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实验中所用MOPA结构的掺Yb保偏光纤激光器的实验装置如图 1所示。其中,种子源采用单频环形激光器,输出波长为1064nm,最大输出功率为2.75W,光谱线宽约为66.7MHz[13]。放大器部分为两级放大结构,一级放大采用10W的914nm LD作为抽运源,增益光纤为8m掺Yb光纤(ytterbium-doped fiber,YDF),光纤纤芯和包层的直径分别为10μm和130μm,吸收系数为1.3dB/m;二级放大采用两个50W的976nm锁波长LD作为抽运源,增益光纤为2.4m掺Yb光纤,光纤纤芯和包层直径分别为20μm和130μm,吸收系数为3.5dB/m。由环形固体激光器输出的种子光经准直、聚焦后耦合进无源光纤中,再经隔离器和合束器的信号端进入双包层掺Yb光纤中。抽运光则经合束器抽运端进入光纤中。这里的隔离器用来避免反向传输的激光对种子源造成损伤。从一级放大输出的种子光经过隔离器和合束器后,进入二级放大增益光纤中,由976nm抽运光进行抽运放大,再经剥模器和端帽后输出。与剥模器一端熔接的输出端帽被切割成8°角,用来抑制自激振荡。此外,各分立器件之间用保偏熔接机FMS100P+进行熔接,实现了放大器系统的全光纤化[14]。
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在光纤MOPA结构中,单频窄线宽连续信号光的输出功率主要受限于SBS。光纤中的SBS阈值受光纤长度、光纤的纤芯直径和损耗等因素的影响[15-17]。SBS的抽运能量的阈值可以用下式表示:
$ {\mathit{P}_{{\rm{SBS, th}}}} \approx \frac{{21{A_{{\rm{eff}}}}}}{{{g_{\rm{B}}}{l_{{\rm{eff}}}}}} $
(1) 式中,gB是布里渊增益的峰值,Aeff是有效纤芯截面积,leff是有效相互作用长度。从(1)式可知,对于一般低损耗的单模光纤,光纤长度达20km左右,阈值将低至数毫瓦[18]。实验中采用减小光纤长度、增大光纤纤芯直径和提高光纤吸收系数等方法来抑制SBS。
一级和二级放大结构采用不同波长的半导体激光器作为抽运源。976nm和914nm都可以作为掺Yb光纤的抽运波长,但二者在抽运效率、所需增益光纤和冷却能力等方面存在显著差异。与914nm抽运场合相比,976nm的吸收系数是914nm的3倍左右,抽运效率高于914nm。因此,若要达到相同的抽运功率吸收效果,914nm抽运场合用到的增益光纤要比976nm的长。另外,914nm附近的吸收带比较宽,波长的稳定性对抽运效果的影响较小。而976nm的吸收带比较窄,抽运波长稳定性对输出功率的影响显著。在一级放大中,所用增益光纤在914nm的吸收系数为1.3dB/m,为保证抽运吸收率高达90%以上,需要8m以上的增益光纤。虽然一级放大用到的增益光纤较长,但一级放大只需将毫瓦量级的种子光放到数瓦,所需抽运功率低于SBS阈值,一般不会出现受激布里渊散射[19]。在二级放大中选用的976nm抽运源为锁波长半导体激光器,具有976nm波长锁定的特点,避免因抽运波长的不稳定而导致输出功率大幅度波动。二级放大与一级放大相比,增益光纤长度短、光纤直径大,可抑制连续光高功率放大时常见的受激布里渊散射。
基于MOPA结构的1064nm单频光纤激光器
1064nm single-frequency fiber lasers based on MOPA structure
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摘要: 为了抑制受激布里渊散射效应, 提高单频窄线宽种子源的放大功率, 采用主振荡功率放大器结构, 并对光纤长度、纤芯直径和抽运参量进行优化, 实现了42W的1064nm信号光输出。实验中, 一级放大采用914nm半导体激光器作为抽运源, 增益光纤芯径10μm, 长度8m;二级放大采用976nm半导体激光器作为抽运源, 增益光纤芯径20μm, 长度2.4m。在种子光功率40mW、一级放大的抽运功率6.8W、二级放大的抽运功率85W时, 得到了42W的1064nm信号光输出。结果表明, 光光转换效率约49.4%, 偏振消光比27.5dB; 输出信号光中心波长1064.5nm, 线宽约70MHz, 保持了种子光的单频特性。在42W连续输出时没有观察到受激布里渊散射, 继续增大抽运功率, 有望实现更高功率的放大。Abstract: In order to suppress the stimulated Brillouin scattering effect and increase the amplification power of single frequency narrow linewidth seed source, master oscillator power amplifier (MOPA) structure was adopted. Fiber length, core diameter and pumping parameters were optimized. Optical output of 42W signal at 1064nm was realized. In the experiment, the first stage amplification used 914nm semiconductor laser as pumping source with gain fiber core diameter of 10μm and length of 8m. The second stage amplifier used 976nm semiconductor laser as the pumping source with gain fiber core diameter of 20μm and length of 2.4m. When seed power is 40mW, pump power of the first stage amplification is 6.8W and pump power of the second stage amplification is 85W, optical output 42W of the signal at 1064nm was obtained. The results show that light-to-light conversion efficiency is about 49.4%, polarization extinction ratio is 27.5dB; central wavelength of output signal light is 1064.5nm, and linewidth is about 70MHz. Single-frequency characteristics of seed light are maintained. Stimulated Brillouin scattering is not observed at 42W continuous output. Continuous increase of pump power is expected to achieve higher power amplification.
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Key words:
- laser technique /
- fiber laser /
- single frequency /
- stimulated Brillouin scattering
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[1] WEI X Ch, OU P, ZHANG Ch X, et al. Single frequency single polarization narrow linewidth fiber laser and its amplification [J]. Laser Technology, 2009, 34(1):5-7(in Chinese). [2] YAO X Q, SUN W, WANG X B. Dual-wavelength single-frequency fiber laser based on ring filter [J]. Laser Technology, 2017, 41(1): 98-100(in Chinese). [3] HU J, ZHANG L, LIU H, et al. High-power single-frequency 1014.8nm Yb-doped fiber amplifier working at room temperature[J].Applied Optics, 2014, 53(22):4972-4977. doi: 10.1364/AO.53.004972 [4] LIU X J, WEI G X, ZHOU B J, et al. 1120nm narrow linewidth ytterbium-doped fiber laser[J]. Laser Technology, 2016, 40(3):349-352(in Chinese). [5] CHEN M H. Research progress of high-power fiber lasers[J]. Laser and Infrared, 2007, 37(7):589-592(in Chinese). [6] ZHANG W N, LI C, FENG Zh M, et al. Short cavity single frequency fiber laser at 1080nm based on highly Yb3+-doped phosphate fiber[J]. Laser & Optoelectronics Progress, 2012, 49(10): 100601(in Chinese). [7] SHI W, FU Sh J, FANG Q, et al. Single-frequency fiber laser based on rare-earth-doped silica fiber[J]. Infrared and Laser Engineering, 2016, 45(10): 1003001(in Chinese). doi: 10.3788/IRLA201645.1003001 [8] QIAN X D, LI Zh R, LIANG X. Synchronization control of MOPA excimer laser system [J].Laser Technology, 2015, 39(2): 233-236(in Chinese). [9] FUJITA E, MASHIKO Y, ASAYA S, et al. High power narrow-linewidth linearly-polarized 1610nm Er:Yb all-fiber MOPA[J]. Optics Express, 2016, 24(23):26255. doi: 10.1364/OE.24.026255 [10] VARONA O, STEINKE M, NEUMANN J, et al. All-fiber, single-frequency, and single-mode Er3+:Yb3+, fiber amplifier at 1556nm core-pumped at 1018nm[J]. Optics Letters, 2018, 43(11):2632-2635. doi: 10.1364/OL.43.002632 [11] FUJITA E, MASHIKO Y, ASAYA S, et al. High power narrow-linewidth linearly-polarized 1610nm Er:Yb all-fiber MOPA[J]. Optics Express, 2016, 24(23):26255. doi: 10.1364/OE.24.026255 [12] GE X, YU J, LIU W, et al. High-power all-fiber 1.0/1.5μm dual-band pulsed MOPA source[J]. Chinese Optics Letters, 2018, 16(2):020010. doi: 10.3788/COL201816.020010 [13] XUE J W, FANG Y J, AN H B, et al. The design of F-P scanning interferometer based on the STC singlechip [J]. Optics & Optoelectronic Technology, 2014, 12(5):8-12(in Chinese). [14] LI X, HANG Zh G, YING L, et al. Research on power alignment technology of large mode field double cladding fiber fusion[J]. Laser Technology, 2017, 41 (3): 337-341(in Chinese). [15] YAMG L, ZHENG J J, HAO L Y, et al. Influence of signal spectral width characteristic on SBS threshold of single frequency fiber amplifier[J]. Chinese Journal of Lasers, 2017, 44(9): 901009(in Ch-inese). doi: 10.3788/CJL201744.0901009 [16] DRAGIC P D. Narrow linewidth fiber laser systems via Brillouin-tailored optical fiber[J]. Proceedings of the SPIE, 2016, 7323:73230W. [17] ZHOU Z Ch, WANG X L, SU R T, et al. Theoretical study on SBS effect suppression of gradient doping gain fibers[J]. Laser & Opto-electronics Progress, 2016, 53(7): 70604(in Chinese). [18] SINCORE A, BODNAR N, BRADFORD J, et al. SBS threshold dependence on pulse duration in a 2053nm single-mode fiber amplifier[J]. Journal of Lightwave Technology, 2017, 35(18):4000-4003. doi: 10.1109/JLT.2017.2729508 [19] WANG X, YANG Y, LIU M, et al. Frequency spacing switchable multiwavelength Brillouin erbium fiber laser utilizing cascaded Brillouin gain fibers[J]. Applied Optics, 2016, 55(23):6475-6479. doi: 10.1364/AO.55.006475 [20] XUE J W, LI K, FANG Y J, et al. High efficiency single frequency ring laser based on magnetic optical rotation glass[J]. Optoelectro-nic Technology, 2016, 36(2):122-125(in Chinese).