Abstract

The importance of nonlinear gain on the behavior of semiconductor lasers is well known. Both self and cross saturation have been included in the laser rate equations to explain, for example, damping of the relaxation oscillations and mode jumping to longer wavelength with increasing current. However, the theoretical analyses^{[1, 2]} which derive nonlinear gain expressions also show that four-wave mixing (FWM) between the longitudinal modes will be present in the laser. In contrast to cross saturation, FWM provides a mode-coupling mechanism which is dependent upon the individual mode phases. Recent experiments indicate the importance of FWM in semiconductor lasers. In one experiment, the double-peaked structure in the field spectra of the nonlasing side modes of a nearly single mode laser is attributed to FWM.^[3] In another experiment, passive FM locking of the longitudinal modes is observed and explained on the basis of FWM.^[4]

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