Self-Injected Optical Frequency Comb Quantum Dash Lasers

Self mode-locked (SML) quantum dash (QDh) lasers emitting at around 1550 nm are compact broadband optical frequency comb (OFC) sources for multi-wavelength communication applications [1,2]. Optical self-feedback (OFB) by auxiliary cavities with lengths in the meter-range is a well-established method for a precise and dynamical mode spacing control of passively mode-locked (PML) semiconductor lasers. Recent works focused on identifying maximum tuning ranges of quantum well PML lasers and suggested that external OFB length, interaction width of fed-back pulse with the pulse inside the laser cavity and the OFB strength appear predominant [3]. In this contribution, we experimentally investigate and explain by modeling the dependence of mode spacing of OFB of SML QDh lasers subject to different cavity lengths on the external optical delay. Here, we study the impact of different OFB strengths and external OFB cavity lengths onto the mode spacing of QDh SML lasers. The three lasers investigated are 1 mm and 2 mm long, corresponding to a mode spacing of 40 GHz and 20 GHz, base on 6 and 9 layers of InAs/InGaAsP Qdhs [1,4] and exhibit as-cleaved facets. In the developed experimental set-up, sketched in Fig. 1a), the laser light is coupled into a lensed single-mode fiber and a part of the light is directed to the hybrid fiber based and free-space OFB cavity.