| The works described in this are under the support of National Key project and the National Natural Science Foundation of China" The Research on Noise Property of Broadband Ytterbium Fiber Amplifier".As the Front-end of a controllable fusion with the highest power and most advanced laser driver in China, the Front-end system should provide several beams of high quality optical shaped pulse for the power amplifier system, the pulse seeds are required to have the energy of mJ level and certain spectrum span, high signal to noise ratio, high beam quality and high stability. Around the pulse shaping technology, which is the key one in Front-end, there are two main structures such as pulse stack shaping and shaping by modulating the continuous laser. We systematically studied the fundamental and key physical problems and gave some simulation results, which could provide important advising for the research of Front-end system. At the same time, we can support the study by providing some crucial apparatus with deep understanding to the entire system.The major results in the dissertation are obtained as the follow:1. In this dissertation, the author analyzed the pulse stack technology of formal Front-end system, set up the theory modeling for this shaping method, proposed a way to use delay-phase area to study the smooth effect of pulse stack. Through the simulation and analysis we suggested the delay of stacking pulse should be equal to the pulse width. This conclusion has been proved to be correct in the experiments. Then we investigated the stack technology by using chirped pulse, which decreases the requirement of delay control compared with the coherent stacking. Employing this kind of pulse stack scheme is also very suitable to apply the smooth by spectrum dispersion (SSD). We also studied the influence of delay error to the smooth result and proposed the pulse selecting criterion in this stack system. A pulse stacker scheme based on Faraday rotation mirror was proposed and discussed to get polarization maintenance stacker using standard single mode fiber. It has been confirmed as the final scheme of the stacker and has been implemented.2. The detail of the propagation process in the Front-end is very important for the experiment. We investigated the properties of the pulse in entire system, studied the change of frequency chirp and pulse width after filter and long distance propagation, and analyzed the influence of this change to shaping result. Theoretical analysis proved the interval of the shaping pulse generated by chirped pulse stack is shorter than 10 ps, which has no harmful influence to application. We also analyzed the mechanism and threshold of some nonlinear effects, such as SBS, SRS, FWM, SPM.3. In laser Front-end system, a stable 1053 nm ultra-short pulse laser with high power is essential for the implementation of pulse stacker. We did some research on passively mode locked laser employing Semiconductor Saturable Absorber Mirror (SESAM). This laser with the features of high stability and repetition rate, high power, low noise and certain operation wavelength has been applied to the formal Front-end. Furthermore, we investigated a kind of Master Oscillator Power Amplifier (MOPA) system-Chirped Pulse Amplifier (CPA), in which the pulse is stretched by Chirped Fiber Bragg Grating (CFBG), and then amplified in the amplifier; at last it is compressed in another CFBG which provides reverse dispersion. Using this system could free the amplification process from nonlinear effects and supply power to the signal effectively. We studied the influence of spectrum and gain characteristic by using CPA.Based on Nonlinear Polarization Rotator (NPR) effect, we obtained a reliable self-starting mode locking of ytterbium (Yb)-doped fiber laser, of which the spectrum bandwidth is over 80 nm. So far as we know, it has the broadest spectrum in the lasers with the same structure. It is estimated that a pulse with several tens of femosecond width will be observed from this laser after compressing out of the laser cavity.4. Studying the fluctuation in shaping pulse in modulation scheme of backup Front-end, it's obtained that lack of high frequency part of the laser pulse causes the fluctuation with the frequency of phase modulator, while the unbalanced sidebands make this effect easy to be observed. The influence of the shape, bandwidth and mismatch of the filer on the pulse fluctuation was also discussed. Furthermore, we proposed a way to suppress the high frequency fluctuation. In the backup Front-end system, a stable, low noise continuous-wave (CW) laser is needed as the system seed. In the dissertation, we introduced a phase shift single frequency CW laser using Distributed Feedback Bragg (DFB) fiber grating as gain media. This laser has some unique features such as narrow line width, high Signal to Noise Radio (SNR), high power stability and so on, which is suitable as the seed of backup Front-end.5. We proposed three schemes of SESAM mode-locking lasers at communication band (1.5 um) based on our research of 1.0um mode-locking lasers. All the three schemes are in F-P configuration, in which one mirror is a SESAM and another either FBG, linear sampling CFBG, or fiber Sagnac loop mirror. The FBG version can be used to study the mechanism of Q switched mode locking or harmonic mode locking, while the CFBG one can generate 0.8nm spaced multi-wavelength mode locking as in DWDM systems, and the last one is a wavelength tunable version by introducing a PC in Sagnac loop mirror. The lasers have potential applications in high speed optical communication, optical sensing, and super continuous light.The experimental results have verified our modeling and simulation. The lasers developed for the Front-end operate with good condition.The innovative results in this dissertation are as followings:1. SESAM based reliable mode locked laser and ultra broadband NPR mode locked laser are developed. Low-noise, smooth- spectrum and high-stability mode locking is achieved by optimization of the cavity. The former has a pulse width of hundreds of femosecond and about 50 MHz repetition rate, whose high stability has been proved to satisfy the requirement of Front-end system. Moreover it could be used as the oscillator of CPA system. While the latter has 3-dB bandwidth of 80 nm and spectrum range of over 200 nm, which has the broadest spectrum in the lasers with the same structure so far as we know.2. We analyzed the features of pulse in transmission and stacking process, built the theory modeling in time and frequency domain. The interval of the shaping pulse generated by chirped pulse stack is shorter than 10 ps, which has no harmful influence to ICF. The estimation method for pulse stacking shaping is established. Parameter selection region is proposed to estimate coherent stacking.3. A numerical simulation is developed for analysis of the origin of high frequency fluctuation in the top of shaping pulse in modulation Front-end system. We studied the influence introduced by filters with different bandwidth, waveform and propose the optimizing way to reduce the fluctuation.
|
PDF Full Text Request