Modal control methods and applications of large mode area optical fibers

光纤激光器以其高功率、高效率、高质量、结构紧凑、稳定可靠等优点，在光通讯、光传感器、先进制造、激光武器和激光聚变驱动器等方面得到广泛应用，已成为当前激光技术的一个重要发展方向。其中，高平均功率和高脉冲功率光纤激光器更是备受关注的研究热点。但输出功率的进一步提高受到非线性效应、光损伤等负载能力的限制。虽然增大芯径可以增大模面积，但往往不能满足单模运转的要求。探索能够同时实现大模面积和单模运转的技术，对高功率光纤激光器发展具有重要意义。 在前人工作基础上，本文以两种电磁场理论方法，即时域有限差分法和光束传播法，研究了大模面积条件下模式的控制理论与技术，取得有重要学术价值和实际意义的创新性成果。本文首次提出并设计了弯曲滤波性能好的多层大平顶模光纤、大平顶模光子晶体光纤、高阶模抑制能力强的百微米螺旋芯光纤、大模场倏逝波耦合的亚波长纤芯光纤束和亚波长空芯光纤等新结构光纤。在国内外学术刊物发表（含接受）13篇学术论文报告，并申请5项发明专利。本文取得的创新性技术进步主要有： 1.对多层结构的大平顶模(LFM)光纤进行建模，详细分析了其传输模式和弯曲损耗。设计出光纤芯径可到40 μm左右、弯曲滤波性能优良的三层大平顶模光纤和大有效面积、弯曲滤波性更强的四层LFM光纤。发表相关论文5篇（Opt. Commun.、Opt. Eng.、Optik、Chin. Phys.等）； 2.将平顶模光纤和光子晶体光纤的概念结合起来，首次提出并分析了大平顶模－光子晶体光纤(LFM-PCF)，这种光纤不用增加纤芯尺寸就可以增大有效模面积，并能输出平顶光束，具有潜在的重要应用前景。相关论文发表于Proceedings of SPIE(国际会议邀请报告)； 3.给出螺旋芯光纤的理论分析方法，优化设计并论证了芯径大于100 μm的螺旋芯光纤可以实现单模运行。相关论文发表于Chin. Phys. Lett.； 4.首次提出利用光纤导引的倏逝波承载高功率激光的物理原理，研究了亚波长直径光纤中大模场的模式特性，并以实验验证了基于倏逝波耦合机制的激光放大和振荡的技术可行性。在此基础上，设计了亚波长多芯光纤和亚波长空芯光纤两类新型结构，分析了模场控制能力，为拓宽应用打下了基础。发表相关论文2篇（Optics Express、Opt. Commun.)，申请发明专利5项； 5.研究了分布式包层泵浦耦合的功率分布及热分布特性，证明这是一种很有效的高功率高效率泵浦耦合方式。从波动理论出发，对锥形光纤功率分布特性进行了分析，提出了优化设计准则。首次基于光束传播法，对锥形光纤、熔锥光纤等光纤组件的模场分布进行了分析，对进一步改善泵浦耦合结构具有重要意义。发表相关论文2篇（中国激光等）。 最后对本论文对工作进行了总结，并展望今后需要进一步开展的工作。Fiber lasers offer a competitive alternative to traditional lasers due to their superior performance: high power, high efficiency, high quality, compactness, stable structure, and low running costs. These characteristics make fiber lasers suitable for communication, fiber sensor, advanced manufacture, laser weapon, laser fusion driver and etc, so the field is a very important one in laser technology. Among these research hotspots, fiber lasers with high average power or high pulsed power output have aroused people’s attention. However, the output power is limited by the nonlinear effects, optical damage and etc. Increasing the core size of the fiber can increase the effective area, but it is not easy to guarantee the fiber operating in singe mode. So it is significant to explore the large mode area fiber with excellent beam-quality output. Based on the previous works, the thesis has focused on studying the modal control methods and applications of large mode area optical fibers using two electromagnetic computation methods, i.e., finite-difference time-domain method and beam propagation method. By theoretical simulation, some innovative and important results have been achieved. Multiple-layer large flattened mode (LFM) fiber with good bend-induced filtering ability, large flattened mode photonic crystal fiber, helical-core fiber with excellent ability to suppress higher-order modes, large mode area subwavelength-diameter fiber bundle coupled by strong evanescent wave and subwavelength-diameter hollow optical fiber have been proposed and designed. Some innovative results have been obtained and presented in our 13 papers (including the accepted) and 5 patents. The main innovative results in our thesis are summarized as follows: 1.Multiple-layer LFM fiber has been modeled and its propagation and bending characteristics have been analyzed. By appropriate design, the core size of three-layer LFM fiber can be scaled up to 40 μm with excellent bend-induced filtering ability. Furthermore, the four-layer LFM fiber also has large mode area and much stronger bend-induced filtering ability. These works have been published in 5 papers, including Opt. Commun., Opt. Eng., Optik, Chin. Phys. and etc. 2.Combining the merits of photonic crystal fiber and LFM fiber, the design guidelines and characteristics of the large flattened mode photonic crystal fiber (LFM-PCF) have been discussed for the first time. By appropriate design, the effective area of the LFM-PCF can be increased by a factor of greater than 2 as compared with conventional index-guiding photonic crystal fiber with the same hole and pitch parameters. The improved effective area, single mode operation and flat-top fundamental mode output make LFM-PCF an ideal candidate in many applications. The results have been published in Proceedings of SPIE (invited report)； 3.The approximation and the improved bending loss model for helical-core optical fiber have been presented. By appropriate design, helical-core optical fiber can operate in singe mode with core size larger than 100 μm. The results have been published in Chin. Phys. Lett.； 4.A new principle that the energy of high power laser is carried by the evanescent wave and guided by the fiber is proposed and the modal characteristics of subwavelength-diameter fiber have been investigated. Based on evanescent-wave-coupled gain, the amplification and oscillation behaviors of the kind of large mode area fiber have been studied experimentally. The modal characteristics of combining multiple subwavelength-diameter fibers or multiple subwavelength-diameter fibers and large core fibers and subwavelength-diameter hollow fiber have been simulated, which establish the basis for further applications. Based on these results, we have published 2 papers (Optics Express and Opt. Commun.) and applied 5 patents. 5.The power and thermal distribution characteristics of distributed pumping scheme have been studied. The results testify that the pumping method is efficient. Based on Maxwell’s equation and the series expansion method, the power distribution characteristics and optimal design guidelines of tapered fiber have been investigated. The modal fields of tapered fiber, fused fiber bundles have been simulated by beam propagation method. The results may provide useful information to improve the fiber pumping architectures. Some results have been published and accepted in Chinese Journal of Lasers and Laser & Optoelectronics Progress. Finally, main achievements of the dissertation are summarized and some problems in current research field are presented. What and how to do in the near future is proposed