基于M型掺镱光纤的近单模2kW光纤放大器

Near-single-mode 2-kW fiber amplifier based on M-type ytterbium-doped fiber

模式不稳定效应和非线性效应已经成为高功率光纤激光器中限制输出功率和光束质量进一步提升的主要障碍.采用改进的化学气相沉积工艺结合溶液掺杂技术制备25/400μm的M型掺镱双包层光纤,纤芯和中间凹陷区域的数值孔径分别为0.054和0.025.基于该光纤搭建976 nm双向泵浦全光纤结构放大器.在泵浦光功率为3283 W时,获得2285 W中心波长为1080 nm的激光输出,3 dB线宽为3.01 nm,测量的光束质量因子为1.42,且未出现受激拉曼散射现象.这是目前基于M型掺镱光纤实现的最高输出功率,通过优化光纤结构参数实现功率进一步提升是有希望的.

High power fiber laser systems have attracted extensive attention due to compactness,good beam quality,efficient heat dissipation and high conversion efficiency.They are widely used in industrial processing,military,medical treatment and other fields.Over the past two decades,owing to the development of double cladding fiber and high-brightness laser diodes,the output power of fiber lasers has been greatly improved.Unfortunately,nonlinear effects(NLEs),such as stimulated Brillouin scattering(SBS)and stimulated Raman scattering(SRS),restrict the further enhancement of the output power of fiber lasers.Apparently,increasing the core diameter is the most common way to suppress NLEs in the fiber,but this causes another limiting factor,i.e.mode instability(MI),resulting in the deterioration of the beam quality and in the limitation of the power scaling.Therefore,it is important and urgent to suppress the NLEs and MI simultaneously in fiber lasers.The M-type fiber,by designing refractive index profile,breaks through the stringent trade-off between mode area and numerical aperture(NA),so it possesses a larger mode area than the step index fiber,which helps to avoid NLEs and expand the power range.The M-type ytterbium doped double-clad fiber is fabricated by the modified chemical vapor deposition(MCVD)process with solution doping technology(SDT),the core/cladding diameter is 25/400μm.The NA of high index ring and index dip in the core are 0.054 and 0.025,respectively.To test the performance of the M-type fiber during high-power operation,a 976 nm bidirectional pumped all-fiber amplifier is constructed.As a result,maximum output power of 2285 W is achieved with an optical-tooptical conversion efficiency of 66.5%under bidirectional pumping scheme,and the measured M^(2) factor is 1.42,the central wavelength and 3 dB linewidth of output laser are 1080 nm and 3.01 nm,respectively.To the best of our knowledge,this is the highest output power in a continuous-wave fiber laser employing an M-type fiber at present.However,the MI effect is observed at the output power of 2252 W.The future work will focus on optimizing the structure of the M-type fiber to achieve a stabler higher-power and higher-efficiency laser output.