Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61735007, 61975061)
Received Date:27 April 2020
Accepted Date:08 July 2020
Available Online:24 November 2020
Published Online:05 December 2020
Abstract:Ytterbium doped fiber lasers (YDFLs) with small volume, good beam quality, good heat dissipation performance and high conversion efficiency are widely used in industrial processing, military, medical and other fields. In past decades, with the development of high-performance double cladding gain fiber and fiber devices, the output power of YDFLs increases rapidly. However, nonlinear effects (NLEs), such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), are produced, which limits the further enhancement of the output power of fiber laser. Large mode area ytterbium-doped fiber (LMAYDF) can effectively increase the nonlinear effect threshold. However, increasing the core diameter will support more high-order modes (HOMs), which may lead the beam quality to deteriorate and induce the mode instability (MI) effect to occur in fiber lasers. Thus, MI and NLEs have become the main limiting factors for the further improving of output power and beam quality in fiber lasers. The confined-doped ytterbium-doped double-clad fiber (CDYDF), by reducing the doping diameter of gain ions in the fiber core, makes the fundamental mode (FM) dominate in mode competition and HOM suppressed to achieve LMAYDF gain control for different modes, thus improving the output power of the fiber laser and maintaining good beam quality. The 33/400 μm confined-doped ytterbium-doped double-clad fiber (CDYDF) is fabricated by modifying the chemical vapor deposition (MCVD) process with solution doping technology (SDT). The Yb3+ doping diameter ratio is 70% and refractive index profile is close to step-index. Utilizing the master oscillator power amplifier (MOPA) system the beam quality optimization effect of confined-doped fiber is verified and optimized to 1.43 as the power increases while the M2 of seed laser is 1.53. An all-fiber structure counter-pumped fiber oscillator is constructed to test the laser performance of home-made confined-doped fiber. When the pump power is ~4.99 kW, laser power of 3.14 kW with a central wavelength of 1081 nm and line width of 3.2 nm at 3 dB is obtained. Moreover, there is no MI nor SRS in the whole experiment. We demonstrate that it is the highest output power based on home-made confined-doped fiber. The above results indicate that confined-doped fibers have the potential to achieve high-power and high-beam-quality fiber laser output. Keywords:confined-doped fiber/ beam quality/ mode instability/ fiber laser
如图5(a)为输出激光功率和光光效率随泵浦光功率的变化示意图, 输出激光功率基本处于线性增长, 且经过拟合后得到的斜率效率为63.6%, 而由于所使用部分掺杂光纤长度较短, 导致泵浦光吸收不够充分, 造成了激光效率较低; 图5(b)所示为后向回光功率随泵浦光功率的变化示意图, 随着泵浦光功率的提升, 回光功率也逐步趋于线性增长. 图 5 (a) 不同泵浦功率下的输出激光功率、效率曲线; (b) 回光功率 Figure5. (a) Dependence of the output power and optical efficiency on the pump power; (b) back light power.
PD1和PD2分别接收CLS2的泄露光和PM靶面的散射光, 在全光纤振荡器达到最高输出功率3140 W时的时频域结果如图7所示. 图7(a)中的时域图表明在最高输出功率时, 两处接收光的相对强度无明显变化. 图7(b)中的频域图显示在最高功率输出时, 未出现高频分量, 表明输出激光功率达到3140 W系统并未出现横向模式不稳定现象. 图 7 输出功率达到3140 W时, PD1和PD2分别接收CLS2的泄露光和PM靶面的散射光的 (a) 时域; (b) 频域 Figure7. When the output power reaches 3140 W, the leakage light of CLS2 and scattering light of PM target surface: (a) Time domain; (b) frequency domain.