Yb-doped double-clad fiber amplifier and its application in the Front-End System of ICF laser driver

惯性约束核聚变（IntertiolConfinementFusion一ICF）是当今国际上的重大科研领域，有极为重要的科学意义和新能源应用前景。ICF实验研究的关键是高功率固体激光驱动器，它不仅要求激光脉冲的能量很高而且对脉冲的频域宽度、时间、空间分布形状也有十分严格的要求。因此用于ICF的激光驱动器是一个复杂的大型系统，一般分为三部分：前端，主放和靶室。其中，前端系统作为一个高质量的种子源，将为后级系统提供一个具有一定能量、带宽、高信噪比和高光束质量的脉冲。前端系统主要包括主振荡器、脉冲整形器、预放大和再生放大器。其中主振荡器是前端一个很重要的子系统，最终辐射靶核的激光脉冲就起源于该子系统产生的低功率光信号。由于用于产生最初信号的DFB掺Yb光纤激光器的输出功率比较低，一般为十几毫瓦量级。因此，必须对这个输出波长为lO53nm的低功率信号进行放大。双包层光纤放大器由于其自身固有的结构优点，可以满足前端对于小信号进行高功率、大能量放大的要求。本论文对满足此要求的掺Yb双包层光纤放大器（DCFA）进行了研究。第一章首先介绍了掺镜光纤放大器在NIF中的应用，并结合神光m的要求提出了连续双包层光纤放大器的应用方案。接着介绍了掺镜光纤放大器的发展，包括：石英光纤中Yb3"的能级结构与光谱特性；双包层光纤特点及包层泵浦技术总结；DCFA在其他领域中的应用；DCFA在国内外的研究进展。第二章对连续掺Yb双包层光纤放大器进行了理论分析。在将Yb离子简化为二能级系统的基础上给出了粒子数速率方程，泵浦光、信号光和放大的自发辐射传播方程组。并对此方程进行了理论模拟，给出了增益和泵浦光功率随光纤长度的变化规律。第三章对异向泵浦的单程连续DCFA进行了实验研究。与单模光纤放大器相比，双包层光纤放大器的核心在于信号光和泵浦光的耦合。我们分别设计加工了泵浦光和信号光的耦合系统，并利用在线放大的方法成功解决了信号光从单模光纤进入双包层光纤纤芯藕合难、监控难的问题。当进入内包层的泵浦功率为10w，发现双程放大结构相比单程放大结构而言具有更大的噪声。在分析总结实验结果的基础上，找到了改善掺Yb双包层光纤双程放大器信噪比，提高放大器增益及系统稳定性的方法。第五章对本论文的主要内容做了总结。Inertial Confinement Fusion (ICF) is an important scientific research field in the world. It is of extreme importance for the applications of new energy source. The key of ICF system is the laser driver, which requires not only a high energy, but also a certain bandwidth and special shape of the laser pulses. Generally speaking, the laser driver is a large-scale and complicated system, which is composed of three major subsystems: frond end, main amplifier chains and target room. The front-end system provides the subsequent amplifiers a laser pulse train with certain energy and bandwidth, high signal to noise ration (SNR) and high quality. The front-end system consists of master oscillator, pulse shaping system, pre-amplifier and regenerative amplifier. The maser oscillator is very important because it is responsible for generating the signal pulse used as a seed in the entire ICF laser system. The DFB fiber laser used as the master oscillator has a low output power, so the signal at the wavelength of 1053nm has to be amplified. Because of the inherent advantages of double-clad fiber, it can meet the requirements of high-power and high-energy amplification. This thesis mainly focuses on how to amplify the signal to a high power by use of double-clad fiber amplifier (DCFA). In the first chapter, the application of DCFA in NIF is introduced. Then a scheme of continuous-wave (CW) Yb-doped DCFA is proposed based on the requirements of Shenguang III. The developments of Yb-doped fiber amplifier (YDFA) are also summarized: the energy configuration and emission spectrum of Yb3+ in silica; the characteristics of double-clad fiber and cladding-pump technologies; the applications of DCFA in other areas; the recent developments of DCFA at home and abroad. In the second chapter, a theoretical analysis is made about DCFA. The Yb + is simplified as a two-level system. Then a set of equations, including the rate equations of populations changing, propagation of signal, propagation of pump and propagation of ASE are given. Based on the above model, a theoretical simulation is made about the variances of pump power and gain along the fiber. In the third chapter, the backward-pumping single-pass CW Yb-doped DCFA is investigated experimentally. Comparing with the single-mode fiber amplifier, the key of double-clad fiber amplifier is the coupling of pump and signal. We designed pump and signal coupling system respectively. The problem of coupling the signal from the single-mode fiber to the core of double-clad fiber is solved successfully by using a new method. When a pump power of 10W is launched into the inner clad and the signal power of lmW is launched into the fiber core, a CW signal output of 1W is obtained. The amplifier gain is 30dB. The gain, noise figure, and spontaneous emission of this backward-pumping single-pass amplifier are studied both experimentally and theoretically. In the fourth chapter, the double-pass DCFA was investigated experimentally. Comparing with the single-pass structure, the signal can be obtained from a single-mode fiber in the double-pass one. But it is relatively difficult to realize double-pass configuration, because it is hard to adjust the coupling system, the high reflecting mirror and the double-clad fiber to the best condition at the same time. In this thesis, we guarantee not only a high coupling efficiency for the pump light, but also a high reflectivity for the signal light and thus realize a double-pass configuration. The gain, noise figure and spontaneous emission are analyzed in detail. We found the noise in this configuration is higher than the single-pass one. Based on the analysis of experimental results, we find methods to improve the SNR, gain and stability of the double-pass Yb-doped double-clad fiber amplifier. In the fifth chapter, the main content of this thesis is summarized