Optical Injection Locking of Longitudinal Modes in a Discrete Mode Laser: application in Gain-Switched Optical Frequency Combs

A detailed experimental study of the effect of external optical injection (OI) on a single-mode discrete mode laser (DML) when the wavelength of the OI is far from that of the solitary DML is presented. We show that the situation observed in Fabry Pérot lasers by Jain et al. extends to single-mode DMLs since injection locking of many longitudinal modes of the device is obtained, giving rise to a wavelength tunability larger than 37 nm. We show that the injection locking range is minimum when injecting close to the longitudinal mode that appears in the free-running DML ( $q=0$ ). We present the observed nonlinear dynamics of the laser when subject to OI with a wavelength close to a longitudinal mode with $q\ne 0$ and in particular, we show that OI induces dynamics simultaneously in several longitudinal modes. Different nonlinear dynamics are observed for positive and negative frequency detunings with respect to the injected mode. We also investigate the effect of OI in optical frequency combs (OFCs) generated by gain-switching the DML in a very wide range of wavelengths of the OI. We show that wavelength tunable injection-locked OFCs are obtained with quasi-continuous wavelength tunability larger than 37 nm.