【作者】 纪帆；

【导师】 明海； 许立新；

【作者基本信息】 中国科学技术大学 ， 光学， 2008， 博士

【摘要】 本论文是在国家重大专项和国家自然科学基金《宽带掺镱光纤放大器噪声特性研究》的研究背景下进行的。作为我国目前功率最高、能量最大、最先进的激光驱动器的前端,其主要设计功能是为后续系统提供数个已初步整形并具有一定能量(mJ)、一定带宽、高信噪比和高光束质量的激光脉冲。目前围绕主要技术—脉冲整形,存在堆积整形和单频连续激光调制整形两套技术方案。作为一种新型的前端系统,对高功率激光器前端系统关键物理问题进行研究和计算机模拟可以为前端系统的研制提供重要的指导意见。同时可以结合本实验室长期从事光纤器件研究的优势,为前端系统提供稳定振荡器等关键器件作为种子光源。本文的主要成果有:1.前端系统要求有很好的激光脉冲整形能力,本文对正式前端的脉冲堆积整形技术进行分析,建立脉冲堆积整形的理论模型,提出使用延时一位相选择区间的方法对堆积平滑效果进行研究,通过理论模拟分析不同路数堆积平滑区间的变化,建议堆积中各路延迟应选择在一倍脉冲半高全宽左右,该结论已经应用于前端系统的建设之中并取得良好效果。啁啾脉冲堆积减小了对光程控制的要求,而且脉冲具有扫频特性,给光谱束匀滑(SSD)技术的实现带来极大的便利,是一种比较优秀的脉冲堆积技术方案。本文深入研究了相干和非相干堆积中延时误差对堆积效果的影响,提出通过选择特定堆积子脉冲形状减小堆积脉冲起伏对延时误差敏感性的方案。提出采用法拉第镜来进行补偿脉冲堆积过程中各路光的偏振差异的方案,通过琼斯矩阵法对脉冲延迟传输过程进行计算,并且在实验上验证了计算结果。2.前端系统需要对脉冲在各个子系统中产生传输行为进行详细了解,文章分析了脉冲在光路中传输的行为,研究了脉冲啁啾和宽度经过滤波及长距离传输的变化情况,分析了这种变化会对堆积脉冲产生的影响。通过理论分析得到了堆积后内部细节起伏和啁啾量的关系,证明了现有的啁啾脉冲堆积技术方案中由各个子脉冲啁啾造成的整形脉冲细节起伏其间隔小于10ps,而应用对这种很快的脉冲起伏并不敏感。同时结合几种非线性效应(受激布里渊散射、受激拉曼散射、四波混频、自相位调制等)的产生机理,使用实际参数计算阈值,并且提出使用啁啾脉冲放大或光子晶体放大器避免这些非线性效应的方案。3.在激光驱动器前端系统中,需要稳定高能的1053nm超短脉冲作为堆积子脉冲,我们进行基于半导体可饱和吸收体(SESAM)锁模激光器的实验,这种高稳定性、具有50MHz高重复频率、高能量、低噪声、特定中心波长超短脉冲激光器已经投入前端作为种子源之用。在后续的实验中,还以其为源研究了啁啾脉冲放大器,即用啁啾光纤光栅对超短脉冲先展宽放大再压缩,不仅能较好避免峰值功率过大带来的非线性效应,还能获得最大有效能量提取的能量密度。在该放大系统中研究了锁模激光通过啁啾放大后光谱和放大特性的改变情况。同时,我们研制了基于非线性偏振旋转(NPR)效应的被动锁模掺镱激光器,结合掺镱光纤增益谱宽的特点,通过优化腔结构,得到了一种超宽带锁模激光器,3-dB光谱宽度超过80nm,光谱范围超过200nm。据我们所知,它具有同类激光器中最宽的光谱。该激光经过腔外压缩预计可以产生几十fs量级的超短脉冲序列。4.针对单频激光调制滤波整形方案中出现的脉冲顶部起伏,提出滤波导致高频分量缺失是产生脉冲顶部高频起伏的根本原因,而滤波中心波长偏离激光中心波长导致在实验上观察到明显的起伏。通过数值模拟给出了不同滤波波形对脉冲高频起伏的影响,并给出滤波带宽、中心波长偏差和顶部起伏大小的关系,提出减小起伏的优化方法。在该备用前端系统中,需要稳定、低噪声的连续光激光器作为光源。本文利用自行刻写的分布反馈布拉格光栅,研制了相移单频光纤激光器,具有线宽窄、信噪比、功率稳定性高等特点,它的光谱信噪比超过70dB,能量达到10mW以上,非常适合作为备用前端系统的种子光源。5.结合高稳定性1.0μm锁模激光器的研制,我们对三种通讯波段的SESAM锁模激光器进行研究。这三种激光器都采用FP腔结构,SESAM作为锁模元件和一个腔镜,另一个腔镜分别为带宽0.2nm左右的FBG、线性采样啁啾光纤光栅和光纤环镜。其功能各描述如下:一为研究谐波锁模和调Q锁模机理,以避免其发生;其二为同时使用采样啁啾光栅达到改变腔内色散和产生多波长锁模目的;最后一种激光器通过控制光纤环镜中偏振控制器的状念来改变激光器工作波长,实现波长可调的宽带锁模激光输出。这三种激光器的研制成功对研究SESAM被动锁模激光器中重要物理现象成因、密集波分复用、超连续光的产生等方面有积极重要意义。通过前端系统的实验结果表明本文理论分析正确,能够很好地指导实验工作。本文中研制的光纤激光器件,已经很好应用于前端系统,充分满足前端的需求。本文创新点:1.分别基于SESAM和NPR效应研制被动锁模激光器,通过引腔型优化,得到脉冲稳定、光谱平坦的锁模脉冲序列。其中基于SESAM的锁模激光器具有高稳定性、高重复频率的特点,可以满足高功率激光前端系统对种子源的要求,也可作为CPA系统的振荡器。基于NPR效应的锁模激光器,据我们所知,具有同类激光器中最宽的光谱。同时将SESAM锁模技术扩展到通讯波段,使用简单结构研制了多波长锁模激光器和波长可变锁模激光器。2.全面分析了脉冲在前端光路中传输、滤波和堆积的行为,证明了现有的啁啾脉冲堆积技术方案可以应用于后续实验当中。对脉冲堆积方案,应用延时和相位变量分离方法进行分析,并使用参数选择区间确定相干和啁啾脉冲堆积延迟范围。提出了在工程上切实可行的堆积脉冲控制方案。3.对调制整形方案中窄带滤波导致脉冲幅度起伏原因进行了分析,系统研究了调制方案中不同滤波器和滤波带宽对顶部起伏的影响,提出减小起伏的滤波器选择优化方法。更多还原

【Abstract】 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.更多还原