低数值孔径部分掺杂纺锤形光纤实现4kW近衍射极限激光输出

4kW Near-Diffraction-Limit Laser Output Based on Low-Numerical Aperture Confined-Doped Long-Tapered Fiber

采用改进的化学气相沉积工艺结合溶液掺杂技术成功制备了一种低数值孔径部分掺杂纺锤形光纤。该光纤的数值孔径约为0.05,镱离子在纤芯中的掺杂直径比约为77%,光纤两端纤芯和包层的直径分别为25μm和400μm,中间部分纤芯和包层的直径分别为37.5μm和600μm。搭建976 nm双端泵浦光纤放大器,该光纤最终实现了4.188 kW的单模激光输出,斜率效率为82.8%,最高功率下的光束质量因子约为1.3,其输出功率的继续提升受限于受激拉曼散射效应。

Objective High-power ytterbium-doped fiber lasers(YDFLs)have good beam quality,high conversion efficiency,high reliability,and good compactness,making them widely usable in industrial processing,military,and national defense fields.The output power of YDFLs has been unprecedentedly improved with the development of double-clad fiber,laser diodes,and passive devices.This unprecedented progress is hampered by nonlinear effects,such as stimulated Raman scattering(SRS)and transverse mode instability(TMI).The most effective and fundamental way to suppress these two effects is to optimize the structure design of active fiber.We propose a low-numerical aperture confined-doped long-tapered(LCT)fiber that combines the advantages of low numerical aperture(NA),Yb^(3+)ions restricted doping,and longitudinal tapered design and can theoretically suppress TMI and SRS effects simultaneously.Methods The LCT fiber is proposed and successfully fabricated using modified chemical vapor deposition(MCVD)in conjunction with a solution doping technique(SDT).The LCT fiber has a core NA of~0.05 and a gain dopant doping diameter ratio of~77%,with a core/cladding diameter of 25/400μm at both ends and 37.5/600μm in the middle.A bidirectional-pumped master oscillator power amplifier(MOPA)system verifies the laser performance of the LCT fiber.Results and Discussions A laser output of 4.188 kW was obtained with a slope efficiency of 82.8%(Fig.3).The intensity of the Raman Stocks light was~18 dB lower than that of the signal laser at 4.188 kW output power,and the M2 factor was about 1.3,maintaining a single-mode output.Further optimization will focus on improving the pump absorption and effective mode area of this fiber to mitigate SRS.Conclusions We present a novel low-NA(0.05)confined-doped long-tapered fiber fabricated using the MCVD process in conjunction with SDT.The Yb-ions doping diameter ratio is~77%,and the middle section core/cladding diameter is 37.5/600μm,tapering to 25/400μm at both ends.In the bidirectional pumping MOPA configuration,a 4.188 kW laser is obtained with a slope efficiency of 82.8%.The M2 factor is about 1.3 at 4.188 kW output power,maintaining a single-mode output.The results above reveal that using low-NA confined-doped long-tapered Yb-doped fiber to achieve high power output with high brightness is a promising prospect.