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We study the coherent multi-mode phenomena in single section Quantum Dot (QD) lasers using a time domain traveling wave approach.
In the conventional Fabry-Perot configuration and close to the lasing threshold, we report a continuous wave solution (CW) instability consisting in several longitudinal modes turned on by the strong carrier grating due to the standing wave pattern. In this regime we found intervals of the bias current where the FP-QD laser spontaneously emits Optical Frequency Combs (OFC) as narrow, equally spaced, spectral lines with locked phases.
Interestingly, in the unidirectional ring configuration, where carrier grating due to standing wave pattern cannot take place, our simulations show the occurrence (at high pump current) of a new type of self-pulsing phenomenon leading to sub-picosecond pulses with THz repetition rate, multiple of the ring free spectral range. The linear stability analysis of the CW solution of the ring laser is in good agreement with the numerical simulation and it allows to establish an analogy between the observed self-pulsing regime and the well known Risken-Nummedal-Graham-Haken instability consisting in the amplification of the Rabi frequency of the system. Systematic simulations also indicate that, contrary to what happens for self-generation of OFC in FP laser, THz self-pulsing is robust and controllable over a wide range of bias currents, device lengths and degree of inhomogeneous gain broadening. Our results on FP lasers well agree with recent experimental evidences.
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