loss in power due to density scattering, diffractive spillover and reflection loss must be less than the power gained by travel through the active medium. Thus diffraction loss is expected to be an important factor, both in determining the start-oscillation condition, and in determining the distribution of energy in the interferometer during oscillation. While it is common practice to regard a Fabry-Perot interferometer as being simultaneously resonant for uniform plane waves traveling parallel to the axis and at certain discrete angles from the axis, this picture is not adequate for the computation of diffraction loss in a maser.