热致超大模场光纤的研究

Very-large-mode-area fibers have attached much attention because of their important role in suppressing nonlinear effects in high-power fiber lasers. In this program, a new type of very-large-mode-area fiber, named as the “thermally-induced very-large-mode-area fiber” (TIVLMAF), will be investigated. Different from the other fibers, the index guiding of the optical field in TIVLMAF is not induced by the dopant, but induced by the thermal effect. The physical mechanism is that because of the effects of the quantum defect and non-irradiance process taking place with lasing of fiber lasers, the temparature of the active fiber core will increase， which will eventually increase the index of fiber core.Then, as long as the increment of the temparature of fiber core is lage enough, the index of fiber core should be large enough to realize the index guiding of optical field in the fiber core. The advantage of TIVLMAF is that we can realize the very-low number-aperture fiber core by controling its temparature, and then, the single-mode area will be increased,correspondingly. This program will make a detailed study on the propagation properties of TIVLMAF with the purpose of revealing its potential in expansion of single-mode area and application in high-power fiber lasers. Both the theoretical study and experimental study will be carried out in the program. In the theoretical study, a three-dimension model of TIVLMAF will be built, which should consider simultaneously all the pertinent physical effects such as the anti-guiding, irregular guiding, thermal effect, mode coupling and competition. In the experimental study, a TIVLMAF amplifier system will be set up to investigate the properties of TIVLMAF. We believe that the study of the program will not only provide a new way to realize very-large-mode-area fibers, but also be of great importance for deeply understanding the thermal effect in fiber lasers; furthermore, the pertinent results will have guiding significance to the development of high-power fiber lasers.

非线性效应是影响高功率光纤激光器功率提升的重要因素，而超大模场光纤是突破这一限制的关键。本项目拟对一种新型超大模场光纤——热致超大模场光纤进行研究，与其它光纤不同的是，该光纤中的波导结构是利用掺杂纤芯中的热效应形成的。其特点在于巧妙的利用光纤激光器中的热效应，对光纤的折射率分布进行调控，使得实现超低数值孔径纤芯成为可能。本项目旨在对热致超大模场掺镱光纤的传输特性（包括模式不稳定性）进行深入研究的基础上，揭示该光纤的纤芯拓展潜力及其在高功率光纤激光器中的应用前景。在理论研究方面，建立能够综合考察弱反波导、非正规波导、热效应、模式竞争和耦合等多种物理因素的热致超大模场光纤的三维模型，对该光纤的本征模式和传输特性进行研究；在实验研究方面，利用光纤放大器系统，对该光纤的增益特性和模式特性进行研究。本项目的开展,将加深对于光纤激光器中热效应的认识,为超大模场光纤的研究提供新的思路。

非线性效应是影响高功率光纤激光器功率提升的重要因素，而超大模场光纤是突破这一限制的关键。本项目对一种新型超大模场光纤——热致超大模场光纤进行了数值和实验研究，该光纤的特别之处在于：其波导结构是利用掺杂纤芯中的热效应形成的，从而巧妙地利用光纤激光器中的热效应，对光纤的折射率分布进行调控，使得实现超低数值孔径纤芯成为可能。本项目开展的主要工作包括：1、建立了用于热致超大模场光纤本征模式分析的二维温度分布数值模型，研究了该光纤的本征模式及其光束质量随热负载的变化规律；2、建立可用于热致超大模场光纤激光器的三维温度分布数值模型，并数值研究了热致超大模场光纤放大器的输出特性；3、对热致超大模场光纤中可能导致模式不稳定性的模式耦合特性进行了研究，该光纤中基模的绝热传输条件，分析了弯曲对于该光纤模式耦合特性的影响；4、对热致超大模场光纤进行了结构设计并进行了试制，验证了拉制热致超大模场光纤的可行性；并开展了实验研究，初步验证了温度对光纤激光器输出模式特性的影响；5、对光纤放大器中的受激拉曼散射效应进行了理论研究，并给出了光纤放大器中拉曼阈值的解析表达式；6、数值研究了热致超大模场光纤放大器在980nm波段的输出特性，验证了其在寄生放大自发辐射抑制方面的优势，并在此基础上，对980nm光纤放大器的输出特性进行了数值研究。本项目的开展,揭示了热致超大模场光纤的传输特性，对加深对于该光纤的认识及其光纤激光器的设计具有重要指导意义；同时，相关结果还将加深对于光纤激光器中热效应的认识,为超大模场光纤的研究提供了新的思路。