Stability of coupled semiconductor diode lasers

This dissertation investigates the stability of self and mutually coupled semiconductor diode lasers. Theoretical analysis is based on coupled rate equations which follow from time-dependent coupled mode theory, including complex coupling coefficients describing optical interactions between the lasers. Phase-locking conditions and small signal stability properties are analyzed for the case of two mutually coupled lasers, including self-coupling. These are found to be strongly influenced by both the linewidth enhancement parameter alpha and the phase of the coupling coefficient. A wide locking bandwidth and large mode suppression are shown to be incompatible due to a non-zero alpha. Two different forms of stability are identified. The model is applied to the cases of one and two diode coupled at a distance. Systematic experiments have been designed to study the stability of these systems as a function of coupling magnitude and phase. Careful spectral and coherence measurements are compared directly to numerical simulations from the rate equation model, including nonlinear gain compression and spontaneous emission noise. Modes, mode selection and dynamic stability properties are shown to be well accounted for by the rate equation approach. An accurate comparison is enabled through independent measurements of the model parameters for the experimental devices. Further, techniques to reliably determine the level of optical coupling have been developed. Three distinctive operating regimes are demonstrated with respect to the coupling magnitude. At low coupling level, mode selection is governed by phase stability. While all oscillating modes are stable, phase-locking is found to be limited by spontaneous emission noise throughout most of this regime. These findings are corroborated through numerical integration of an approximate form of the rate equations. In contrast, at large coupling levels, mode selection and dynamic stability are dominated by threshold gain considerations; where the most stable mode minimizes the population inversion. Here, the effect of alpha on the stability properties is minimal. <Intermediate levels of coupling are characterized by extreme dynamic instability, resulting in spectral broadening and a near total loss of coherence in the coupled system. &This regime is interpreted as a transition between phase- and gain-dominated modes of operation, initiated by the system's inability to damp relaxation oscillations.