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1、上海交通大学 博士学位论文 光纤激光器中有理数谐波锁模技术的研究 姓名：汪平河 申请学位级别：博士 专业：光信息科学 指导教师：夏宇兴;詹黎 20040922 I 光纤激光器中有理数谐波锁模技术的研究 摘 要 高速超短脉冲光源在物理研究和光通讯领域都有重要的应用 有 理数谐波锁模技术是实现高速超短脉冲光源的重要方法之一 本文对 光纤激光器中的有理数谐波锁模技术进行了实验和理论两方面的研 究同时对有理数谐波锁模技术应用中的难点问题锁模所产生的光 脉冲序列强度分布不均匀的问题进行了研究 首先我们介绍了激光器的基本原理 并重点对激光器以及光纤激 光器中的锁模技术进行了详细的介绍 我们首次报道了环形光
2、纤激光器中的任意分子数有理数谐波锁 模现象在传统的有理数谐波锁模技术中调制频率 m f 和激光器的 腔频 c f 间的关系为(1) mc fnp f=+,其中 n 为大于等于零的整数 p 为大 于 1 的整数我们在实验中发现当调制频率() mc fnm p f=+时m 为 小于 p 的整数锁模现象也能发生我们称之为任意分子数有理数 谐波锁模通过实验现象总结了任意分子数有理数谐波锁模所产生 的脉冲序列的强度分布规律并实现了分子数为 5 的高达 22 阶的有 理数谐波锁模脉冲产生并获得任意分子数 11 阶的有理数谐波锁模 脉冲的完全产生实验结果表明采用合适的分子数可以部分的改 善有理数谐波锁模所产
3、生的脉冲序列的强度分布 特别是分子数等于 一时功率最小的光脉冲的功率得到了较大的提高 同时我们对任意分 II 子数有理数谐波锁模技术进行了定性的解释 采用主动锁模均匀加宽激光器的时域分析理论 对任意分子数有 理数谐波锁模技术进行了详细的分析 首先我们发展了锁模的时域分 析理论使之能被应用于任意分子数有理数谐波锁模脉冲的模拟采 用该理论我们模拟出了任意分子数有理数谐波锁模脉冲序列模拟 的结果和实验结果吻合较好验证了相关的实验规律和理论的正确 性通过对模拟分析可知造成任意分子数有理数谐波锁模脉冲序列 强度分布的主要原因是调制频率的变化即纵模间位相差的变化对 锁模脉冲的模拟表明: 和传统的有理数谐波
4、锁模技术相比采用任意 分子数有理数谐波锁模技术可以得到更高次的有理数谐波锁模脉冲 通过对模拟出的脉冲序列进行傅立叶变换得出相应的频谱进行频 谱分析我们得出了有理数谐波锁模时噪声的产生规律随着谐波次 数的增加分子数的增大噪声随之增加这也可能是在以前的实验 中难以观测到非一分子数有理数谐波锁模脉冲产生的原因之一 文中 还对时域分析理论中误差的来源进行了分析 所产生的锁模光脉冲序列的强度分布不均匀是限制有理数谐波 锁模技术应用的主要问题采用非线性偏振旋转技术我们研究了任 意分子数有理数谐波锁模技术中 分子数对脉冲序列强度均匀化的影 响实验的结果表明采用任意分子数有理数谐波锁模技术可以得到 更高次的等
5、强度分布的脉冲序列实验中我们得到了最高次数为 10 次的等强度分布的有理数谐波锁模脉冲序列 造成这种结果的原因在 于采用适当的分子数进行有理数谐波锁模可以部分的改善脉冲序 III 列的强度分布 采用传统的有理数谐波锁模技术在 1996 年就实现了 200GHz 超短脉冲光源 同时采用传统的有理数谐波锁模技术获得了最高为二 十二次的有理数谐波锁模脉冲采用任意分子数有理数谐波锁模技 术可以在以上两个方面获得突破同时任意分子数有理数谐波锁模 技术对解决有理数谐波锁模脉冲序列的强度分布不均匀问题提供了 新的方法 另外我们在有理数谐波锁模技术研究中的理论工作为有理 数谐波锁模技术实用化和相关的研究领域提
6、供了一些可借鉴的内容 和新思路 关键词分子数 有理数谐波锁模光纤激光器高重复频率 脉冲等强度分布 IV Study on Rational Harmonic Mode-Locking Technique in Fiber Laser Abstract Ultra-fast short pulse source is important for both physics research and optical communication network. the rational harmonic mode-locking (RHML) technique is an important wa
7、y that generates the short pulse source. The thesis investigates RHML technique in fiber lasers theoretically and experimentally. At the same time, we study the uneven amplitude issue of the pulse train generated in the fiber laser by RHML technique. At first, we introduce the basic theory of lasers
8、, put emphasis on the mode- locked pulse lasers and fiber lasers. We report for the first time to our best knowledge, the arbitrary numerator RHML phenomenon in a ring fiber laser firstly. In conventional RHML, the relationship between the modulator m f and the fundamental cavity frequency c f of th
9、e fiber laser is (1) mc fnp f=+, where n and p are both integers. In the experiment, we found that RHML took place when the modulation frequency of the modulator () cm fpmnf+=, where m is an integer less than p. We call it arbitrary numerator RHML. The experiments demonstrate that the generations of
10、 up to 22nd order RHML pulse train when numerator =5, and the generation of the 11th order RHML pulses with an arbitrary numerator in fiber-ring lasers. The results show that the amplitude V distributions in non-one numerator high order RHML pulse trains are strongly modulated by the numerator=1 low
11、 order RHML. The experiments also demonstrate that the amplitude distribution of the pulse train can be improved partially by choosing a proper numerator. Using the time domain analysis theory of the active mode-locked homogeneous lasers, we study the mechanism of the arbitrary numerator RHML pulse
12、generation in detail. At first, we developed the theory of the time domain analysis and used it in the analysis of the arbitrary numerator RHML pulse generation. The simulated results are in good accord with the experiments. The relative experimental regular and the validity of the theory are proved
13、. Through the stimulation, we can conclude that, compared with the conventional numerator=1 RHML technique, the higher order RHML pulse train can be gotten by non-one numerator RHML. Analyzing the stimulation, we find that the reason that induces the uneven amplitude distribution of the pulse train
14、is the modulation frequency, which is the same as the phase difference among the modes. By Fourier analysis of the pulse train generated by the stimulation, we get the rule of noise, that the noise become larger when the order and the numerator tend to be larger. That may be possible of the fact tha
15、t it is difficult to observe the generation of non-one numerator RHML pulse in the previous works. In the thesis, we discuss the error source of the stimulation. The uneven amplitude distribution of the pulse train generated by RHML may be one of the most important problems for practical application
16、. Using nonlinear polarization rotation, we study the influence of the numerator to the amplitude VI distribution of the pulse train. The experiment shows that compared with the conventional RHML, the higher order equal- amplitude-distribution pulse train can be gotten by arbitrary numerator RHML. The up to 10th order RHML pulse train with uniform pulse amplitude is experimentally demonstrated when the numerator=3. The reason is that the amplitude distribution of the pulse train can be imp
