Selectivity of open-ended interferometric resonators

Several different versions [1]-[3] of the open-ended Michelson-type interferometric resonator have been proposed as axial-mode selectors for wide-band lasers, primarily for the purpose of restricting laser oscillations to a single frequency. Their frequency selectivities are compared in terms of their loss discrimination against the nearest unwanted mode, by evaluating the transmission-loss fraction via the open port of the beam splitter with a given number of axial modes in a free spectral range of the mode selector. Vernier-type interferometric resonators, in which the straight and folded cavities are of nearly the same length, are found to have substantially the same selectivity when they have an active element in the common arm [1] as when they have active elements in one or both of the two branch arms [2]. Resonators in which one of the coupled cavities is much smaller than the other [3] have considerably better selectivity, with up to four times as much loss discrimination against the nearest unwanted mode, because of the greater frequency separation between noncoincident eigenmodes of their decoupled cavities. The ring-laser counterpart of each version has the same frequency-selective properties with twice as much internal gain.