-
纳秒脉冲光纤振荡器的实现主要依靠调Q技术和增益开关技术[15-18]。调Q是通过调节谐振腔的损耗来获得激光脉冲的技术,包括主动调Q和被动调Q。主动调Q技术是在谐振腔内通过主动元器件(如电光调制器EOM、声光调制器AOM)来引入损耗而实现Q值调节;被动调Q技术则采用被动元件(如可饱和吸收体)来对Q值进行调制[19]。增益开关是一种利用足够短的泵浦脉冲来捕捉激光驰豫振荡尖峰,从而实现脉冲输出的技术;当泵浦脉冲具有固定的重频和脉宽,激光器就可以稳定输出脉冲[20-21]。
主动调Q是一种较为成熟的纳秒脉冲实现技术,具有脉宽和重复频率可调的优点。基于主动调Q技术,掺镱光纤纳秒脉冲激光在其典型波长1064 nm、强吸收强发射波长976 nm,以及其峰值拉曼增益波长1120 nm都得到了发展。在长波长方面,2013年国防科技大学Wang J H等人采用AOM搭建了1120 nm全光纤单模主动调Q激光器,其结构如图1所示,实现了重频1 kHz、平均功率111 mW、脉宽140 ns的激光输出,此时1060 nm 处ASE低于信号峰40 dB;重频为10 kHz时,脉宽为181 ns、平均功率达291 mW[22]。在短波长方面,2014年天津大学Fang Q等人搭建了单频主动调Q全光纤978 nm激光器:高反光栅和部分反射光栅分别刻在非保偏和保偏的石英光纤上,利用一个3 mm长的压电元件在腔内周期性地压缩掺镱石英光纤来产生应力双折射以改变腔内光的偏振状态,实现对激光腔Q因子的调制。该结构可以产生脉冲宽度数十纳秒到数百纳秒、重复频率20~800 kHz的单频光纤激光脉冲[23]。此外,2019年中国科学院安徽光学精密机械研究所Yao B等人搭建了主动调Q环形腔全光纤激光器,其结构及输出特性如图2所示。该团队采用双包层掺镱光纤同时作为包层光剥离器(CPS)输入端光纤和增益光纤(纤芯/包层直径10/130 μm)来减少增益光纤与CPS之间的熔点,从而减弱反向ASE;此外,通过将一段长度优化的单包层高掺杂镱光纤熔接到合束器输入端,并利用反向传输的ASE进行自泵浦,避免了ASE的增益自饱和,放大了窄带光栅反馈的初始弱信号,最终获得中心波长1064.5 nm、重频175 kHz、脉宽9 ns、平均功率1.3 W的激光脉冲[24],该脉冲宽度是调研到的主动调Q技术获得的最窄脉冲。
腔内主动调Q技术的脉冲宽度通常在几十到几百纳秒,但很难实现脉宽<50 ns的激光脉冲,研究人员将AOM或EOM置于谐振腔外,通过对连续激光进行调制斩波实现较窄的脉冲输出,EOM斩波可实现脉宽<10 ns的输出。近年来,国防科技大学对于纳秒光纤激光的研究就主要采用斩波技术来实现窄脉冲[25-26],2018年该单位Huang L等人利用电光强度调制器(EOIM)对基于超短腔的连续线偏振单频激光进行调制(结构如图3所示),产生了脉宽4 ns、重频10 MHz、平均功率0.48 mW的纳秒脉冲,进一步采用纤芯/包层直径6/125 μm单模双包层保偏掺镱光纤进行预放大,并使用AOM滤除脉冲之间的泄漏光来提高消光比,得到了2 mW的平均功率[27]。通过连续激光斩波技术可以实现短脉冲宽度,但其平均功率较低,需要额外的预放大级来实现纳秒脉冲平均功率的提升。
被动调Q技术可采用半导体可饱和吸收镜、石墨烯、掺杂稀土元素光纤等作为可饱和吸收体,实现结构紧凑的全光纤激光器。可饱和吸收镜、石墨烯等可饱和吸收体材料损伤阈值较低,不利于高功率的纳秒脉冲输出[28],掺杂稀土元素光纤作为可饱和吸收体可以有效提高被动调Q光纤激光器的输出功率,是近年来被动调Q技术的研究热点,其掺杂元素主要包括:Yb[29],Sm[30],Bi[31],Cr[32],Ho[33]。2011年,美国桑迪亚国家实验室Soh D B S等人数值模拟了图4的被动调Q全光纤激光器,其增益介质为纤芯/包层直径20/125 μm掺镱光纤,可饱和吸收体为绝热锥向非泵浦的单模掺镱光纤(纤芯直径5 μm),通过这种模场失配实现可饱和吸收光纤的快速漂白,模拟得到脉冲宽度14 ns、脉冲能量0.5 mJ、重复频率200 kHz的激光脉冲[34]。2014年,加拿大瑞尔森大学Lu Y等人利用纤芯/包层直径为7/128 μm的掺镱可饱和吸收光纤,纤芯/包层直径为15/130 μm的掺镱增益光纤搭建被动调Q激光器,实现了脉宽140 ns、重频100 kHz、平均功率14 W的脉冲输出[35],是当时采用模场失配法结合镱饱和吸收光纤实现的最高功率。该单位在2015年搭建图5的单模-多模-单模(SMS)被动调Q掺镱光纤激光器,抑制了自相位调制(SPM)和受激拉曼散射效应(SRS),实现了重频100 kHz、平均功率9.15 W、脉宽100 ns的脉冲输出[36],该SMS结构的线宽小于传统结构的1/3,其SRS信噪比也是传统结构的2倍。
可饱和吸收光纤结合模场失配法实现被动调Q的方案在大功率输出时,会造成熔接锥区的高温。为突破熔接锥区高温引起的功率限制,研究人员提出了基于光纤可饱和吸收体的双谐振腔,在实现被动调制时进行多次波长变换,结合被动调Q及增益开关特点来输出稳定纳秒脉冲[37]。2013年,北京工业大学Jin D C等人搭建双腔结构,如图6所示:通过泵浦外腔增益光纤产生的1040 nm激光,经内腔未泵浦的掺镱光纤调制实现被动调Q,由于外腔采用两个高反光纤布拉格光栅,外腔调Q脉冲在腔内不断振荡并泵浦内腔掺镱光纤,实现内腔1081 nm的激光脉冲输出,内腔脉冲输出后经过外腔增益光纤又得以放大,可获得高功率激光脉冲;在泵浦功率4.8 W时,可输出重频30 kHz、最大输出功率1.8 W,脉宽45 ns的激光脉冲[38],该脉宽是调研到的掺镱全光纤被动调Q激光输出的最窄脉宽。2015年,该单位将内腔和外腔掺镱光纤都扩展为纤芯/包层直径10/130 μm,实现了平均功率21 W、脉宽49 ns、重频113.6 kHz脉冲输出,此为目前全光纤掺镱激光被动调Q技术的最高平均功率;纤芯/包层直径均为20/130 μm时,单脉冲能量可进一步扩展到484 μJ[39],为调研所知的全光纤纳秒脉冲振荡器最高能量。
除了调Q技术外,通过对泵浦脉冲进行快速调制的增益开关技术也可实现全光纤纳秒脉冲输出。2019年,斯洛文尼亚卢布尔雅那大学Agrez V等人将增益开关技术与全光纤主振荡器和泵浦-恢复放大器结构相结合,即通过在主振荡器后增加一段额外的增益光纤来吸收残余泵浦并放大种子激光,解决了增益开关模式下产生短脉宽需要短振荡腔长造成的效率降低问题,其结构如图7所示。此外,他们利用掺镱光纤的窄带吸收峰在不同泵浦波长下吸收系数的变化来实现脉冲宽度调谐,获得了脉冲随需应变、脉宽精确调谐的高适应性光纤激光器。通过泵浦-恢复放大器结构,他们将效率从无该结构下的10%~30%提高到了65%~75%;在保持峰值功率约1 kW时,其重频在1 kHz~1 MHz可调;重频1 MHz时,可输出波长1030 nm、平均功率30 W、脉宽37.5 ns的激光脉冲[40]。据我们所知,该平均功率是目前全光纤纳秒脉冲激光振荡器可实现的最高功率输出。
表1展示了近年来全光纤纳秒脉冲振荡器的发展情况,其脉宽在几十纳秒到几百纳秒都得到了发展,平均输出功率可达瓦级,峰值功率可达千瓦级,为后续高功率大能量全光纤激光放大器的发展提供了坚实的基础。调Q技术是目前全光纤种子源中使用较为广泛的技术,主动调Q可以实现脉宽和重复频率的调节,但需要在腔内插入声光或电光器件,或利用额外机电器件对光栅进行应变调谐,使得结构较为复杂,损耗较高;由于多纵模振荡及自脉冲效应引入的噪声、Q开关快速切换引入的微扰等复杂机制影响,脉冲会出现多峰毛刺等现象[41]。此外,利用AOM或EOM对连续光进行斩波,也可实现纳秒脉冲输出,特别是利用EOM斩波,能实现<10 ns的纳秒脉冲,但其输出功率较低,需要进行预放大才可作为高功率高能量放大器的种子源。被动调Q技术具有结构更为紧凑等优点,但其重频和脉宽难以主动控制,受传统可饱和吸收体的影响,其输出功率较低,而且稳定性不够高;近年来,基于掺杂稀土元素增益光纤较高的损伤阈值,将其作为可饱和吸收体,可实现平均功率20 W级的输出。增益开关技术可实现脉宽和重频可调的窄脉冲输出,但其平均功率较低,新发展的利用额外增益光纤吸收残余泵浦来对种子进行放大可实现30 W平均功率输出。
表 1 全光纤纳秒激光振荡器研究进展
Table 1. Research progress of all-fiber nanosecond laser oscillator
Measure Year[Ref.] Average power Pulse duration Repetition rate Pulse energy Peak power Active Q-switching 2013[22] 111.0 mW 140 ns 1 kHz 111.0 μJ 0.8 kW 291.0 mW 181 ns 10 kHz 29.1 μJ 0.2 kW 2014[23] 11.3 mW 40 ns 100 kHz 113.0 nJ 2.8 W 2019[24] 1.3 W 9 ns 175 kHz 7.4 μJ 0.8 kW Passive Q-switching 2010[42] 9.9 mW 430 ns 9 kHz 1.1 μJ 2.6 W 2011[28] 12.0 mW 70 ns 257 kHz 46.0 nJ 0.7 W 2013[38] 1.8 W 45 ns 30 kHz 62.0 μJ 1.4 kW 2014[35] 14.0 W 140 ns 100 kHz 141.0 μJ 1.0 kW 2015[36] 9.2 W 100 ns 100 kHz 92.0 μJ 0.9 kW 2015[39] 6.0 W 143 ns 12 kHz 484.0 μJ 3.4 kW 21.0 W 49 ns 114 kHz 187.0 μJ 3.8 kW Gain-switching 2019[40] 30.0 W 38 ns 1 MHz 30.0 μJ 0.8 kW -
全光纤纳秒脉冲振荡器产生的脉冲能量及平均功率较低,无法应用于工业加工等领域,因此需要采用额外光路进行放大。主振荡功率放大技术(MOPA)是实现全光纤纳秒脉冲高功率、大能量输出的主要手段(结构如图8(a)所示),但是其平均功率或能量的进一步扩展都受限于SBS、SRS等非线性效应:SBS效应产生的Stokes光沿光纤反向传播,会造成激光振荡器或放大器稳定性的降低,甚至破坏光纤及器件,从而限制激光功率及能量的进一步提升[43];SRS效应会将信号激光能量大幅度转移到频率下移的Stokes光中,使信号光功率及转换效率降低,而拉曼频移引起的量子亏损会造成严重的热效应,导致光纤及器件烧毁[44]。尽管MOPA技术通过逐级增加增益光纤的有效模场面积可在一定程度上抑制非线性效应,但由于量子亏损及光子暗化产热与大模场面积光纤支持的多个模式共同作用引发TMI,使基模能量周期性的耦合到高阶模中,泄漏到包层的高阶模被CPS滤除造成信号光的能量损失,而纤芯中遗留的高阶模也会破坏光束质量[45-48]。因此,该部分主要概述了通过对这些非线性效应及热效应的抑制而实现大能量、高平均功率全光纤纳秒脉冲放大器的技术方案。
图 8 低重频纳秒脉冲放大器及同步脉冲泵浦技术示意图。(a)全光纤MOPA结构图;(b)不同放大级泵浦脉冲的时间序列
Figure 8. Low repetition rate nanosecond pulse amplifier and its synchronous pulse pumping technology. (a) Schematic diagram of all-fiber MOPA structure; (b) Time series of pump pulses at different amplification stages
大脉冲能量的全光纤纳秒脉冲放大器可以通过两类方案实现,一是利用连续泵浦光实现重频10 kHz级的激光脉冲,连续光泵浦偏向于重复频率较高的放大器,而在Hz级的低重频放大器中,由于脉冲提取效率较低会引起放大的自发辐射(ASE),因此重频几十Hz级的脉冲放大器常通过同步脉冲泵浦技术实现(如图8(b)所示)。此外,大脉冲能量的纳秒光纤放大器多在主放大级前使用空间光路来滤除ASE,而全光纤结构的大脉冲能量纳秒激光研究较少。2014年,天津大学Fang Q等人搭建MOPA光纤激光器,主放大采用0.68 m纤芯/包层直径50/400 μm双包层掺镱光纤作为增益纤,获得了单脉冲能量2.3 mJ、脉宽3 ns、峰值功率697 kW的脉冲输出[49]。据笔者所知,该团队实现了50 μm芯径增益光纤输出的最高峰值功率。2013年,清华大学Zheng C等人使用超辐射发光二极管(SLD)作为宽谱种子源来搭建六级放大系统,所有放大级都采用同步脉冲泵浦的方法来抑制ASE,获得了脉宽12 ns、重频20 Hz、单脉冲能量31 mJ、谱宽10 nm的1063 nm脉冲输出[50]。2018年,该课题组再次将SLD种子进行6级全光纤放大实现了30 mJ的单脉冲能量,最后采用空间滤波片滤除ASE后将信号光耦合到纤芯/包层直径200/600 μm超大模场面积(XLMA)光纤中,并结合低重频同步脉冲泵浦技术来抑制ASE和解决热问题,获得了峰值功率8 MW,单脉冲能量100 mJ,脉宽10 ns,重频10 Hz的纳秒脉冲输出[51],是国内在纳秒光纤激光方面报道的最高单脉冲能量和峰值功率。
高平均功率的全光纤纳秒脉冲放大器常使用MHz量级的重复频率来降低脉冲峰值功率,以避免非线性效应。北京工业大学、国防科技大学等单位都进行了相应的研究,特别是国防科技大学利用EOM对连续单频激光调制斩波得到脉宽<10 ns纳秒种子源,采用MOPA结构实现了窄线宽全光纤放大器在高平均功率方面的诸多突破。2013年,北京工业大学Chi J J等人采用非线性偏振旋转被动锁模的掺镱单模光纤激光器作为种子源,经过放大实现了重频26.3 MHz、平均功率120 W、脉宽0.62 ns的亚纳秒脉冲输出[52]。国防科技大学主要从非保偏和保偏两方面对全光纤纳秒脉冲放大器进行了研究。在非保偏方面:2012年,该单位Su R T等人通过EOM调制连续波得到纳秒脉冲种子,搭建MOPA系统实现了重频10 MHz,平均功率505 W、脉宽6 ns的脉冲输出[53];2014年,该团队通过缩短信号激光与斯托克斯光相互作用长度来抑制SBS,其结构及输出特性如图9所示,最终实现了重频10 MHz、平均功率913 W、脉宽3 ns的激光脉冲[54],其平均功率是目前脉宽<10 ns的最高单纤功率。在保偏方面:2015年,他们采用强度调制联合相位调制技术抑制SBS,通过电光强度调制使放大信号脉宽小于SBS产生所需的声子寿命,结合电光相位调制展宽光谱线宽,使光信号能量分布在更多频率中来降低光功率谱密度,进一步提高了SBS阈值,最终获得了重频20 MHz、平均功率293 W、脉宽3.5 ns的保偏激光输出[55];2018年,他们搭建了如图10(a)的全光纤线偏振窄线宽纳秒脉冲放大器,利用大模场直径短增益光纤结合小于声子寿命的窄脉冲宽度来抑制SBS,实现了重频10 MHz、平均功率466 W,脉宽4 ns的激光输出[27],该平均功率是脉宽<10 ns量级的最高保偏功率输出。此外,该课题组还研究了亚纳秒量级的全光纤保偏激光系统:2015年,Ma P F等人使用窄带线偏振增益开关半导体激光器作为种子源进行放大,其结构及输出特性如图11所示,通过选择915 nm泵浦波长代替976 nm波长来降低光纤产热,使受限于TMI的平均功率从380 W提升到了608 W,其重频为10 MHz,脉宽为0.81 ns[56],该平均功率是目前报道的亚纳秒级全光纤激光器最高输出。
图 11 全光纤亚纳秒激光放大器及输出特性。(a)结构示意图;(b) 976 nm泵浦下光束质量变化;(c) 915 nm泵浦下光束质量变化;光束质量的改善表明TMI得到抑制[56]
Figure 11. All-fiber sub-nanosecond laser amplifier and its output characteristics. (a) Schematic of structure; (b) Changes of beam quality at 976 nm pumping wavelength; (c) Changes of beam quality at 915 nm pumping wavelength. The improvement of beam quality shows that TMI is suppressed[56]
由于工业加工等领域的需求,国内外关于全光纤纳秒脉冲激光器的研究更倾向高平均功率和大脉冲能量的协同发展。国外,2013年,英国SPI公司Malinowski A等人使用直接调制半导体激光器作为种子源搭建MOPA系统,通过利用AOM对预放大脉冲前沿进行整形来防止脉冲迅速压缩,实现了平均功率265 W、单脉冲能量10.6 mJ、脉宽500 ns的脉冲输出[57];2014年,德国Rofin公司Dinger R等人采用粉末烧结法制备芯径300 μm掺镱光纤,实现了平均功率400 W、单脉冲能量40 mJ、脉宽12 ns的激光输出[58]。该公司在2017年再次利用粉末烧结法制备了超大模场面积镱/铈共掺光纤作为主放大级增益光纤,通过使用种子预整形技术和超大模场面积光纤实现了平均功率1500 W、单脉冲能量150 mJ、脉宽90 ns以及平均功率1150 W、单脉冲能量115 mJ、脉宽30 ns的输出[59]。尽管Rofin公司的高指标纳秒脉冲激光放大器内部结构并不透明,但考虑300 μm光纤芯径非线性效应阈值非常高,文中暂将其处理为全光纤结构,因此其平均功率和单脉冲能量是调研到的纳秒量级全光纤激光放大器的最高功率和能量输出。
国内,中国工程物理研究院、北京航天控制仪器研究所、华中科技大学等单位均研究了同时具备高平均功率、大脉冲能量的全光纤纳秒激光放大器。2016年,中国工程物理研究院Li Z B等人发现较小的增益光纤盘绕直径具有更高的斜效率,他们认为这是因为小盘绕直径使得高阶模损耗增加,并使基模比高阶模具有更高的重叠因子所致,在主放大增益纤为纤芯/包层直径48/400 μm双包层掺镱光纤时,实现了平均功率188 W、脉冲能量4.5 mJ、脉宽101 ns的激光输出[60]。2018年,北京航天控制仪器研究所Li P等人采用双向泵浦技术缓解了单向泵浦引入的ASE和热量集中的问题,研制了平均功率302 W、单脉冲能量3 mJ、脉宽203 ns的纳秒脉冲激光器(见图12),并用于除锈效果探究[61]。2019年,华中科技大学Wang S J等人采用自主研发的纤芯/包层直径100/400 μm掺镱光纤作为主放大光纤,获得了60 kHz重频下平均功率761 W、单脉冲能量12.6 mJ、脉宽350 ns的脉冲输出,但功率扩展受限于光纤的高温;在重频30 kHz下,实现了平均功率526 W、单脉冲能量17.5 mJ、脉宽210 ns的激光输出,但是功率扩展受限于1030 nm的寄生振荡[62]。该团队在2021年将主放大级泵浦波长由975 nm改为915 nm,对寄生振荡和光纤温度进行了改善,其结构如图13所示,实现了平均功率1000 W、单脉冲能量16.7 mJ、脉宽260 ns的激光输出[63],该平均功率是目前国内实现的纳秒量级全光纤激光放大器的最高指标。
表2展示了近十年来全光纤纳秒激光放大器的研究进展。随着高功率半导体激光器和大模场、超大模场面积光纤的发展,全光纤纳秒激光放大器已经实现了单纤百毫焦级、平均功率千瓦级的输出,在峰值功率上更是出现了单纤兆瓦级输出,脉宽范围也跨越了亚纳秒到百纳秒量级。但这些激光器中高平均功率输出常常伴随较低的单脉冲能量或较低的峰值功率,大脉冲能量的输出常常伴随着较低的平均功率,难以实现集高平均功率、大脉冲能量和高峰值功率一体的高指标输出,而激光清洗这类应用常需要高平均功率来提高清洗效率,又需要较大的脉冲能量来达到加工阈值,因此发展同时具备高功率、大能量的纳秒激光脉冲具有实际的应用价值。SRS等非线性效应及TMI等热效应是限制集高平均功率、大脉冲能量、高峰值功率一体的纳秒全光纤激光放大器发展的主要因素。在抑制SRS等非线性效应的技术层面,最直接的方式是通过使用短的大芯径高掺杂增益光纤,但较大的模场面积又将导致TMI的出现。因此,针对这些非线性效应及热效应的抑制发展了更为独特的手段:(1)利用光谱选择性光纤(如特殊设计的3 C光纤或W型光纤)滤除SRS产生的斯托克斯光,以及利用长周期光纤光栅(LPFG)或啁啾倾斜光纤光栅(CTFBG)等滤波器件对斯托克斯光形成高损耗,来实现SRS抑制[64-67];(2)通过声光剪裁或特种光纤(如锥形光纤)改变布里渊频移来降低布里渊增益峰值,以及采用相位调制技术展宽种子光线宽等方式实现SBS效应的抑制[68-71];(3)通过声光偏转器或光子灯笼等器件改变种子光中的模式激发,以及采用特种光纤(3 C光纤,大间距微结构光纤等)保证单模激光运行等技术手段来抑制TMI[72-75]。然而,这些抑制技术研究大多在连续光纤激光中,在纳秒脉冲方向的可行性仍有待考量。
表 2 全光纤纳秒脉冲激光放大器研究进展
Table 2. Research progress of all-fiber nanosecond pulse laser amplifier
Year[Ref.] Active fiber Average power Pulse duration Repetition rate Pulse energy Peak power 2010[76] Dcore=20 μm 21.07 W 100 ns 200 kHz 0.1 mJ 1 kW 2012[77] Dcore=30 μm 300.8 W 8 ns 10 MHz 30 μJ 3.75 kW 2012[53] Dcore=30 μm 505 W 6 ns 10 MHz 50.5 μJ 7.9 kW 2013[50] Dcore=200 μm 0.62 W 12 ns 20 Hz 31 mJ 2.58 MW 2013[78] Dcore=30 μm 102 W 14.9 ns 100 kHz 1.02 mJ 68 kW 2013[57] Dcore=50 μm 265 W 500 ns 25 kHz 10.6 mJ 21.2 kW 2014[79] Dcore=30 μm 25.3 W 0.223 ns 100 MHz 0.253 μJ 1.13 kW 2014[54] Dcore=30 μm 913 W 3 ns 10 MHz 91.3 μJ 28.6 kW 2014[58] Dcore=300 μm 400 W 12 ns 10 kHz 40 mJ 3.5 MW 2014[49] Dcore=50 μm 23 W 3 ns 10 kHz 2.3 mJ 697 kW 2014[52] Dcore=30 μm 120 W 0.62 ns 26.3 MHz 4.56 μJ 7.35 kW 2015[55] — 293 W 3.5 ns 20 MHz 14.65 μJ 3.9 kW 2015[56] Dcore=30 μm 608 W 0.81 ns 10 MHz 60.8 μJ 128 kW 2016[60] Dcore=20 μm 188 W 101 ns 40 kHz 4.5 mJ 46.5 kW 2017[59] — 1500 W 90 ns 10 kHz 150 mJ 1.7 MW 1150 W 30 ns 10 kHz 115 mJ 3.5 MW 2018[61] Dcore=30 μm 302 W 203 ns 100 kHz 3 mJ 15 kW 2018[80] Dcore=25 μm 189 W 250 ns 200 kHz 0.95 mJ 3.8 kW 2018[27] Dcore=30 μm 466 W 4 ns 10 MHz 46.6 μJ 8.8 kW 2019[62] Dcore=100 μm 526 W 150 ns 30 kHz 17.5 mJ 116 kW 761 W 280 ns 60 kHz 12.6 mJ 45 kW 2021[63] Dcore=100 μm 1000 W 260 ns 60 kHz 16.7 mJ 64 kW
Recent advances in nanosecond-pulsed Ytterbium-doped all-fiber lasers
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摘要: 掺镱全光纤纳秒脉冲激光器发展迅猛,已经为诸多领域开辟了新的道路,特别是高平均功率、大脉冲能量的纳秒脉冲光纤激光器在激光清洗等领域得到了广泛应用。多路光纤激光合束是实现高平均功率、大脉冲能量激光输出的主要手段,其结构复杂程度取决于单模块激光器的输出特性,提升单模块纳秒脉冲全光纤激光器输出特性对于激光清洗等领域具有重要意义。文中总结了单模块掺镱全光纤纳秒脉冲激光器的研究进展,分析了当前限制其功率和能量进一步提升的主要因素。首先,从主动调Q、被动调Q以及增益开关技术三个层面回顾了纳秒脉冲掺镱全光纤振荡器的研究进展;其次,从大脉冲能量、高平均功率、两者协同发展三个指标层面总结了纳秒脉冲掺镱全光纤放大器的研究现状;最后,从限制高指标掺镱全光纤激光器输出特性的因素出发,展望了其在未来功率和能量提升上的发展趋势。Abstract: Ytterbium-doped all-fiber nanosecond-pulsed lasers have developed rapidly in recent years and have opened up new horizons in many fields. Especially in the field of laser cleaning, there is a strong demand for nanosecond pulsed fiber laser with high-power and high-energy. Multi-channel fiber laser beam combining is the main means to achieve high-power and high-energy laser output. The complexity of the structure depends on the output characteristics of a monolithic laser. Improving the output characteristics of monolithic nanosecond pulsed all-fiber laser is very important for laser cleaning and other applications. In this paper, the research progress of monolithic Ytterbium-doped all-fiber nanosecond pulsed lasers is summarized, and the main factors that limit its power and energy expansion are analyzed. Firstly, the recent advances of nanosecond pulsed Ytterbium-doped all-fiber oscillator is reviewed with active Q-switching, passive Q-switching and gain-switching technology. Then, the research status of nanosecond pulse Ytterbium-doped all-fiber amplifiers is summarized with large pulse energy, high average power and collaborative development of the two. In the end, the development trend of Ytterbium-doped all-fiber laser in scaling of power and energy is prospected from the factors limiting the output characteristics.
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图 11 全光纤亚纳秒激光放大器及输出特性。(a)结构示意图;(b) 976 nm泵浦下光束质量变化;(c) 915 nm泵浦下光束质量变化;光束质量的改善表明TMI得到抑制[56]
Figure 11. All-fiber sub-nanosecond laser amplifier and its output characteristics. (a) Schematic of structure; (b) Changes of beam quality at 976 nm pumping wavelength; (c) Changes of beam quality at 915 nm pumping wavelength. The improvement of beam quality shows that TMI is suppressed[56]
表 1 全光纤纳秒激光振荡器研究进展
Table 1. Research progress of all-fiber nanosecond laser oscillator
Measure Year[Ref.] Average power Pulse duration Repetition rate Pulse energy Peak power Active Q-switching 2013[22] 111.0 mW 140 ns 1 kHz 111.0 μJ 0.8 kW 291.0 mW 181 ns 10 kHz 29.1 μJ 0.2 kW 2014[23] 11.3 mW 40 ns 100 kHz 113.0 nJ 2.8 W 2019[24] 1.3 W 9 ns 175 kHz 7.4 μJ 0.8 kW Passive Q-switching 2010[42] 9.9 mW 430 ns 9 kHz 1.1 μJ 2.6 W 2011[28] 12.0 mW 70 ns 257 kHz 46.0 nJ 0.7 W 2013[38] 1.8 W 45 ns 30 kHz 62.0 μJ 1.4 kW 2014[35] 14.0 W 140 ns 100 kHz 141.0 μJ 1.0 kW 2015[36] 9.2 W 100 ns 100 kHz 92.0 μJ 0.9 kW 2015[39] 6.0 W 143 ns 12 kHz 484.0 μJ 3.4 kW 21.0 W 49 ns 114 kHz 187.0 μJ 3.8 kW Gain-switching 2019[40] 30.0 W 38 ns 1 MHz 30.0 μJ 0.8 kW 表 2 全光纤纳秒脉冲激光放大器研究进展
Table 2. Research progress of all-fiber nanosecond pulse laser amplifier
Year[Ref.] Active fiber Average power Pulse duration Repetition rate Pulse energy Peak power 2010[76] Dcore=20 μm 21.07 W 100 ns 200 kHz 0.1 mJ 1 kW 2012[77] Dcore=30 μm 300.8 W 8 ns 10 MHz 30 μJ 3.75 kW 2012[53] Dcore=30 μm 505 W 6 ns 10 MHz 50.5 μJ 7.9 kW 2013[50] Dcore=200 μm 0.62 W 12 ns 20 Hz 31 mJ 2.58 MW 2013[78] Dcore=30 μm 102 W 14.9 ns 100 kHz 1.02 mJ 68 kW 2013[57] Dcore=50 μm 265 W 500 ns 25 kHz 10.6 mJ 21.2 kW 2014[79] Dcore=30 μm 25.3 W 0.223 ns 100 MHz 0.253 μJ 1.13 kW 2014[54] Dcore=30 μm 913 W 3 ns 10 MHz 91.3 μJ 28.6 kW 2014[58] Dcore=300 μm 400 W 12 ns 10 kHz 40 mJ 3.5 MW 2014[49] Dcore=50 μm 23 W 3 ns 10 kHz 2.3 mJ 697 kW 2014[52] Dcore=30 μm 120 W 0.62 ns 26.3 MHz 4.56 μJ 7.35 kW 2015[55] — 293 W 3.5 ns 20 MHz 14.65 μJ 3.9 kW 2015[56] Dcore=30 μm 608 W 0.81 ns 10 MHz 60.8 μJ 128 kW 2016[60] Dcore=20 μm 188 W 101 ns 40 kHz 4.5 mJ 46.5 kW 2017[59] — 1500 W 90 ns 10 kHz 150 mJ 1.7 MW 1150 W 30 ns 10 kHz 115 mJ 3.5 MW 2018[61] Dcore=30 μm 302 W 203 ns 100 kHz 3 mJ 15 kW 2018[80] Dcore=25 μm 189 W 250 ns 200 kHz 0.95 mJ 3.8 kW 2018[27] Dcore=30 μm 466 W 4 ns 10 MHz 46.6 μJ 8.8 kW 2019[62] Dcore=100 μm 526 W 150 ns 30 kHz 17.5 mJ 116 kW 761 W 280 ns 60 kHz 12.6 mJ 45 kW 2021[63] Dcore=100 μm 1000 W 260 ns 60 kHz 16.7 mJ 64 kW -
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