Strategies for developing tunable multiwavelength mode-locked semiconductor fiber ring lasers

Compact optical sources that generate picosecond pulses at multiple wavelengths are of interest for numerous applications in optical instrumentation, fiber optic sensing, and optical communications. In recent years, numerous methods have been demonstrated to obtain multi-wavelength, mode-locked (ML) operation from erbium-doped fiber lasers (EDFLs) and semiconductor fiber ring lasers (SFRLs). In contrast to EDFLs, the use of semiconductor optical amplifiers (SOAs) allows for stable, multi-wavelength emission at room temperature with narrow wavelength separation since they are not constrained by the EDF homogenous broadened linewidth, and for operation over a wide wavelength band. To increase the functionality for some applications, it is also important to be able to tune the output wavelengths of the optical pulse source. In this paper, we provide an overview of our on-going work on developing tunable multi-wavelength, ML-SFRLs. In terms of achieving multi-wavelength operation, we have used multi-wavelength filters based on a high-birefringence Sagnac loop and superimposed fiber Bragg gratings. In terms of short pulse generation, we have explored two different methods for mode-locking: the use of an intra-cavity electro-optic modulator and the injection of an external optical control signal to modulate the gain of the SOA via cross-gain modulation. Finally, in terms of wavelength tunable operation, we have exploited dispersion tuning, i.e. the use of a dispersive cavity and changing the modulation frequency of the mode-locking element. We present and discuss our results for two different ML-SFRL configurations.