Research On Key Technologies Of Novel Fiber Lasers

Due to the variety of generated solitons, soliton mode-locked fiber lasers have been applied to many applications. For instance, dissipative solitons (DSs) with high energy and a large chirp can be generated, which makes soliton mode-locked fiber lasers be considered as good pump sources for supercontinuum (SC) generation; a soliton pair which is formed by two solitons with small time space and strong interactions can also be generated, which makes them valuable in many potential applications such as high-speed photography, ultra-fast dynamics researches and so forth. The development law of mode-locked fiber lasers indicates that a discovery of a novel phenomenon always promotes the development of mode-locked fiber lasers, and enhances the understanding of the theory of mode-locked fiber lasers.This dissertation mainly focuses on the technologies of soliton mode-locked fiber lasers and one of their major applications-SC generations. The main research efforts are summarized as follow.1. A DS passively mode-locked fiber laser based on a semiconductor saturable absorber mirror (SESAM) is studied. Firstly, theoretical study is carried out by numerical simulations on its typical operation states: single DS mode locking and multiple DSs bound state mode locking. Then a passively mode-locked fiber laser based on a SESAM is built, which can generate DSs when the pump power is 88 mW. With the pump power of 175 mW and 265 mW, the fiber laser can be operated at three states:multi-pulse mode locking, harmonic mode locking (HML) and bound state mode locking. Transitions among these three operation states can be achieved by only adjusting a polarization controller (PC) inside the laser cavity. A correlative relationship among those states (N pulses/Nth-order HML/ bound state of N pulses) is revealed based on the experimental results. Experimental results validate and develop the simulation results2. Theoretical and experimental researches on mode locking states of soliton pairs (a soliton pair is composed of two bound solitons) in a passively mode-locked fiber laser based on a SESAM are demonstrated.Experimental results show that the discovered correlative relationship still exsits in the soliton pairs in the mode-locked fiber laser with pump powers at 34.81 mW, 50.76 mW and 64.05 mW, respectively. When the pump power is 34.81 mW, a more stable mode locking state based on soliton pairs is observed, which further validates that discovery. When the fiber laser is operated on two-pulse (each pulse is composed of a soliton pair) mode locking state, novel dynamic spectra (unusual modulation envelopes) are observed by adjusting the PC inside the laser cavity. The period of modulation envelopes decreases along with the increasing time space between two solitons. Numerical simulation results agree well with the experimental results, based on which we suggest that the birefringence inside the laser cavity may cause the novel phenomenon.More experimental results confirm that guess and can also be used to explain other observed phenomena mentioned above.3. An experimental research on a high power, blue-enhanced,ultra-wide SC fiber laser is demonstrated. An all-fiber SC generation system is built, including a picosecond pulse seed fiber laser, an all-fiber four-stage pulse power amplifying system based on a master oscillator power amplifier (MOPA) structure, and a piece of photonic crystal fiber(PCF). Experiments have been done in two actively mode-locked fiber lasers with a polarization-maintaining (PM) cavity and a non-PM cavity,respectively. According to the experimental results, the phenomenon of the mode locking state of high order HML suddenly turns into fundamental mode locking state when the voltage driving the modulator is altered is observed. This phenomenon has been studied and been explained numerically; the simulation can also guide the design of the seed fiber laser. By introducing an extra chirp to the seed pulses,optimizating pump powers of laser diodes in every stage of the amplifying system to avoid nonlinear effects, and controlling the fiber mode to ensure high amplification efficiency, and so forth, the output average power of the generated SC can be increased from 3.8 W to 30.6 W, and its spectrum can be broadened from 418 nm?2400+ nm to 385 nm?2400+ nm.