Study on dynamic behavior of ring-cavity quantum-cascade lasers with group velocity dispersion

Abstract

Quantum cascade lasers (QCLs) have been the common mid-infrared sources because of their high output power and agile wavelength range owing to the intersubband transitions. Previous research on QCLs shows that the instability and dynamic behaviors are more complicated than conventional semiconductor lasers due to interaction among various intracavity effects, such as material nonlinearities, unique gain and dispersions. Group velocity dispersion (GVD) plays an important role in the study of QCLs because of its effects on pulse spectrum in both unsaturation and saturation regime. To investigate the effect of group velocity dispersion in QCLs, both instability analysis and time-domain numerical simulation are presented in this thesis. A model that accounts for GVD effect and a background saturable absorber is developed based on Maxwell-Bloch equations. Stability analysis of a ring cavity QCL is performed through the linearization of dynamic equations. Depending on the results of stability analysis, the numerical simulation for time-domain pulse progression based on the split-step method is demonstrated. It is found that the instabilities of QCL with GVD are highly related to the pumping strengths, and stronger saturable absorber effect tends to make the system more stable. Numerical simulations show that Rabi side modes are enhanced by saturable absorber effect and pumping strength; GVD effect lowers the intensity of stable pulses and shrinks the pulse width. The split-step method successfully combines nonlinear and linear effects in the dynamic equations and finally leads to stable results.