Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61805280, 62035015, 61806217), the Science Research Plan of National University of Defense Technology, China (Grant No. ZK19-07), and the Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology, China (Grant No. SKL2020ZR07)
Received Date:12 April 2021
Accepted Date:11 May 2021
Available Online:07 October 2021
Published Online:20 October 2021
Abstract:In recent years, the high-power single-frequency fiber lasers have developed rapidly, and they have been used in nonlinear frequency conversion and gravitational wave detection. The main factors limiting the output power of single-frequency fiber lasers are the nonlinear effect and transverse mode instability (TMI) effect. In general, large-core fibers can mitigate nonlinear effects while small-core fibers help to suppress the TMI effect. Owing to the core diameter varying in the longitudinal direction, tapered double clad fiber (T-DCF) is a promising solution to simultaneously suppress the nonlinearity and TMI effects. In the present study, we have fabricated a piece of 2.2-m-long Ytterbium-doped T-DCF. The core diameter and the cladding diameter of this fiber vary gradually from 30.3 μm to 49.3 μm and from 245 μm to 404 μm, respectively. Using this homemade fiber, we constructe an all-fiberized single-frequency master oscillator power amplifier system, which is pumped by laser diodes with a central wavelength of 976 nm. The seed of the system has a central wavelength of 1064 nm, and output power of 30 mW. The T-DCF is coiled on a piece of cooling plate, whose output end is cleaved at a 8° angle. The laser is output to free space and collimated by a free-space collimator. After the collimator, dichroic mirror is utilized to strip out the residual pump power for measuring power, spectrum, time-domain signal and beam quality. The output power increases linearly with the pumping power increasing. When the pumping power is 502 W, the output power reaches 400 W. And there is no stimulated Brillouin scattering (SBS) nor TMI under the power level. The corresponding slope efficiency is 81.7% while the M2 is measured to be 1.29, exhibiting the single-mode output characteristic of the system. When the output power is further increased to 418 W, the TMI effect is observed, which limits further the power scaling of the single-mode output. To the best of our knowledge, this is the highest output power of single-frequency fiber laser based on home-made gain fibers. The results indicate that T-DCFs can simultaneously suppress the nonlinearity and TMI, thus providing a useful reference for further power scaling of single-frequency fiber lasers. Higher output power is expected by optimizing the parameters of T-DCF and the structure of system. Keywords:single-frequency/ high power fiber lasers/ long tapered fiber/ transverse mode instability
基于该光纤的全光纤结构的单频光纤激光器系统如图2所示. 单频种子激光(seed)中心波长为1064 nm、线宽约为20 kHz、输出功率为30 mW. 种子激光经过隔离器(ISO)和两级预放大器(two-stage pre-amplifiers)后功率约为5 W, 然后通过耦合器(tapper)和合束器(combiner)注入到主放大器, 其中耦合器后向的输出臂用于监测主放大器中产生的后向回光(backward monitor), 以判断主放大器中SBS效应是否达到阈值. 主放大器采用前向泵浦的方式进行激光放大, 泵浦源为6个最大输出功率为95 W、中心波长为976 nm的激光二极管(LD). 所用(6+1)×1合束器的信号输入臂和输出臂的纤芯/内包层直径分别为15 μm/130 μm和30 μm/250 μm. 长锥形光纤的大部分区域盘绕在水冷盘上, 弯曲半径约为10—15 cm, 尾纤脱离水冷盘, 固定于精密调节架上. 为避免端面反馈对系统的不利影响, 尾纤末端切割成8°斜角. 放大后的激光经斜角输出至自由空间, 并经空间准直器准直. 此时的输出光束中含有未被光纤吸收的剩余泵浦光. 利用二色镜滤除剩余泵浦光后, 进行输出功率、光谱、时域、光束质量等参数的测量. 其中测量光束质量时需要调整光路, 加入足够的衰减器件以确保进入到光束质量测量仪中的光功率在仪器的承受范围. 图 2 基于长锥形双包层光纤搭建的单频光纤放大器的实验装置图 Figure2. Experimental setup of single frequency fiber amplifier based on tapered double clad fiber.
3.结果与分析实验中利用光电探测器监测功率计靶面的散射光, 以判断是否有TMI效应出现. 当输出光功率至400, 418和434 W时, 系统时域结果如图3(a)、图3(c)和图3(e)所示, 频域结果如图3(b)、图3(d)和图3(f)所示. 图3(a)和图3(b)中的时频域结果表明在输出功率为400 W时, 光电探测器接收到的散射光的强度无明显波动, 表明此时系统并未出现TMI效应. 根据图3(c)—(f)结果可知, 当输出功率增加至418 W时域信号开始在ms量级的尺度上出现轻微的波动, 对应的频域曲线上开始出现明显的高频分量; 当输出功率进一步增加至434 W时, 时域信号的波动更加明显. 因此, 这些现象预示着输出功率在400 W时, TMI效应尚未出现, 当输出功率进一步增加, TMI效应出现. 图 3 不同输出功率下, 光电探测器接收光信号的时频域 (a)输出功率为400 W时的时域; (b) 输出功率为400 W时的频域; (c) 输出功率为418 W时的时域; (d) 输出功率为418 W时的频域; (e) 输出功率为434 W时的时域; (f) 输出功率为434 W时的频域 Figure3. The detected scattering light signals under different output power levels: (a) Time domain when output power reaches 400 W; (b) frequency domain when output power reaches 400 W; (c) time domain when output power reaches 418 W; (d) frequency domain when output power reaches 418 W; (e) time domain when output power reaches 434 W; (f) frequency domain when output power reaches 434 W.
测得输出功率和回光功率随泵浦光功率的变化情况如图4所示. 从图4中可以看出, 随着泵浦功率增大, 输出功率近似呈线性增长, 整个过程没有观察到功率下降现象. 当泵浦功率为0 W时, 输出激光功率为4.5 W; 当泵浦功率为502 W时, 输出激光功率为400 W, 对应的斜率效率约为81.7%. 该输出功率下对应的后向回光仅为8.4 mW, 约为前向输出功率的0.021‰. 在整个放大过程中, 回光没有出现非线性增长的迹象, 这表明SBS效应得到了良好的抑制. 图 4 输出功率、回光功率随泵浦光功率的变化 Figure4. Output power and backward power versus pump power.
注入主放大器前的种子光和经过主放大器后不同输出功率下的光谱如图5所示. 其中, 图5(a)是经过预放大器后、注入主放大器前的种子光的光谱, 光谱的信噪比约为24 dB; 图5(b)—(d)分别是输出功率为109, 255, 和400 W时的光谱. 从图5(b)—(d)可以看到, 在不同输出功率下光谱中无泵浦光成分, 说明剩余泵浦光已经被二色镜充分滤除. 同时, 光谱中无放大自发辐射(ASE)成分. 伴随着输出功率的增加, 光谱的信噪比也逐渐增加, 400 W输出功率下的信噪比约为32 dB. 图 5 种子光及经过主放大器后不同输出功率下的光谱 (a) 种子光; (b) 输出功率109 W; (c) 输出功率255 W; (d) 输出功率400 W Figure5. Spectra of the seed light and the output laser with different power lever: (a) Seed light; (b) output power of 109 W; (c) output power of 255 W; (d) output power of 400 W.