Small-Signal Analysis of Internal (Coupling-Type) Modulation of Lasers

Abstract

The internal modulation of lasers by a variation of cavity losses is analyzed in terms of the normal modes of the system. Time‐dependent perturbation theory is used to describe the results of resistive mode coupling when the losses are modulated with small signals. For the single‐mode oscillator, an expression is obtained for the amplitude distortion in the modulation index of the light produced by a coupling‐type internal modulator when the losses are modulated at a frequency commensurate with the separation between longitudinal modes of the laser cavity. Low distortion is obtained when the modulating frequency is noncommensurate with the longitudinal‐mode frequency separation. The internal coupling‐type modulator in its present form can provide small amounts of modulation over bandwidths limited to the separation between adjacent interferometer cavity normal modes. For a multimode oscillator, modulation of the internal losses at a frequency equal to the separation between adjacent longitudinal modes produces a pulse‐modulated output wave. The average intensity is unchanged and the peak intensity is increased over the intensity of the unmodulated laser by a factor equal to the number of oscillating modes.