| 题名 | 光纤激光特性改善及相干组束技术研究 |
| 作者 | 何兵 |
| 学位类别 | 博士 |
| 答辩日期 | 2007 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 楼祺洪 |
| 关键词 | 光纤激光器 模式选择 相干组束 数值模拟 空间滤波器 |
| 其他题名 | Study on improvement of properties of fiber lasers and beam combination techniques |
| 中文摘要 | 光纤激光器由于其结构紧凑，使用轻便灵活，转换效率高，光束质量好，输出功率高等许多优点，在很多领域，如工业加工，通讯，医疗，科学研究和军事等具有广泛应用。随着应用领域的扩展，不但需要高功率而且还需要高质量的激光束。最近，单根光纤激光器输出功率迅猛提高，已经超过千瓦量级。但是单根光纤的最高功率输出是受到非线性效应，如受激拉曼散射（SRS）和受激布里渊散射（SBS）的限制，并且随着功率的提高，光束质量也变得越来越差。两种技术：模式选择技术和相干组束技术，可以解决上面所说的限制和存在的问题。模式选择技术可以提高输出光束的质量，而相干组束技术可以在提高输出功率的同时保证好的光束质量。本文中，对模式选择技术和相干组束技术进行详细的理论和实验研究。 第一章综述了脉冲光纤激光器的发展历史、发展趋势和实现脉冲输出的几种形式，展望了高功率双包层光纤放大器在工业、军事、医疗中的应用前景。对光纤激光器的模式选择技术和相干组束技术也进行的综述，详细分析了不同技术的优缺点。 第二章提出了一种新型的对于单光纤激光器的模式选择技术。根据阶跃折射率光纤中的模式理论，推导了光纤中所存在的不同模式的光束质量因子表达式，证明了高阶模式的光束质量因子比基模高，也就是说在自由空间传输时，高阶模式具有更大的发散角。在这种新技术中，模式选择的实现是基于这样一个事实：就是在腔中，如果置入一个直径合适的小孔可以对基模引入低损耗而对高阶模引入较高的损耗。这种新的小孔选模技术有很多优点，例如原理简单，结构紧凑，和其它模式选择技术兼容性好等等。利用这种技术，我们在实验中获得单光纤输出23.4W，对应的光束质量因子M2为1.20。整个光纤激光器系统在近衍射极限输出状态下操作时，使用的小孔直径为4mm。 第三章对利用自傅立叶变换（S-F）腔对掺镱双包层光纤激光器组束进行了数值模拟研究。结果证明这种自傅立叶变换（S-F）腔不但适合于一维光纤激光器阵列的相干组束，同样也适合二维光纤激光器阵列。只要通过合理的设计，光纤激光器阵列输出的光经腔内一次往返以后再次到达输出面，这时光场的强度分布中的极大值点正好对应光纤的纤芯，这充分说明了光纤输出的光在腔内经过一次往返以后，虽然不同激光器的间光场相互作用光场进行了重新分布，但是能够再次进入光纤的纤芯，实现相干相干组束。对比几种不同光纤阵列的数值模拟结果，发现对于相同光纤数目情况下，S-F腔相干组束技术对于二维阵列的耦合效果要比一维阵列的效果好。而对于二维阵列来说，光纤数目越多能量再次进入纤芯的比例就越高，说明S-F腔技术非常适合于大量光纤二维排列的相干组束。 第四章对自成像腔相干组束技术进行了详细的理论研究。根据标量衍射理论，推导了傅立叶透镜后焦面上强度分布的表达式。两光纤激光器数值模拟的结果显示，异相模式的强度极大值正好位于同相模式的强度极小值处，基于这一重要事实，我们可以设计一滤波器放置在同相模式的极小值处，这样就能对同相模式引入低损耗而对异相模式引入较高损耗，从而实现模式控制。对于四光纤激光器二维阵列情况，尽管不同光纤激光器之间模式关系较复杂，但是数值模拟结果说明，在同相模式和异相模式之间具有与两光纤激光相同的特点。毫无疑问，我们也可以设计一个滤波器用于四光纤激光器自成像腔相干组束的模式控制。 第五章利用一个自成像共焦腔和一个滤波器，实验证明了两个掺镱双包层光纤激光器的位相锁定。观测到稳定的高对比度干涉条纹。当使用两根大芯双包层光纤时，相干输出功率得到极大提高。相干条纹的对比度高达59%，而非相干图样的对比度仅为6%。中心干涉条纹宽度与理论计算结果吻合得很好。对于同相模式，位相锁定光纤阵列输出功率达到113W，对应的斜率效率为38.5%。该结果经中科院上海科技查新咨询技术中心水平检索，达到国际先进水平。 |
| 英文摘要 | Because of the advantages in compactness, lightness, conversion efficiency, beam quality, output power etc., fiber lasers are becoming the geometry of choice for many applications, including industry, communication, medicine, science research, military affairs and so on. With the development of applied realms, higher quality and higher power of laser beam is required. The output power of a single fiber lasers has been improved rapidly and has exceeded kilowatt magnitude. However, the ultimate output from a single fiber laser is limited by the nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) and beam quality became worse and worse with increasing output power. Two techniques, mode-selection and coherent beam combination, to overcoming these above limitations and problems have been used. Mode-selection can improve beam quality and beam combination is an effective geometry which can improve output power with excellent beam quality. In this thesis, theoretical and experimental study on fiber lasers based mode-selection techniques and coherent beam combination techniques. In the first chapter, the history, current status, and future prospects of rare-earth doped fiber laser and fiber amplifier were introduced. The recent techniques of coherent beam combination and mode-selection of fiber lasers are also briefly introduced. The advantage and drawback of various techniques are analyzed in detail. In the second chapter, a novel technique of a single fiber laser for mode-selection is presented. According to mode theory in the step-index fiber, the expressions of beam quality factor for different modes are derived. It is demonstrated that beam quality factor of high order mode is higher than that of the fundmental mode. In this method, mode-selection is realized by based the fact that an aperture with adapted diameter in cavity could introduce low loss for the fundament mode but high loss for high order modes. This new method possesses many advantages including simple, compactness, compatibility and so on. By using this method, 23.4W output from a single fiber laser with 1.20 of M2 is obtained. When the fiber laser system is operating on nearly diffraction-limited output state, the diameter of aperture is 4mm. In the third chapter, we simulated coherent beam combination of Yb-doped double-clad-fiber (DCF) lasers theoretically by using a self-Fourier (S-F) cavity. This system suits both of 1D array and 2D array. The intensity maximum spot of intensity distribution pattern correspond to the position of fiber cores, which shows that the light from fiber lasers can be coupled into fiber cores through a single round trip. In comparison with several fiber arrays, we find that coupling effect of 2D array is much better than that of 1D array with the same number of fibers. For 2D array, we also find that light energy coupled into cores increase with the number of fibers increasing. In the fourth chapter, theoretical study on self-imaging resonator of coherent beam combination is done in detail. According to the scalar diffraction theory, the formula of intensity distribution on the back focal plane of Fourier lens is derived. Numerical simulation of two fiber lasers shows that the intensity maximum positions of out-of-phase mode just lie on the intensity minimum of in-phase mode. With the key fact, a spatial filter, which introduces low loss for in-phase mode but high loss for out-of-phase mode, can be used for controlling mode, if the filter were be placed at the intensity minimum of in-phase mode. For four fiber lasers with 2D arrangement, although mode relation of four fiber lasers are complicate, numerical simulation of four fiber lasers shows that the relation between out-of-phase mode and in-phase mode is the same with two fiber lasers case. There is no doubt that a spatial filter for four fiber laser array can be designed. In the last chapter, phase locking for two Ytterbium-doped double-clad fiber lasers has been demonstrated by using a self-imaging confocal resonator with a spatial filter. Steady high-contrast interference stripes are observed. The coherent output power is improved greatly when two large-core Yb-doped double-clad-fiber lasers is used in this experiment. The visibility of coherent stripes is 59% while that of incoherent stripes is 6%. The width of the central strip is well in agreement with the calculated result. For in-phase mode, the output power of the phase locking fiber laser array is up to 113W and corresponding slope efficiency is 38.5%. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15464]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 何兵. 光纤激光特性改善及相干组束技术研究[D]. 中国科学院上海光学精密机械研究所. 2007. |
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