Gain and noise in subthreshold longitudinal laser modes

A theoretical study is presented of the gain profile and noise spectrum in the vicinity of the longitudinal-mode frequency separation of a single-mode laser amplifier of class A or class B. The gain and the noise are associated with the adjacent subthreshold modes on either side of the lasing mode; coupled equations of motion for these three modes have been solved. The amplitudes of the contributions of each individual subthreshold mode to both the gain and the noise are generally anticorrelated. Thus striking cancellation effects should occur in self-heterodyne measurements from the superposition of these contributions. The theoretical predictions are compared with the results of a parallel experimental study on a single-mode argon-ion laser amplifier. Measurements of the gain profiles and noise spectra close to the longitudinal-mode separation are reported for a range of laser output powers. Results are presented for simultaneous detection of the contributions of the two adjacent modes; for detection of only one adjacent mode, the other being suppressed with a Fabry-Pérot étalon; and again for detection of both modes but with their phase relative to the central laser beam modified by a Fabry-Pérot phase shifter. The predicted cancellation effects are clearly confirmed, and the gain profiles and noise spectra are in good agreement with the theoretical expressions. The noise-cancellation effect is demonstrated by the generation of an optical ‘‘antinoise’’ beam that is nearly 180° out of phase with respect to a broadband noisy input beam.