Self-generation of optical frequency combs in single section QD lasers: theory and numerical modelling (Conference Presentation)
Optical Frequency Combs (OFC) generated by semiconductor lasers at optical communication wavelengths are promising laser sources for high capacity optical interconnects exploiting WDM techniques; very often they are integrated with Silicon Photonic integrated circuits to realize compact and low-cost transmitters. Quantum Dot (QD) or Quantum Dash (QDash) single section Fabry-Perot lasers have turned to be a good candidate for this application because they can generate a comb of self-locked optical lines using just one laser diode operating in CW and no saturable absorber section. In this talk we review the state-of-art of these devices and their applications, evidencing also the analogies with single section Quantum Cascade Lasers, that, as for QD and QDash lasers, generates optical combs in the mid-IR or THz range. We will focus on the understanding of the physical effects that can explain the self-locking of the lasing lines and we will compare the self-locking mechanism in Quantum Dot and Quantum Well lasers. We will then present the numerical simulation tool we have developed to simulate the self-locking in Quantum Dot Fabry-Perot lasers. Our model is based on a Time-Domain Traveling-Wave (TDTW) approach that properly accounts for coherent radiation-matter interaction in the semiconductor active medium and includes the carrier grating generated by the optical standing wave pattern in the laser cavity. We show that the latter is the fundamental physical effect at the origin of the multi-wavelength spectrum appearing just above the laser threshold, but it is not enough for forcing the self-locking of the optical lines. The self-mode-locking regime associated with the emission of OFC is achieved for higher bias currents and it ascribed to nonlinear phase sensitive effects as Four Wave Mixing (FWM). To quantify the locking of the lines we have calculated some indicators that are obtained by the post processing of the calculated optical electric field of the laser output. These indicators are the RF spectrum at the beat note, the optical linewidth of the lasing lines and the Relative Intensity Noise (RIN) spectrum for both the total power and the power of each line. Varying the CW injected current above threshold we have observed three different regimes: in the first one, at low current, the laser is dominated by multi-wavelength emission with rather wide RF beat note and high low frequency RIN, this regime corresponds to an unlocked regime. In the optical spectrum we observe an optical line and side bands due to FWM components. In the second regime, at much higher current, the RF beat note is extremely narrow and the low frequency RIN of each line reduces significantly; in the optical spectrum the lines narrow and the side-bands disappear. This is a self-locked regime. In an intermediate current range, we have a transition regime where the state (locked or unlocked) depends on the initial conditions. Our results explain in detail the behavior observed experimentally by different research groups and in different QD and Quantum Dash (QDash) devices.