| Picosecond fiber laser is widely studied because of high stability, easy starting, short pulse and low cost. The output picosecond pulse has become a hot topic due to virtues of high peak power, high repetition rate, long transmission distance, high precision and large power. Combined with the saturable mirror(SESAM mirror) and fiber, picosecond fiber laser made of SESAM mirror shows much importance in many fields. Therefore, the numerical research on its theory can make the experiment simple and also directly reflect the impact of the parameters on the pulse. So this kind of numerical research is very necessary.SESAM mirror often consists of the saturable absorber and semiconductor reflector. The absorption rate of the mirror is varied with the light intensity. Before the saturable absorption intensity, the absorption decrease as the light intensity increase. Thus, it can act as a mirror, and good pulse mode-locked can be achieved. As a result, the structure can be simplified.In this thesis, a picosecond laser in a linear cavity based on SESAM mirror is mainly studied. Based on theoretical formula, impacts both on the pulse waveform and power were numerically discussed from YB fiber, SESAM mirror, FBG. The main contents are as follows:1. The structure and principle of the fiber laser is introduced. The numerical equations are deduced in detail. The transmission characteristics of the fiber is analyzed. Impacts on the pulse’s width and shape are discussed from dispersion and nonlinear of the fiber.2. The structure of gain fiber is analyzed in detail. The reason why Yb3+ acts as the gain medium is explained. Based on, the power in the cavity and the output power of the pulse are systematic simulated from the rate equation by MTLAB software. Parameters such as the reflect rate between the two mirrors, pump power, the doping concentration, the length of fiber are considered.3. Advantages in structure and development of SESAM mirror are described. From its locking characteristics, impacts both on the modulation of SESAM and its pulse are numerically studied using the fox-li iteration. The linear Schrodinger equation and the transport equation are adopted. It is indicated that a perfect pulse is obtained when the modulation depth △R is beyond 3.8%.4. The characteristics, production and the corresponding specific formulas are given. Changes in the length even distortion of the pulse are investigated by variation of the length or the coupling coefficient of FBG. The propagation characteristics of pulse are also analyzed. The results show that a good output pulse can be obtained when 4.9 nm≤LFBG≤7.9 nm and 8.1 nm≤LFBG≤11.1 nm. Keywords: picosecond fiber laser, SESAM mirror, FBG, pulse, numerical analysis... |
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