The calculated gain spectrum of a semiconductor laser based on a generic modal gain model reveals a gain peak that is higher than measured. Analyses of the model and the gain mechanism in semiconductor lasers suggest that the accuracy of the model can be improved by re-formulating the carrier occupation probabilities associated with the model. As a result, good agreement between the optical gain spectrum calculated using the revised model and measured results is achieved.
Evaluation of the influence of laser noise on the precision of single-mode rate equations in modeling laser diode (LD) behavior is performed. The inclusion of Langevin noise sources in simulations is found to increase the relaxation damping significantly. This phenomenon explains the reported truncations of higher-order bifurcations leading to chaos and the enhancement of period-doubling when laser noise is taken into account in simulations. Numerical analysis of the nonlinear dynamics of directly-modulated LD is performed, and results show that laser noise has been mistaken as an important factor which enables an agreement between calculated and measured results to be achieved. Instead, simulations carried out to investigate the effect of current-dependent gain suppression on the nonlinear behavior of LD reveal that temperature plays an important role in modeling LD behavior. Hence, accurate electro-thermal modeling is critical in reproducing measured behavior of LD in simulations.
A novel single-moded self-sustained pulsation mechanism suitable for single-section long-wavelength semiconductor lasers with wavelength constraints is proposed. It is based on the principles of gain-switching and the dispersive property of the active layer in laser diodes (LDs). Numerical simulations using a carrier heating model illustrate the mechanism. Results indicate that if a LD is engineered such that its lasing wavelength, over a range of bias currents, experiences an optical gain that is slightly above threshold, self-gain-switching occurs giving rise to self-pulsations. The self-gain-switching is brought about by wavelength shifting due to index-change induced by carrier density fluctuations and current heating. Depending on the magnitude of the bias current the self-pulsating waveform can be sinusoidal or pulse-like.
A numerical analysis is carried out to investigate the influence of injecting either unmodulated or modulated light from an external laser diode (LD) into a 1.55 micrometers InGaAsP distributed feedback (DFB) self-pulsating (SP) LD. Injection of weak unmodulated light causes the SP-LD to behave mostly chaotically apart from some regions of self-sustained-pulsations (SSP) and multiple-peak periodic oscillations. Increasing the injected optical power has the effects of changing the behavior of the optically-injected SP-LD from chaos to multiple-peak periodic oscillations followed by high-frequency (HF) self-oscillations before reaching an equilibrium state (i.e. a steady level). The SP-LD is found to exhibit quasi-periodic behavior when injected with lightly modulated light. At relatively higher modulation index, the output of the optically-injected SP-LD becomes chaotic.
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