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掺镱光纤激光器(ytterbium-doped fiber lasers, YDFLs)以其体积小、光束质量好、散热性能好和转化效率高等优点[1,2], 被广泛应用于工业加工、军事和医疗等领域[3-5]. 随着高性能双包层增益光纤及光纤器件的发展, 高功率掺镱光纤激光器输出功率得到迅速提升[6-9]. 然而, 非线性效应(nonlinear effects, NLEs), 如受激布里渊散射(stimulated Brillouin scattering, SBS)、受激拉曼散射(stimulated Raman scattering, SRS)等的产生, 限制了光纤激光器输出功率进一步提升[10]. 大模场面积掺镱光纤(large mode area ytterbium-doped fiber, LMAYDF)能有效提高非线性效应阈值, 然而, 增大纤芯直径将支持更多高阶模式(higher order mode, HOM), 这可能会导致光纤激光器光束质量恶化和模式不稳定(mode instability, MI)效应的产生[11-14]. 减小纤芯内增益离子掺杂直径可有效地抑制LMAYDF中HOM成分, 从而提高光纤激光输出功率, 同时保持良好的光束质量.
部分掺杂光纤即通过对纤芯中增益离子进行直径裁剪, 使得基模(fundamental mode, FM)在模式竞争中处于优势地位, 同时抑制HOM增益, 实现LMAYDF对不同模式的增益控制, 进而提高光纤激光器输出功率. 多项理论研究已表明[15,16], 部分掺杂光纤能有效提高光纤激光器输出激光的HOM阈值. 然而, 关于部分掺杂光纤实验部分报道较少, 这主要是由于其制备工艺相对困难. 2012年, 芬兰nLIGHT公司采用直接纳米粒子沉积(direct nanoparticle deposition, DND)工艺制备出纤芯/内包层尺寸分别为41/395 μm部分掺杂光纤, 采用放大自发辐射光源验证光纤性能, 输出激光光束质量因子（M2）约为1.3[17]. 与主流稀土掺杂光纤制备工艺-改进的化学气相沉积(modified chemical vapor deposition, MCVD)工艺相比, DND工艺所需设备昂贵、工艺复杂, 因此, 采用MCVD工艺制备部分掺杂光纤成为亟待解决的问题. 2018年, 华中科技大学Liao等[18]利用MCVD工艺制备出纤芯35 μm、Yb3+掺杂直径比为71.4%的部分掺杂光纤, 实验中测得M2随功率提升从2.8优化至1.5. 在大功率部分掺杂光纤激光输出方面, 日本藤仓公司于2016年采用自研部分掺杂光纤获得具有高SRS抑制的2 kW单模激光输出[19]; 2018年, 该公司利用部分掺杂光纤将近衍射极限激光输出功率提升至5 kW, 同时SRS抑制比达到45 dB[20]; 最近, 该公司又将部分掺杂光纤激光输出功率提升至8 kW, SRS抑制比为22 dB[21], 但上述三次报道中均未具体介绍制备工艺. 显然, 掌握部分掺杂光纤制备工艺, 实现国产化, 并应用到大功率光纤激光器中, 将具有极高的工业和国防价值.
本文采用MCVD工艺结合溶液掺杂技术(solution doping technique, SDT)制备出纤芯和包层直径分别为33/400 μm部分掺杂光纤, 增益离子在纤芯中占比约为70%, 纤芯数值孔径约为(numerical aperture, NA) 0.06. 接着利用主振荡功率放大(master oscillator power-amplifier, MOPA)系统结构验证部分掺杂光纤M2优化作用. 最后搭建915 nm反向泵浦全光纤激光振荡器, 获得了3.14 kW激光输出, 且未出现SRS和TMI现象.
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采用MCVD工艺结合SDT制备部分掺杂预制棒, 主要步骤如下: 疏松(soot)层沉积、溶液掺杂、除羟基干燥、玻璃化和高温缩棒. 沉积温度会影响soot层的孔径大小和均匀性, 进而影响后续溶液浸泡过程中孔径对稀土离子的吸附能力, 最终影响掺杂浓度和折射率剖面.
部分掺杂光纤纤芯可根据是否含有Yb3+离子分为有源区和无源区, 为了获得近似阶跃型折射率剖面, 需要保证纤芯有源区与无源区折射率匹配, 因此溶液掺杂成为关键的一步[17]. 图1(a)为部分掺杂光纤纤芯剖面设计, 纤芯中有源区掺杂离子主要有Yb3+, Al3+和Ce3+, 这三种离子都会提高纤芯有源区NA, 而NA过高又很难保证良好的光束质量和激光亮度, 因此需要掺入F–降低NA[22]. 纤芯无源区采用Al3+和F–相互配比, 用于匹配有源区折射率, 以保证最佳的阶跃型剖面. SHR-1602光纤分析仪测得部分掺杂光纤折射率剖面如图1(b)所示, 可以看出, 纤芯的折射率剖面较为平坦, 且纤芯有源区与无源区折射率匹配度高, 经过计算, 总体纤芯的NA约为0.06.
图 1 部分掺杂光纤 (a) 纤芯离子分布设计; (b) 折射率剖面
Figure 1. Confined-doped fiber: (a) Designed distribution of ions in core; (b) refractive index profile.
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MOPA系统用于验证部分掺杂光纤(confined-doped Yb-doped fiber, CDYDF)高输出功率时的光束质量优化作用, 系统结构如图2所示[23]. 种子光和泵浦光由(6 + 1) × 1泵浦信号合束器(pump signal coupler, PSC)注入光纤, 该部分掺杂光纤在976 nm处的吸收系数为0.77 dB/m, 使用长度为27 m, 保证了足够的泵浦吸收, 弯曲直径为11.0—23.5 cm, 包层光滤除器(cladding light stripper, CLS)将泄露到包层中的光滤除, 经端帽(end cup, EC)输出的光由准直器模块和缩束模块注入到光束质量分析仪(M2-200S)中, 测试其光束质量因子.
图 2 高功率光束质量MOPA结构测试系统
Figure 2. High-power beam quality testing system with MOPA.
测得部分掺杂光纤M2和输出功率随泵浦功率的变化如图3所示. 由图3可以看出, 部分掺杂光纤具有明显的光束优化作用. 具体地, 未注入放大级泵浦光时, 测得种子光M2为1.53, 逐渐提高泵浦功率, 输出激光功率呈线性增长, 而M2值逐渐下降, 当输出功率达到1.2 kW时, 输出激光M2为1.43.
图 3 部分掺杂光纤光束质量和输出功率
Figure 3. Beam quality factor and output power of confined-doped fiber.
3 kW反向泵浦掺镱全光纤激光振荡器结构如图4所示. 高反射光纤布拉格光栅(high reflectivity fiber Bragg grating, HR FBG, 中心波长为1080 nm, 3 dB带宽约为3 nm, 反射率约为99.9%)、增益光纤和输出耦合光纤布拉格光栅(output coupler fiber Bragg grating, OC FBG, 中心波长为1080 nm, 3 dB带宽约为1 nm, 反射率约为10%)构成激光谐振腔. 每7个中心波长为915 nm半导体激光器(laser diode, LD)经过7 × 1的泵浦合束器(pump coupler, PC)合束到(6 + 1) × 1 PSC的泵浦臂中, 泵浦光将全部经过OC FBG注入到腔内. 增益光纤采用国产部分掺杂双包层掺镱光纤, 对45 m长部分掺杂光纤进行如图4跑道式盘绕, 光纤两端与光栅尾纤熔接的熔点放入在内圈, 并加以充分水冷. 在HR FBG的腔外端, 连接CLS1和石英光束输出头(quartz beam header, QBH), 用于监测回光功率. 由于受水冷板跑道总长度和光纤吸收系数的影响, 因此在OC FBG的输出端, 传能光纤的内包层中会积累大量的剩余泵浦光和少量的高阶模式激光. 为了更加有效地滤除包层光, 采用两段式包层光滤除手段, 大大提高输出功率并降低实验风险. 最后只留下存在于纤芯中的激光, 经QBH输出并由万瓦功率计(power meter, PM)监测激光功率. 在CLS2和PM靶面位置分别放置了光电探头1, 2 (photo detector 1, 2, PD 1, 2), 并将转换的电信号输入到示波器的输入信道中.
图 4 反向泵浦全光纤激光振荡器结构
Figure 4. Scheme of backward pumped fiber laser oscillator.
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
Figure 5. (a) Dependence of the output power and optical efficiency on the pump power; (b) back light power.
测得的输出激光光谱随输出功率变化如图6所示. 从图6可以看出, 随着输出激光功率逐步增长, 3 dB线宽从112 W时的0.5 nm展宽到3140 W时的3.2 nm. 输出功率达到3.14 kW时, 光谱上1064 nm和1098 nm附近出现两个边带, 预示着四波混频和自相位调制的存在, 且光谱中无拉曼光成分, 说明无SRS效应.
图 6 不同功率下输出激光光谱特性
Figure 6. Output laser spectra at different output 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
Figure 7. 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.
由于反向泵浦激光振荡器系统中低反光栅承受功率受限, 在振荡器输出为3 kW时已经表现出高温, 鉴于安全的原则, 本次实验没有能够测试该光纤振荡器的光束质量, 也没有进行长时间的高功率拷机实验以验证部分掺杂光纤在3 kW振荡器系统中的功率稳定性. 但是可以预测, 部分掺杂光纤在3 kW时将会有更加显著的光束质量优化作用.
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部分掺杂光纤是实现光纤激光器高功率和高光束质量输出的有效途径. 本文基于MCVD工艺结合SDT制备33/400 μm部分掺杂掺镱双包层光纤, 镱离子掺杂直径比为70%. 利用MOPA系统验证部分掺杂光纤光束质量优化作用, 种子光束质量为1.53, 随着泵浦功率增长, 输出激光光束质量逐渐优化至1.43. 搭建915 nm反向泵浦全光纤激光振荡器, 在泵浦功率约为4.99 kW时, 获得3.14 kW中心波长为1081 nm激光输出, 3 dB带宽为3.2 nm, 实验中并未出现SRS和TMI效应, 这是目前基于国产部分掺杂光纤实现的最高输出功率. 进一步提高泵浦功率, 优化激光器系统将有望实现国产部分掺杂光纤更高功率的稳定激光输出.
感谢国防科技大学奚小明老师、王鹏博士、叶云博士、徐小勇老师、张坤老师等为本次实验提供的帮助.
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模式不稳定效应和非线性效应成为光纤激光器输出功率和光束质量进一步提升的主要限制因素. 采用改进的化学气相沉积工艺结合溶液掺杂技术制备33/400 μm部分掺杂掺镱双包层光纤, 镱离子掺杂直径比为70%, 折射率剖面近似阶跃型. 利用主振荡功率放大系统验证部分掺杂光纤光束质量优化作用, 种子光束质量为1.53, 随着泵浦功率增长, 输出激光光束质量逐渐优化至1.43. 搭建915 nm反向泵浦全光纤结构激光振荡器. 实验中, 在泵浦光功率约为4.99 kW时, 获得3.14 kW中心波长为1081 nm的激光输出, 3 dB带宽为3.2 nm, 且未出现模式不稳定和受激拉曼散射现象, 这是目前基于国产部分掺杂光纤实现的最高输出功率. 以上结果表明, 部分掺杂光纤在实现高功率且高光束质量光纤激光输出中具有潜力.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.
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Keywords:
- confined-doped fiber /
- beam quality /
- mode instability /
- fiber laser
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图 1 部分掺杂光纤 (a) 纤芯离子分布设计; (b) 折射率剖面
Fig. 1. Confined-doped fiber: (a) Designed distribution of ions in core; (b) refractive index profile.
图 3 部分掺杂光纤光束质量和输出功率
Fig. 3. Beam quality factor and output power of confined-doped fiber.
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
Fig. 5. (a) Dependence of the output power and optical efficiency on the pump power; (b) back light power.
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Google Scholar
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Google Scholar
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