An investigation of the frequency stability and temperature characteristics of 1.5 µm coupled-cavity injection lasers

It has been demonstrated, both theoretically and experimentally, that a coupled-cavity injection laser is capable of oscillating with a single-frequency output. However, because of the nature of the constructive and destructive interference of the optical fields inside the coupled cavities, the frequency of the lasing mode is subject to change due to variations in the refractive indexes of the active media, which in turn are functions of both injected carrier density and junction temperature. In this paper, the frequency stability (mode hopping) and temperature characteristics of such lasers are computed numerically for two specific cases based on a rate equation formalism previously reported. The behavior of experimental lasers fabricated as cleaved-coupled-cavity (C
³
) devices showed good agreement with the computed result, except that the measured light-versus-current (
L-I
) curves were "kinky." Each kink was found to signify mode hopping, which is believed to be caused by incomplete electron pinning above lasing threshold. The speed of mode switching from the initial lasing mode to the adjacent lasing mode was less than 1 ns and a hysteresis was observed when both cavities were operated with large gains. Experimental results have shown that when the C
³
laser is operated in a steady-state single-frequency output the photon fluctuations are significantly reduced, which implies that the coupled-cavity laser can be used as a CW signal source, in conjunction with an external modulation, which is free from partition noise at gigabit per second data rates.