Comment on 'Discrimination Against Unwanted Orders in the Fabry-Perot Resonator'

In the above paper, 1 Kleinman and Kisliuk state that "Fox and Li have investigated these configurations and the corresponding frequencies and losses for interferometers consisting of perfectly reflecting plates in air. In the usual laboratory interferometer the Fox and Li modes cannot be resolved because of insufficient reflectivity of the plates. Therefore the role played by these modes in optical masers is not settled." Unfortunately these statements might be interpreted to mean that there is doubt as to the validity of the normal mode concept applied to maser interferometers. We should like to correct the impression that the analysis of Fox and Li2 was limited to perfectly reflecting mirrors. As a matter of fact, the reflectivity of the mirrors is completely unimportant in determining the normal modes, providing only that it is uniform over the mirrors. It is quite true that in most solid state masers the inhomogeneities of the medium appear to create so much chaos in the radiation fields that correlation with a simple theoretical picture is often hard to demonstrate. However, gas masers appear to behave in a reasonably ideal way, and both the near-field and far-field radiation patterns for these masers appear to confirm the normal mode picture. In the case of such a maser equipped with plane mirrors, Herriott 3 has observed 1.3 mc beats which correspond well with the expected difference frequency between the dominant (even-symmetric) mode and the lowest order odd-symmetric mode.