【作者】 刘静；

【导师】 明海； 许立新；

【作者基本信息】 中国科学技术大学 ， 光学， 2013， 博士

【摘要】 本文工作是在国家自然科学基金《高功率啁啾脉冲在单模光纤中的非线性传输特性研究》项目的支持下完成。激光约束惯性核聚变是高功率激光技术的重要应用领域之一,其高功率激光驱动器利用种子源加功率放大的结构来获取高能量输出。现已建成的驱动器装置多采用全光纤化前端系统提供种子光源,用氙灯泵浦块状钕玻璃实现后续多级放大。然而,固体激光器较低的能量转换效率以及较高的热效应,限制了该系统的性能进一步提升。最近,研究人员提出了一种基于光纤放大网络的全光纤激光驱动器设想。由于高功率激光光纤系统的能量转化效率高、结构紧凑、易于维护、热效应低、光束质量好且可高重频工作等优点,该驱动器非常适合于未来的聚变能源应用。在高功率光纤系统中,一个需要研究的基本问题是高功率脉冲在光纤中的传输与放大特性。本文分别就啁啾堆积脉冲的非线性传输、高功率脉冲的受激布里渊散射效应抑制以及高功率光纤放大器中的信噪比提高等问题进行了系统研究,取得了一些重要成果。本文研究内容及主要成果：1.针对高功率激光驱动器前端系统中的啁啾脉冲堆积整形方案,研究啁啾堆积脉冲在光纤中的非线性传输特性。分析堆积脉冲的啁啾堆积特性,以及脉冲堆积后时域及频域出现周期起伏的原因；通过与线性啁啾脉冲的非线性传输特性的对比,指出啁啾堆积脉冲光谱展宽相对剧烈,其原因是脉冲内不同频率成分间发生的四波混频效应；研究各参数对于四波混频相位失配的影响,从而得到在不同参数下堆积脉冲时域和频域的演化规律。2.分析高功率脉冲在光纤中的受激布里渊散射特性,提出利用入射脉冲的线性啁啾特性来抑制受激布里渊散射。研究入射光的瞬时频率变化对于受激布里渊散射过程的影响,证明线性啁啾可以有效抑制受激布里渊散射,且阈值功率随线性啁啾系数的增加而线性增长；分析了入射光脉宽以及光纤长度对于受激布里渊散射的影响,指出入射脉冲越窄,啁啾变化对于阈值功率的影响越大,且对于固定的入射脉冲,存在抑制受激布里渊散射的最佳光纤长度。3.分析高功率脉冲光纤放大器的特性,提出利用高峰值功率脉冲泵浦来有效提高其输出信噪比。研究光纤放大器的瞬态特性随泵浦峰值功率的变化,当泵浦峰值功率越高、建立同等储能所需时间越少、而对放大自发辐射的抑制效果越好；通过与低峰值功率脉冲泵浦以及连续泵浦的对比,证明了高峰值功率脉冲泵浦方式可以有效提高光纤放大器的输出信噪比、放大效率,并且可以减少输出信号的形变；分析泵浦脉冲各参数变化对放大结果的影响,在不影响放大器增益的前提下,存在最优泵浦脉冲参数可以获得最佳信噪比输出。本文的主要创新点：1.利用时间分辨光谱研究啁啾堆积脉冲的啁啾特性以及非线性传输演化特性。结果证明：堆积脉冲由于其啁啾堆积特性而在时域以及频域上出现周期性起伏；啁啾堆积脉冲在光纤中传输时,脉冲内部发生四波混频使得光谱在传输初期迅速展宽而后减缓展宽速率。2.提出了利用线性啁啾特性来抑制高功率脉冲的受激布里渊散射的方法。结果证明：当入射脉冲存在线性啁啾时,其在光纤传输中的受激布里渊散射效应可以被有效抑制,阈值功率随线性啁啾系数的增加而线性增长；入射光越窄,线性啁啾系数变化对阈值功率的影响越大；当入射脉冲固定时,存在抑制受激布里渊散射的最佳光纤长度。3.提出了利用高峰值功率脉冲泵浦来提升高功率光纤放大器的输出性能的方法。结果证明：高峰值功率脉冲泵浦下,光纤放大器具有高放大效率、高输出信噪比以及低信号形变等优势；且存在最优泵浦脉冲参数可以在不影响放大器增益的前提下获得最佳信噪比输出。更多还原

【Abstract】 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.更多还原