Research On Mode-locked Fiber Laser With Low Repetition Rate

Due to its unique features, ultra-short pulse source have been widely used in many fields. To produce the ultra-short optical pulses, people often utilize the mode-locking technology. Because the fiber lasers own high gain, good beam quality, small size, flexibility and easy connection with optical fiber communication systems, many researches are focused on the mode-locked fiber laser.As the idea sources for the dense wavelength division multiplexing systems (DWDM) and optical time division multiplexing (OTDM), the high-repetition-rate ultra-short optical pulses sources have already attracted many close attentions and got rapid development. Compared with the high-repetition-rate ultra-short optical pulses, the repetition rate of the low-repetition-rate optical pulse source is much lower, so the peak power of per pulse is increased at the same average power level. Furthermore, because of the low thermal parasitics and sample damage, this kind of sources are playing an increasingly important role in micromachining, biomedical detection, optical detection, laser radar and other applications. This paper is in the research of the mode-locked fiber laser with low-repetition-rate, several work is carried out as follow:(1)Firstly, this thesis introduces the characteristics and development of fiber laser and highlights the significance of studying ultra-short pulse fiber laser. Then, the working principles of fiber laser, the theoretical foundation of mode-locking technologies and the various configurations of different mode-locked fiber lasers are present. Finally, the passive mode-locked fiber lasers using non-linear optical loop mirror are discussed in detail.(2)Based on the structure of passive mode-locked fiber laser using the nonlinear optical loop mirror, a variable ratio coupler nonlinear optical loop mirror (VRC-NOLM) is designed, and a new type of mode-locked fiber laser is proposed. By inserting a segment of nonlinear optical fiber to the VRC-NOLM, optical pulse with low repetition rate (200kHz) is obtained and the experiment results are analyzed.(3) At the same time, the width of the output pulse can be controlled by changing the output coupler ratio of the variable ratio coupler.(4)By adding a segment of single mode optical fiber in the fiber laser (about 1 km), this laser not only realizes a lower repetition rate (100kHz) of the pulse sequence, but also gets pulse splits and form a multi-pulse output. Because the laser resonator is too long, the stability of the system needs to be further improved.(5)The main idea of the paper is summarized. Based on the work which has already been completed, the further researches on low-repetition-rate mode-locked fiber laser are present.