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Discrimination Against Unwanted Orders in the Fabry-Perot Resonator

01 March 1962

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The Fabry-Perot interferometer has recently become important as a resonant cavity for electromagnetic radiation at optical frequencies. 1,2 ' 3 ' 4 The nature of the modes of such a cavity has been discussed by Schawlow and Townes 1 and by Fox and Li.5 The modes may be specified by three quantum numbers, one of which is the familiar order number giving the separation of the plates in units of the half-wavelength. The other two quantum numbers specify the possible field configurations across the plates, which are essentially identical in each order. 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. On the other hand, fine structure which could be due to various Fabry-Perot orders has been seen in the output of both the gas6 and the ruby 7 optical maser. It has been pointed out,1'8 that the optical maser is inherently a multimode device, and that the excitation of many modes can lead to undesirable effects in the noise, stability, and ultimate usefulness of the device. Therefore it is proposed here that it would be useful to discriminate 453