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1992年,Pinto等人首次利用主动声光调制技术在Tm: YAG振荡器中实现了2 μm波段35 ps锁模脉冲输出[5],开启了2 μm掺铥超短脉冲产生的研究热潮。但是,主动锁模受限于声光调制的响应时间和调制频率,只能得到皮秒量级超短脉冲,且调制器电路复杂,成本较高,所以主动锁模逐渐被被动锁模所替代。被动锁模基于可饱和吸收体的非线性光学可饱和吸收效应,具有效率高、响应快、操作方便、容易实现的优点,并且可饱和吸收体价格相对低廉,大大降低了生产成本,是近年来2 μm超短激光脉冲产生的主要方式。克尔透镜锁模由于具有亚飞秒超快非线性响应速度成为产生更短周期量级激光脉冲的主要锁模方式。目前,基于可饱和吸收体被动锁模是2 μm超短脉冲产生的主要技术,按照文献报道频率依次有半导体可饱和吸收镜(Semiconductor Saturable Absorber Mirror, SESAM)、碳纳米管(Carbon Nanotubes, CNTs)和氧化石墨烯(Graphene Oxide, GO)等。这里介绍几种常用锁模材料在2 μm掺铥全固态超短脉冲振荡器中的研究进展。
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2 μm波段超短脉冲振荡器最常用的可饱和吸收体是SESAM。SESAM被动锁模激光谐振腔结构简单、操作方便、运行稳定,可商业化推广,目前已广泛用来产生皮秒和飞秒激光。1992年,U. Keller 团队首次将 SESAM 应用于被动锁模振荡器中[6]。2010年,A. A. Lagatsky等人首次利用SESAM在Tm, Ho: NaY(WO4)2钨酸盐晶体中得到2 μm波段191 fs锁模脉冲,随后开启了利用SESAM实现2 μm锁模的研究高潮[7]。2012 年,上海交通大学马杰等人首次利用LD泵浦Tm: CLNGG无序晶体实现了479 fs锁模运转[8],其实验装置如图1所示,泵浦源为商用的790 nm 铟钾砷激光二极管,锁模器件为SESAM,实验结果如图2所示,Tm: CLNGG无序晶体振荡器输出脉宽为479 fs,重复频率为99 MHz,平均输出功率为288 mW。LD泵浦大大降低了掺铥振荡器的价额,为SESAM锁模的LD泵浦全固态商业化2 μm超短脉冲振荡器提供了可行的技术方案。
2017年,Zhou等人利用Tm: CYA 晶体作为增益介质结合SESAM锁模技术首次实现了瓦级的锁模振荡器的运转,平均输出功率高达1.35 W,是利用SESAM在块状晶体获得2 μm锁模激光的最高输出功率[9],该结果表明SESAM可支持高功率锁模运转。2021年,Wang等人通过SESAM锁模方式在Tm, Ho: CALO振荡器中获得了46 fs的激光[10],这也是目前利用SESAM锁模获得的最短脉冲。尽管SESAM在2 μm波段固体激光锁模中有着较为广泛的应用,但是SESAM制备工艺复杂,成本较高,带宽有限,从而限制了它的应用范围。表1概括了近五年来基于SESAM锁模的掺铥固体振荡器的研究进展。
表 1 基于SESAM锁模全固体掺铥振荡器总结
Table 1. Summary of all solid state Tm-doped oscillators mode-locked by SESAMs
Laser medium Average power/mW Pulse width/fs Ref. Tm: CYA 1350 49000 [9] Tm, Ho: CALO 121 46 [10] Tm: LuAG 98 13600 [11] Tm: YAP 730 1700 [12] Tm, Ho: CALYO 27 87 [13] Tm, Ho: CALGO 376 52 [14] Tm, Ho: CNGG 36 73 [15] Tm: LuYO3 133/51 59/54 [16] Tm: LuAG 1210 38000 [17] Tm: YLF 95 31000 [18] Tm: YAG 150 30000 [19] Tm: LuAG 232 2700 [20] Tm: YLF 165 94000 [21] Tm: CALGO 337 650 [22] Tm: (Lu, Sc)2O3 175/34 230/74 [23] -
碳纳米管是继SESAM后最早应用于锁模的新型可饱和吸收体材料,具有制作成本低、响应时间短、工作波段范围广的特点。根据管壁的层数,碳纳米管可以分为单壁碳纳米管(Single-Walled Carbon Nanotubes, SWCNTs)和多壁碳纳米管(Multi-Walled Carbon Nanotubes, MWCNTs)。相对于双壁碳纳米管,单壁碳纳米管的响应时间更快,恢复时间更短,调节波长更宽,应用更为广泛。2009年,Cho W B等人首次利用SWCNTs作为锁模启动元件,通过钛宝石振荡器泵浦Tm: KLu(WO4)2晶体,最终实现了输出功率为240 mW,脉宽为 9.7 ps的脉冲输出[24]。2012年,刘杰等人首次利用双壁碳纳米管在2 μm固体振荡器中实现锁模激光运转[25]。2019年,Zhao等人基于SWCNTs在Tm: LuYO3陶瓷振荡器获得2 μm波段的激光,脉宽为57 fs[26],这也是目前利用SWCNTs锁模获得的最短脉冲。但是单壁碳纳米管也有缺点,因其带宽受到其管径的限制,会在腔内造成相当大的散射损耗[27]。表2概括了近五年来基于碳纳米管锁模的掺铥固体振荡器的研究进展。
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石墨烯是二维纳米结构碳材料,其损伤阈值比碳纳米管高,同时因为电阻率低、电子迁移率高、支持高功率运转、恢复时间短和饱和强度低等优点[32],近年来在二维锁模材料中崭露头角。其吸收光谱覆盖500~2000 nm,所以石墨烯可以运用在大部分脉冲振荡器中。2011年,Liu 等人首次将氧化石墨烯GO作为可饱和吸收体在Tm: YAP振荡器中实现被动锁模[33]。2019年,笔者所在实验室首次采用Tm: LuAG在全固态振荡器中实现了瓦级调Q锁模,其中最高功率可达1740 mW[34]。2017年,Wang Y等人以GO作为可饱和吸收体在Tm: MgWO4振荡器得到锁模脉冲,脉冲宽度为86 fs,这是目前为止基于GO可饱和吸收体获得的最短脉冲[35]。目前,GO作为可饱和吸收体已经广泛用于各种固体振荡器中,但是由于其非线性光学响应弱,调制深度浅,并且在制备中尺寸难以控制,无法进行大规模量产,进而限制了其在商业方面的应用。表3总结了近五年来基于石墨烯锁模的全固体掺铥振荡器的研究进展。
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除以上锁模材料外,过渡金属硫化物、黑磷等材料近年来也用来产生2 μm超短脉冲。过渡金属硫化物具有损伤阈值高、非线性吸收特性好、宽带可调等优点,并且在近红外与中红外波段饱和特性优于石墨烯,弛豫时间在飞秒量级[38]。2014 年,Xu等人首次将过渡金属硫化物MoS2作为可饱和吸收体应用于超快固体振荡器[39]。2020 年,Li等人基于MoS2利用LD泵浦在 Tm: YAG晶体上首次实现被动锁模,其中最窄脉冲宽度为280 ps,最大平均输出功率为0.2 W[40]。过渡金属硫化物虽有很好的非线性特性,但制备较为复杂,限制了其应用。黑磷是一种具有恢复时间短、各向异性、大开关比和高承载流动性的纳米材料,与硫化物相比更容易得到。2015年,山东大学B. Zhang等人首次将黑磷作为可饱和吸收体应用于固体振荡器中[41]。2016年,Xian等人首次成功地将黑磷应用于固体振荡器产生1053 nm的飞秒脉冲,最高平均功率可达0.82 W,相应脉宽为272 fs[42]。目前,利用黑磷作为锁模材料获得2 μm波段超短脉冲的报道主要集中在掺Tm3+光纤振荡器中,关于黑磷锁模固体振荡器的报道甚少。这是由于黑磷在空气中容易被氧化,因此制备工艺还需要进一步改进。
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2 μm超短脉冲固体激光研究在最近几年取得了丰硕的成果,其中获得更短的激光脉冲是主流研究方向,尤其是产生周期量级锁模脉冲是更具挑战性的课题。目前,实现亚100 fs脉冲输出均采用可饱和吸收体的被动锁模技术,锁模启动元件一般采用SESAM、SWCNTs和GO三种宽带可饱和吸收体,输出锁模脉冲受限于这些吸收体的非线性响应时间,这些吸收体的非线性响应时间为几十飞秒到亚皮秒之间[43-44],因此基于可饱和吸收体的被动锁模技术限制了亚50 fs输出脉冲的产生,不再适合更短周期量级脉冲的产生。克尔透镜锁模非线性瞬态响应时间为亚飞秒(<1 fs),且不受光谱带宽限制,利用克尔透镜自锁模技术是周期量级脉冲产生的最佳方案。但要实现2 μm波段的克尔透镜锁模,面临几方面的挑战:(1) 克尔透镜效应的大小与ω−4成正比,其中ω是与激光波长平方根成正比的腔模半径。因此,波长越长,克尔效应越弱,相对于800 nm钛宝石和1030 nm掺镱振荡器克尔透镜锁模,2 μm波段锁模克尔效应要弱很多[45-46]。(2) 要实现自聚焦克尔透镜锁模运转,腔内功率必须大于自聚焦临界功率[47],自聚焦临界功率正比于波长[48],相对于钛宝石和掺镱振荡器克尔透镜锁模,2 μm激光腔内自聚焦临界功率高出近一倍。基于以上原因,目前2 μm波段仅有五例克尔透镜锁模报道,第一例是日本电子通讯大学激光科学研究所利用1.6 μm同带泵浦技术在Tm: Sc2O3中实现了克尔透镜锁模运转,输出最短脉冲为166 fs[45];第二例是土耳其科萨大学激光研究组在腔内插入二级非线性系数很高的ZnSe晶体实现克尔透镜锁模,输出脉冲宽度为514 fs[49]。第三例是马克斯-伯恩研究所借助SWCNTs辅助启动获得克尔透镜锁模,在Tm: MgWO4中实现了锁模运转,输出脉冲为76 fs[50]。第四例是日本电子通信大学激光科学研究所提出的一种基于Tm: Lu2O3陶瓷和Tm: Sc2O3单晶的组合增益介质克尔透镜锁模振荡器[51]。其实验装置图如图3所示。实验结果如图4所示。在波长为2.1 μm的情况下,获得了41 fs的脉冲输出,平均输出功率为42 mW,对应的脉冲宽度不到6个光学周期,这也是目前获得的2 μm波段的最短脉冲。第五例是最近德国Max-Born非线性光学研究所利用掺钛蓝宝石激光器为泵浦源在倍半氧化物陶瓷 Tm: (Lu, Sc)2O3激光器中首次实现克尔透镜锁模,输出脉冲宽度为58 fs,输出功率高达220 mW[52]。
截至目前为止,还没有用常规泵浦源790 LD直接泵浦实现克尔透镜锁模的报道,利用790 nm LD泵浦源通过纯克尔透镜锁模获得周期量级脉冲成了目前诱人的挑战和亟待解决的问题。
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激光介质的各种特性决定了锁模启动难易程度和更短脉冲的色散补偿。激光介质的输出光谱调谐越宽、光谱调制越小,越容易补偿色散,获得更短脉冲;介质二阶非线性系数大小决定克尔透镜锁模启动的难易程度,系数越大越是容易启动克尔透镜锁模运转;激光介质热导率越大,越支持更高功率的锁模运转。目前2 μm超短激光脉冲在多种掺铥和铥钬共掺的固体振荡器中已实现锁模运转,并且通过色散补偿技术致力于更短脉冲产生研究。现阶段寻找光谱特性和导热特性等更优异的基质成为激光增益材料研究的重要方向,平滑的宽带光谱特性支持更短的脉冲和更稳定的锁模运转,高导热系数支持更高的锁模输出功率。根据激光介质分类,笔者分别介绍各种掺铥和铥钬共掺增益介质的超短脉冲全固态振荡器最新研究现状。
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单晶是实现2 μm波段脉冲激光最常见的增益介质,由于其化学性质稳定、荧光谱线较窄,发光效率高容易实现大功率运转,被广泛用于连续和脉冲振荡器中,近五年来单晶研究进展如表4所示。在石榴石晶体中常见的是镥铝石榴石(LuAG)[11]和钇铝石榴石(YAG)[53],目前可实现的最短脉宽分别是13.6 ps和21.3 ps。在铝酸盐晶体中常见的介质是铝酸钇(YAP)[12,54]和铝酸钇钙(CaYAlO4, CALYO)[13],目前可实现的最短脉冲分别为1.7 ps和87 fs。在硅酸盐晶体中常见的介质是硅酸钇镥(LuYSiO5, LYSO)[55],输出的最短脉冲为19.6 ps。在钨酸盐类激光介质中钨酸钇钾(KY(WO4)2, KYW)[56]、钨酸镥钾(KLu(WO4)2, KLuW)[57]、钨酸钇钠(NaY(WO4)2, NYW)[7]、钨酸镁(MgWO4)[50]较为常见,目前达到的最短脉冲分别为:549 fs、141 fs、191 fs和76 fs。无序晶体介质输出光谱均比较平滑易调制,色散补偿容易获得更短激光脉冲,其中铝酸钆钙(CALGO)[14]、钙锂铌镓晶体(CLNGG)[28]、钙铌镓晶体(CNGG)[15]较为常见,输出最短脉冲分别为52 fs、67 fs和73 fs。氟化物晶体中常见的晶体主要包括氟化钇锂(LiYF4, YLF)、氟化镥锂(LiLuF4, LLF)。2018年,笔者所在实验室首次在 LLF晶体中实现2 μm锁模[58]。目前氟化物振荡器以Tm:YLF[49]、Tm, Ho:LiLuF4[59]、Tm:LLF[58]为代表,其中输出脉冲分别为514 fs、4.7 ps和14 ps。表4总结了近五年单晶介质超短脉冲掺铥振荡器输出指标。
表 4 单晶介质超短脉冲全固态掺铥振荡器总结
Table 4. Summary of ultrashort pulse all solid state Tm-doped oscillators with single crystal gain medium
Laser medium Mode-locked device Average power/
mWPulse width/
fsRef. Tm: CYA SESAM 1350 49000 [9] Tm, Ho: CALO SESAM 121 46 [10] Tm: LuAG SESAM 98 13600 [11] Tm: YAP SESAM 730 1700 [12] Tm, Ho: CALYO SESAM 27 87 [13] Tm, Ho: CALGO SESAM 376 52 [14] Tm, Ho: CNGG SESAM 36 73 [15] Tm: LuAG SESAM 1210 38000 [17] Tm: YLF SESAM 95 31000 [18] Tm: YLF SESAM 165 94000 [21] Tm: CALGO SESAM 337 650 [22] Tm, Ho: CLNGG SWCNTs 123 98 [28] Tm: CNNGG SWCNTs 22 84 [29] Tm, Ho: CNGG SWCNTs 67 76 [30] Tm: CLNGG SWCNTs 54 78 [31] Tm: MgW Graphene 39 86 [35] Tm: YAP Graphene 256 35000 [36] Tm: YAG MoS2 200 280000 [40] Tm: Sc2O3 KLM 440 166 [45] Tm: YLF KLM 14.4 514 [49] Tm: MgW KLM 100 89 [50] -
玻璃虽然热导率比激光晶体低,但是玻璃具有各向同性[60]、介稳性、熔点不固定、性质变化的可逆性和连续性[61]的特点,并且玻璃容易制备,容易加工,可调谐范围宽,在2 μm波段脉冲激光产生中具有明显的优势。1964年,干福熹等人在掺钕的硅酸盐玻璃中得到激光的输出[62]。但是硅酸盐玻璃声子能量太高使能级之间无辐射跃迁几率上升导致中红外光消失,所以硅酸盐玻璃不适合作为2 μm波段脉冲激光的增益介质[63]。研究表明:碲酸盐玻璃在所有氧化物中其声子能量最低,稀土粒子溶解能力强,因此碲酸盐玻璃基质是一种很有潜力的中红外激光基质材料[64]。2010年,F Fusariy等人在Tm: GPNG和Tm, Ho: TZN增益介质中实现锁模运转,脉冲宽度分别为410 fs和630 fs[65]。由于玻璃导热性差,易脆裂等特性限制了2 μm波段的锁模应用,研发适合固体振荡器的玻璃基质是亟待解决的问题。
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自1964年世界上第一台陶瓷振荡器问世[66]后,陶瓷便进入了大家的视野。激光陶瓷的光学性质和激光晶体的类似,但是陶瓷还具有制备工艺简单、容易获取、成本低和热导率高的优点,在2 μm波段固体超快激光中有着广阔的应用前景,近五年来2 μm波段陶瓷固体振荡器研究进展如表5所示。常见的激光陶瓷基质有YAG陶瓷[67]和氧化物陶瓷,氧化物陶瓷介质包括LuYO3[16]、Lu2O3[68]和倍半氧化物键合陶瓷Tm: (Lu, Sc)2O3[52]。其中YAG陶瓷最短脉冲输出为2.1 ps,氧化物陶瓷最短脉冲输出为54 fs、180 fs和58 fs。氧化物陶瓷介质具有优异的热传导性能和机械性能,并且1 980 nm以上具有十分宽的增益带宽,避免了水分子Q调制对锁模的影响,适合于高功率锁模运转,同时具有很高的二阶非线性系数(是YAG介质的2倍)[69],具有很强的克尔效应和自相位调制,是克尔透镜锁模最具潜力的首选材料。表5总结了各种掺铥陶瓷固体振荡器的最短脉冲输出指标。
表 5 各种陶瓷介质超短脉冲全固态掺铥振荡器总结
Table 5. Summary of ultrashort pulse all solid state Tm-doped oscillators with ceramic gain medium
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2 μm全固态超短脉冲掺铥振荡器发展的另一个趋势是更高的输出功率,尤其是瓦级输出功率。优异的光束质量和高的输出功率十分适合作为3~8 μm超短脉冲激光产生的泵浦源。2017年,Zhou等人基于SESAM锁模技术在Tm: CYA振荡器上首次实现了瓦级的锁模振荡器的运转,平均输出功率高达1.35 W [9]。实验所用Z型激光腔由四个反射镜构成,泵浦源为790 nm LD,实验装置如图5所示。
实验稳定的锁模波长可以从1 874~1 973 nm调谐,可调谐的波长范围可达100 nm,最大输出功率高达1.35 W。可调谐锁模Tm: CYA振荡器在不同输出镜下的输出功率如图6所示。
表6总结了最近几年输出功率大于500 mW全固态振荡器的具体指标。在以下基质中,无论单晶还是陶瓷介质,石榴石晶体(LuAG)和铝酸盐晶体(CYA)均具有高功率输出,主要是这两种晶体均具备大的增益发射截面和高的热导率,十分适合高功率输出。由于玻璃热导率不高,加上热膨胀时易于脆裂,所以不适合高功率超短激光脉冲输出,目前该类超短脉冲全固态振荡器最高输出功率均在100 mW以下。
表 6 功率高于500 mW的超短脉冲全固态掺铥振荡器总结
Table 6. Summary of ultra short pulse all solid state Tm-doped oscillators with output power higher than 500 mW
Research progress of 2 μm ultrashort pulse all solid state thulium doped oscillator (Invited)
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摘要: 2 μm激光处于水的吸收峰,对人眼安全而且处于大气窗口波段,在空间通讯、遥感探测、环境监测、激光制导、红外对抗、外科手术等领域具有重要的应用价值。随着各类掺铥和铥钬共掺激光介质的不断丰富及锁模技术不断发展,2 μm波段超短脉冲全固态振荡器成为最近几年激光技术的研究热点之一。文中系统分析了2 μm波段激光基质材料和锁模技术,概括了近年来国内外2 μm 超短脉冲全固态掺铥振荡器的最新进展,并对代表性实验进行了分析介绍,最后对2 μm波段超短脉冲全固态掺铥振荡器的发展前景做出总结与展望。Abstract: There is a absorption peak of water molecular and a transmission window of atmosphere in the 2 μm spectral range, which has important application in space communication, remote sensing detection, environmental monitoring, laser guidance, infrared countermeasure, surgical operation and so on. With the development of Tm-doped and Tm, Ho co-doped laser host materials and mode-locked technologies, ultrashort pulse all solid state oscillator at 2 μm has become one of the research hotspots of laser technology in recent years. In this paper, the Tm-doped laser host materials and the mode-locked technologies in the 2 μm spectral range were systematically analyzed, the latest development of ultrashort pulse all-solid-state Tm-doped oscillators at home and abroad was summarized, and the representative experiments were analyzed and introduced. Finally its development prospect was summaried.
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表 1 基于SESAM锁模全固体掺铥振荡器总结
Table 1. Summary of all solid state Tm-doped oscillators mode-locked by SESAMs
Laser medium Average power/mW Pulse width/fs Ref. Tm: CYA 1350 49000 [9] Tm, Ho: CALO 121 46 [10] Tm: LuAG 98 13600 [11] Tm: YAP 730 1700 [12] Tm, Ho: CALYO 27 87 [13] Tm, Ho: CALGO 376 52 [14] Tm, Ho: CNGG 36 73 [15] Tm: LuYO3 133/51 59/54 [16] Tm: LuAG 1210 38000 [17] Tm: YLF 95 31000 [18] Tm: YAG 150 30000 [19] Tm: LuAG 232 2700 [20] Tm: YLF 165 94000 [21] Tm: CALGO 337 650 [22] Tm: (Lu, Sc)2O3 175/34 230/74 [23] 表 2 基于CNTs锁模全固态掺铥振荡器总结
Table 2. Summary of all solid state Tm-doped oscillators mode-locked by CNTs
表 3 基于GO锁模全固体掺铥振荡器总结
Table 3. Summary of all solid state Tm-doped oscillators mode-locked by GO
表 4 单晶介质超短脉冲全固态掺铥振荡器总结
Table 4. Summary of ultrashort pulse all solid state Tm-doped oscillators with single crystal gain medium
Laser medium Mode-locked device Average power/
mWPulse width/
fsRef. Tm: CYA SESAM 1350 49000 [9] Tm, Ho: CALO SESAM 121 46 [10] Tm: LuAG SESAM 98 13600 [11] Tm: YAP SESAM 730 1700 [12] Tm, Ho: CALYO SESAM 27 87 [13] Tm, Ho: CALGO SESAM 376 52 [14] Tm, Ho: CNGG SESAM 36 73 [15] Tm: LuAG SESAM 1210 38000 [17] Tm: YLF SESAM 95 31000 [18] Tm: YLF SESAM 165 94000 [21] Tm: CALGO SESAM 337 650 [22] Tm, Ho: CLNGG SWCNTs 123 98 [28] Tm: CNNGG SWCNTs 22 84 [29] Tm, Ho: CNGG SWCNTs 67 76 [30] Tm: CLNGG SWCNTs 54 78 [31] Tm: MgW Graphene 39 86 [35] Tm: YAP Graphene 256 35000 [36] Tm: YAG MoS2 200 280000 [40] Tm: Sc2O3 KLM 440 166 [45] Tm: YLF KLM 14.4 514 [49] Tm: MgW KLM 100 89 [50] 表 5 各种陶瓷介质超短脉冲全固态掺铥振荡器总结
Table 5. Summary of ultrashort pulse all solid state Tm-doped oscillators with ceramic gain medium
表 6 功率高于500 mW的超短脉冲全固态掺铥振荡器总结
Table 6. Summary of ultra short pulse all solid state Tm-doped oscillators with output power higher than 500 mW
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