The operation of coupling-type internal modulators is reviewed briefly, and the bandwidth limitations are examined. It is found that excitation of natural modes of the resonator will normally restrict the signal bandwidth to half the longitudinal mode spacing. However, the bandwidth can be extended if the maser is permitted to oscillate in only one mode, if only a small fraction of the internal energy is coupled out, and if large attenuation can be introduced at appropriate resonator mode frequencies.
I. P. Kaminow, "Internal Modulation of Optical Masers (Bandwidth Limitations)," Appl. Opt. 4, 123-127 (1965)