Study On Novel Mechanisms And Techniques For Improving Performance Of Mode-locked Fiber Lasers

In this dissertation, the extended nonlinear Schr?dinger equation is derived, based on which several operation mechanisms and cavity structures of mode-locked fiber lasers based on all-solid Yb-doped photonic bandgap fiber (AS-Yb-PBGF) have been simulated numerically by split-step Fourier method and their relative characteristics also have been discussed. Additionally, spectral filtering regimes of all-normal-dispersion (ANDi) mode-locked fiber lasers have been investigated theoretically and numerically. In order to verify the simulation results, we constructed ANDi mode-locked fiber lasers with linear cavity and ring cavity, respectively, in experiments. The detailed contents can be classified as follows. 1. Based on AS-Yb-PBGF providing laser gain, negative dispersion and nonlinearity, we numerically constructed several passively mode-locked fiber lasers with different cavity structures and operation regimes, and discussed their operation characteristics and relationships with some laser parameters, which is specialized as follows. (a) Employing a single segment of AS-Yb-PB??o build a linear cavity mode-locked fiber laser model, there is not any fiber elements with norma??spersion in the cavity, thus the laser operates in negative dispersion region resulting in the generation of conventional soliton pulse trains. By shortening the length of the AS-Yb-PBGF, 500 MHz soliton pulse repetition rate was obtained. (b) A passively mode-locked fiber ring laser model was constructed, in which NPE was used to initiate and sustain mode-locking operation. The output soliton pulse which is nearly transform-limited pulse has the width of 224 fs, and repetition rate is 200 MHz. (c) By lengthening SMF in the cavity, self-similar mode-locked fiber ring laser was built. Simulation results show that the pulse characteristics are dependent on the length of the SMF in the cavity, and appropriate SMF length could lead to the generation of the parabolic pulse. (d) A weak-breathing mode-locked fiber ring laser model was presented. In order to make the laser operate in high normal dispersion region, a spectral filter was introduced in the cavity. By the investigation of the effects of the filtering bandwidth on the laser performances, an optimal filtering bandwidth was achieved. (e) It is a general knowledge that large third-order dispersion (TOD) will lead to pulse distortion in dispersion-managed soliton lasers. By introducing a spectral filter in a dispersion-managed soliton fiber laser model, the resulting pulse quality was improved, and the pulse duration was narrowed. (f) Under the condition of 10 nm gain bandwidth, the effects of TOD in AS-Yb-PBGF on the output pulse energy enhancement and pulse duration narrowing were discussed. 2. The effects of spectral filtering bandwidth and filtering position on pulse shaping regimes of highly chirped pulses in ANDi fiber lasers were discussed with emphasis. Furthermore, integrated impacts of four structural parameters, such as filtering bandwidth, filtering position, output position and output coupling ratio, on the laser operation performances have been studied systematically. The optimized filtering position and output position were obtained. At the same time, we got the optimal filtering bandwidth and output coupling ratio for achieving higher pulse energy and stronger pulse peak power. In ANDi fiber lasers, the integrated impacts of the four structural parameters, especially the filtering position effect, have not been reported in earlier studies, thus our work is of importance for constructing high energy, high peak power ANDi mode-locked fiber lase?? 3. An ANDi mode-locked fiber laser with linear cavity was built in experiment, in which SESAM w??sed as mode-locking element and end mirror. Another end mirror was to use high reflectivity, narrow bandwidth fiber Bragg grating. The output pulse energy is 423 pJ, with average power of 7.4 mW. The pulse repetition rate is 17.5 MHz. As using the output pulse from a 50% fiber coupler as seed pulse, amplified pulse average power is up to 140 mW. The energy per pulse is 10.4 nJ, with pulse width of 5.53 ps and peak power of 1.57 kW. 4. Employing two quarter-wave plates and one half-wave plate to realize NPE mode-locking, an ANDi fiber ring laser was constructed. In the laser a 10 nm bandwidth fiber filter was spliced in the cavity. As varing the filter position the optimal filtering position has been found, namely, the location before the gain fiber. Therefore, the experimental result agrees with that of simulations, which verifies the necessary to choose the filter position in ANDi fiber lasers. Under 320 mW pump power, the output pulse with 92.2 mW average power, 3.5 nJ pulse energy and 6.2 ps pulse width has been obtained. Additionally, this fiber laser has high stability, whose SNR is up to 80 dB. The rms amplitude noise measured is ~0.06% and the rms timing-jitter is ~200 fs for the frequency range from 6 Hz to 30 kHz.