基于级联跃迁的2.8μm低掺铒氟化物光纤激光器数值分析与优化

Numerical analysis and optimization of 2.8μm lightly-erbium-doped fluoride fiber laser based on cascaded transition

2.8μm和1.6μm激光级联跃迁的工作方式,可以有效解决低掺铒氟化物光纤中自终止效应导致的2.8μm激光功率提升难题.建立基于低掺铒氟化物光纤2.8μm和1.6μm激光级联跃迁的中红外光纤激光器数值模型,系统分析了2.8μm和1.6μm激光波长对2.8μm激光功率和转换效率的影响.计算结果表明,选取1610 nm作为级联激光工作波长,能有效平衡2.8μm激光下能级^(4)I_(13/2)粒子向基态^(4)I_(15/2)和激发态^(4)I_(9/2)的跃迁过程,实现2.8μm波段激光输出功率和效率的提升.此外,计算了1.6μm激光腔反馈对2.8μm激光功率和效率的影响,结果表明,仅通过光纤端面提供的弱反馈即可实现1.6μm激光振荡,从而获得高效率2.8μm激光输出.

Er^(3+)-doped ZBLAN fiber laser is a promising approach to producing 2.8μm mid-infrared(MIR)laser.The long lifetime of the lower-laser-level^(4)I_(13/2)often results in serious self-terminating effect which harms the laser power and efficiency significantly,especially for the active fiber with low dopant concentration which is preferred for weak thermal issues but cannot depopulate the lower-laser-level effectively via the up-conversion process.The 1.6μm lasing(^(4)I_(13/2)→^(4)I_(15/2))in Er-ZBLAN fiber could deplete the population on^(4)I_(13/2).Therefore,cascaded 2.8μm and 1.6μm lasing in Er^(3+)-doped ZBLAN fiber provides a promising solution to the self-termination effects on laser power scaling.Moreover,the^(4)I_(13/2)→^(4)I_(15/2)1.6μm laser also has some overlap with the^(4)I_(13/2)→^(4)I_(9/2)excited state absorption(ESA)spectrum.The ions on the^(4)I_(9/2)level would then relax to the upper-laser-level of 2.8μm lasing(^(4)I_(11/2)),and results in enhanced laser efficiency.In general,the 1.6μm cascaded lasing in 2.8μm Er-ZBLAN fiber laser involves both lasing and ESA.The two processes have different spectra and different influences on the 2.8μm laser gain.Therefore,there should exist an optimal wavelength of the 1.6μm laser,which would balance the two processes,ensuring the lower-laser-level depopulation while maximizing the ion recycling.Therefore,we develop a comprehensive numerical model of cascaded 2.8μm and 1.6μm lasers based on Er-ZBLAN fiber.After the numerical model is verified by the previous experimental results,the effects of MIR and 1.6μm lasing wavelengths on the power and conversion efficiency of 2.8μm laser are investigated in depth.The results show that a suitable trade-off between the two processes can be reached with the cascaded lasing wavelength of 1610 nm,for the optimized 2.8μm laser power/efficiency.Moreover,the influence of 1.6μm laser cavity feedback on the power/efficiency characteristics of the 2.8μm laser is also investigated.It is found that the feedback at 1.6μm is very low,even only 4%is provided by the Fresnel reflection of the fiber facet,which can effectively generate 1.6μm laser and significantly improve the efficiency of 2.8μm laser.