单模热致超大模场掺镱光纤放大器的数值研究

Modeling the single-mode thermally guiding very-large-mode-area Yb-doped fiber amplifier

非线性效应是限制光纤激光器功率提升的重要限制因素,而超大模场光纤对于非线性效应的抑制具有重要意义.热致超大模场光纤是一种新型超大模场光纤,其利用热透镜效应实现低数值孔径波导结构,从而在保证光束质量的前提下实现超大模场输出.不过,现阶段对于热致超大模场光纤激光器的研究较为有限.本文提出了单模超大模场掺镱光纤放大器的速率方程模型,该模型由稳态速率方程和热传导方程组成.利用该模型,对前向抽运单模热致超大模场光纤放大器进行了数值研究.研究表明:信号光模场直径随着信号光功率的增加而增加,这体现了热致超大模场光纤在非线性效应抑制方面的优势.研究还揭示了最佳光纤长度及其产生的物理机制,发现最佳光纤长度与注入抽运光功率有关,其随着注入抽运光功率的增加而减小;不过,当注入抽运光功率足够大时,最佳光纤长度随注入抽运光功率变化不大.此外,还对输出光场的模式进行了探讨,验证了其在保证超大模场输出的同时,实现高斜率效率输出的可行性.相关研究对于热致超大模场光纤放大器的设计具有指导意义.

The very-large-mode-area（VLMA） fiber is of great importance for suppressing the nonlinear effects which are considered as main limitations to the power scaling-up of high-power fiber lasers and amplifiers.The thermally guiding（TG） VLMA fiber is a novel VLMA fiber,the waveguide of which is formed by the thermal lens effect.Then,a low numerical aperture can be realized,which is promising to achieve the expanding of mode area with a high-quality beam.In order to study the performance of TG VLMA fiber in a fiber amplifier,we present a rate-equation model of the single-mode ytterbium-doped TG VLMA fiber amplifier,which consists of the steady-state rate equations and thermal transferring equations.With this model,the forward-pumped single-mode TG VLMA fiber amplifier is numerically studied.It is found that the diameter of fundamental mode field rises with the increase of the signal power,which shows the superiority of the TG VLMA fiber in suppressing the nonlinear effect in the fiber amplifier.The optimum fiber length and pertinent physical mechanism are also investigated.It is revealed the optimum fiber length is related to the input pump power,and it decreases with the increase of input pump power.However,when the input pump power is large enough,such a variation of optimum fiber length will become weakened.The numerical results also illuminate that the thermal load at the optimum length of TG VLMA fiber should not change too much with the input pump power.Moreover,the mode of output optical field is also discussed.It is found that the thermal load at the optimum length may not be large enough to realize a core-confined mode.In order to ensure that the core-confined mode can be output,the thermal load at the end of the fiber amplifier should be larger.It requires that the fiber length used in the amplifier should be shorter than the optimum fiber length,which will induce the decrease of the output signal power to some extent.In spite of that,the numerical results reveal that the decrease of output signal power should not be much,and the pertinent slope efficiency is not obviously lowered,either.Thus,it is verified that the core-confined mode with a VLMA can be obtained from the TG VLMA fiber amplifier with high slope efficiency.The pertinent results have significant guidance in the design of TG VLMA fiber amplifier.