Nonlinear Dynamics in Semiconductor Lasers - Abstract
Wang, Cheng
Directly-modulated quantum-dot (QD) lasers are promising candidates for the low-cost high-speed optical communications. However, the existence of the wetting layer (WL) associated with the excited states (ES) is known to cause severe limitations in the modulation bandwidth [1, 2]. In order to overcome such a problem, the injection-locking technique is an attractive approach for improving the modulation characteristics [3, 4]. For instance, the modulation response of an injection-locked laser can exhibit a broadband and relatively flat response under the zero detuning condition [4], while under positive detuning operation, it shows a higher resonance frequency but with a bandwidth limited by the so-called pre-resonance frequency dip [3]. Although it has already been demonstrated that larger bias current is beneficial to suppress the dip [4], it remains crucial to identify other key parameters for controlling its magnitude. To this end, this paper aims to theoretically analyze the role of the carrier capture and relaxation times, the linewidth enhancement factor (LEF) as well as the gain compression on the frequency dip degradation. The model is based on a set of five rate equations: one for the ground state (GS) photons, another one treating the phase difference between the slave and the master lasers while three rate equations are used to describe the carrier dynamics in WL, ES and GS, respectively. The modulation response is obtained through a small signal analysis of the differential rate equations. Simulations prove that fast carrier capture rate diminishes the dip, and hence well enhances the bandwidth. This result also means that shallow QDs with fast carrier capture rate are more favourable for the dip suppression and the bandwidth enhancement. Besides, simulations show that fast carrier relaxation rate also contributes to the dip suppression while small LEF mitigates the dip. However, once the dip is located beyond the 3-dB level, larger LEF can boost the modulation bandwidth with a higher relaxation peak. Finally, calculations point out that the gain compression effect aggravates the dip and reduces the peak, leading to the decrease of the modulation bandwidth. References [1] C. Wang et al., IEEE J. Quantum Electron., vol. 48, pp. 1144-1150, (2012). [2] L. Asryan et al., Appl. Phys. Lett., vol. 98, pp. 131108, (2011). [3] A. Murakami et al., IEEE J. Quantum Electron., vol. 39, pp. 1196-1204, (2003). [4] E. K. Lau et al., IEEE J. Quantum Electron., vol. 44, pp. 90-99, (2008).