Dynamic mode-spacing control of an optical frequency comb quantum dash semiconductor laser emitting at 1535 nm

Semiconductor photonics based broadband optical frequency combs are attractive sources for ultra-compact spectroscopy and data communication applications. In order to dynamically control their intrinsically fixed optical mode comb spacing, recently optical self-feedback has been demonstrated to be a viable and effective method [Fiala et al, DPG Spring Meeting Berlin (2018), DY 69.18]. The impact of selected optical feedback delays and strengths on the mode beat line width and on the comb spacing of single-section quantum dash lasers emitting at 1535 nm are investigated experimentally. For fiber-based feedback cavity lengths ranging from 6 m to 73 m the dynamic mode spacing control spanning from 2 MHz to 22.5 MHz is discussed. The mode beating line width can be simultaneously reduced from 20 kHz (free-running) to 2.5 kHz. Experimental results are validated by a simple stochastic time-domain model.