Theoretical Analysis Of High Power Pulse Propagation And Amplification In Optical Fibers

Laser driven inertial combinement fusion is the most important application of the high power laser system, which usually adopts master-oscillator power amplifiers to achieve the extremely high power output. The laser drivers which have been built almost all use an all fiber frontier system to apply the seed pulse, and the flash lamp pumped neodymium glass to offer the multi-stage amplification. But the low amplification efficiency and the intense thermal effect limits the system to improve further. Recently, an all-fiber laser driver based on the amplification network of high power fiber lasers is proposed. Because of the advantages of high energy conversion efficiency, compactness, easy-maintenance, low thermal effects, low insertion loss and good beam quality, this all-fiber laser driver is suitable for the fusion energy application. It is necessary to analyze the propagation and amplification process of a high power pulse in optical fibers. In this dissertation, we study the nonlinear propagation of the stacking chirped pulse, the suppression of stimulated Brillouin scattering in a optical fiber seeded with a linearly chirped pulse and effective improvement of the signal-to-noise ratio in a high power pulsed pumping fiber amplifier respectively, and obtain some significant results.The major research works and results are as followings:1. We presents a systematic analysis on the nonlinear propagation of the high power stacking chirped pulse in optical fibers. Using spectrogram, we analysis the stacking chirp characters of the stacking pulse, and explain the reason of the fluctuation structure in time domain and frequency domain; Compared with the propagation results of a linearly chirped pulse, the effect of stacking chirp characters on the propagation is revealed. We point out that the four-wave mixing effect within the stacking pulse would induce strongly spectrum broadening.2. We propose that the linear chirp character of the input pulse can suppress the Stimulated Brillouin Scattering (SBS) process in the high power fiber system. Using the transient coupled complex amplitude equations, we study the effect of the chirp characters of the input pulse on the SBS process. The results shows that the threshold power of SBS would increase linearly with the linear chirp value of the input pulse; the effect of the pulse width of input pulse and the fiber length on the SBS process is also discussed. For a fixed injected pulse, there is a optimal fiber length for the best SBS suppression.3. We propose a high power pulsed pump method to efficiently improve the output signal-to-noise ratio (SNR) of a high power fiber amplifier. Using the transient rate equations, the transient process of a fiber amplifier under different pump power is discussed. The high pump power is approved to be more effective on suppressing the amplified stimulated emission; Compared with a low power pulsed pump and a continue wave (CW) pump, the high power pulsed pump method has the advantages of improved amplification efficiency, increased output SNR and weaken pulse distortion. Without the sacrifice of the signal gain of fiber amplifier, the pump parameters can be optimized to obtain the optimal SNR output.Highlights of the dissertation are as followings:1. The spectrogram method is used to analysis the stacking chirp characters and nonlinear propagation of a stacking chirped pulse. The stacking chirp character of a stacking pulse induce the fluctuation structure appearing in time domain and frequency domain. And the four-wave mixing effect within a stacking pulse makes the spectrum strongly broadening.2. The linear chirped pulse is suggested to suppress the SBS effect in the high power fiber system. The threshold power of SBS is demonstrated to be increased linearly with the linear chirp value of the input pulse. For a fixed injected pulses, there is a optimal fiber length for the best SBS suppression.3. A high peak power pulsed pump method is proposed to effectively improve the output SNR of a high power fiber amplifier. This pump method is approved to be effective on improving the amplification efficiency and the SNR of the output signal, as well as decreasing the distortion of the output signal. Without the sacrifice of signal gain, the pump parameters can be optimized to obtain the best SNR output.