Optical Solitons In Fiber Lasers Mode-locked With Nonlinear Polarization Rotation Technique

Optical fiber solitons have many applications in optical communication systemsbecause they can maintain their waveforms over long distances by balancing the effectsof nonlinearity and dispersion. Now, optical solitons have become a research focus ofnonlinear fiber optics. Compared with other soliton sources, fiber lasers mode-lockedwith nonlinear polarization rotation (NPR) technique have advantages of simplestructure and wide bandwidth. The lasers can generate femtosecond soliton pulses withvarious soliton phenomena, which are ideal platforms for studying the optical soliton.In this dissertation, a comprehensive investigation on soliton characteristics in thefiber laser mode-locked with NPR technique has been presented. Firstly, we analyzedthe NPR mode-locking mechanism and the cavity transmission properties. Themode-locking area and tunable filtering effect of the fiber laser were also discussedbased on the cavity transmission rate. Secondly, a passively mode-locked fiber laser wasconstructed by using the NPR technique in the experiment. The laser was operated inthe negative net cavity dispersion regime. An ultrashort soliton with center wavelengthof1566.5nm and pulse width of295fs was obtained at a pump power of around30mW.Bound soliton, multi-soliton and harmonic mode locking, have been observed in thelaser by appropriately increasing the pump power.In the numerical research, a “pulse tracing” method was adopted to simulate thesoliton operation of fiber lasers mode-locked with NPR technique. Based on a fiberlaser with negative net cavity dispersion, various soliton operations such as singlesoliton, bound soliton and multi-soliton were numerically observed. The solitondynamics in the laser cavity were investigated. In addition, the effect of theerbium-doped fiber gain coefficient, the phase difference caused by the polarizationcontroller and the cavity fiber birefringence on the laser operating state were alsodiscussed. By changing the length of fibers with different dispersion in the cavity, thedark soliton operation of fiber laser with positive net cavity dispersion was numericallysimulated. A stable dark soliton with pulse width of5ps was observed. At last, we presented a technique for the generation of high repetition rate soliton pulse in fiberlasers. By incorporating a segment of high birefringence fiber in the fiber lasermode-locked with NPR technique, a spectral filter was induced in the laser cavity due tothe fiber birefringence filter effect. It was numerically found that soliton pulses atrepetition rate of345GHz could be obtained. Further studies have shown that therepetition rate of solitons depended on the bandwidth of the fiber birefringence filtereffect and could be varied by changing the length of the high birefringence fiber.