Manipulation of Generation Regimes of a Hybridly Mode-Locked Erbium Fiber Laser by Adjusting Polarization-Dependent Loss in a Cavity with a Distributed Polarizer

The generation of ultrashort pulses (USPs) in a hybridly mode-locked erbium-doped fiber laser is investigated, and the possibility is demonstrated of controlling USP generation regimes by varying polarization-dependent loss in a cavity with a distributed fiber polarizer. Hybrid mode-locking in the cavity was achieved using a combination of two types of saturable absorbers (SAs), including an artificial absorber based on the nonlinear polarization evolution (NPE) mechanism, and a natural absorber made of a thin polymer film with single-walled carbon nanotubes (SWCNTs). The anomalous group velocity dispersion (GVD) in the cavity at –0.19 ps2 enabled the generation of solitons with a low pump power threshold at a wavelength of 976 nm (48 mW). The intracavity transmittance of both SA types was explored, and the evolution of the laser generation regime during the pump power variation was studied as well. Experiments have shown that, as the pump power increases, a transition occurs to the generation of “soliton molecules” consisting of two pulses. In addition, the possibility was investigated of manipulation over USP generation regimes by varying the polarization-dependent loss introduced by NPE through polarization controller adjustment inside the cavity. The results demonstrate the advantage of hybrid mode-locking due to reduction in its sensitivity to changes in polarization- dependent loss and fluctuations in laser pump power compared to the NPE mechanism, as well as the possibility of obtaining various USP generation regimes at a fixed pump power.