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高功率中红外光纤激光光源在分子光谱学、光通讯、生物医疗、遥感、环境监测以及国防安全等领域有着重要的应用[1-8]。拉曼激光光源是实现中红外激光输出的一种主要技术手段,其是利用光纤中的受激拉曼散射过程实现增益,具有增益谱带宽、可实现级联运转等特点,原则上可在中红外玻璃光纤材料透过窗口范围内实现任意波长激光输出[9]。此外,基于光纤中拉曼孤子自频移效应产生宽调谐范围的拉曼孤子激光是另一种获得中红外波段激光光源的重要方式。当孤子激光(脉冲宽度为~1 ps或更短)在具有负色散的中红外光纤中传输时,在脉冲内拉曼散射效应作用下,拉曼孤子激光脉冲的低频成分被放大,孤子光谱向长波方向移动,获得宽波长调谐范围的中红外拉曼孤子激光输出[9-10]。
作为拉曼光纤激光光源的重要组成部分之一,增益光纤的基质材料选择以及光纤设计与制备对光源的性能有着重要影响。石英、氟化物、硫系、碲酸盐等玻璃光纤是研制拉曼激光光源的几种主要增益介质。表1总结了几种典型玻璃材料的透过窗口和拉曼增益特性[9,11-12]。可以看出,石英玻璃的红外透过窗口窄,这限制了其在中红外波段拉曼激光光源研制方面的应用。而氟化物、硫化物和碲酸盐等玻璃光纤在中红外波段具有相对较宽的透过窗口和较大的拉曼增益系数,是研制中红外拉曼激光光源的主要介质材料。
表 1 几种典型近红外和中红外玻璃光纤材料的透过窗口及拉曼增益特性[9,11-12]
Table 1. Transparency windows and Raman gain characteristics of a few key near-infrared and mid-infrared glass optical fiber materials[9,11-12]
Glass Transparency window/
μmPeak Raman gain at 2.0 μm/
×10−13 m·W−1Peak Raman shift/
cm−1Nominal Raman gain bandwidth/
cm−1Silica 0.35-2 0.5 442 200 Fluoride 0.22-4.5 0.57-2.1 570 50 Arsenic sulfide (As2S3) 1.5-6.5 21.5-28.5 345 90 Arsenic selenide (As2Se3) 1.5-9.5 100-255 226 60 Tellurite 0.5-4.5 4.5-26 750 140 利用上述几种玻璃光纤作为增益介质,研究者已报道了工作波长位于近红外和中红外波段的拉曼激光光源,如图1所示[13-24]。在近红外波段拉曼激光光源研究方面,利用低损耗石英光纤作为增益介质,已实现平均功率千瓦量级和波长调谐范围覆盖1~2 μm的拉曼激光光源[13-17]。与之相比,中红外波段拉曼激光光源输出功率较低,其工作波长有待进一步向长波区拓展。在基于氟化物玻璃光纤的拉曼激光光源研究方面,Fortin等研制出平均输出功率为3.7 W、工作波长位于2231 nm的拉曼光纤激光器[18]。Tang等利用拉曼孤子自频移技术研制出波长调谐范围覆盖2~4.3 μm的中红外拉曼孤子激光光源[19]。与氟化物玻璃相比,硫系玻璃具有更宽的中红外透过窗口和更大的拉曼增益系数。Barnier等利用硫化砷玻璃光纤作为增益介质,搭建了基于嵌套光栅的两级级联拉曼激光器,实现了工作波长位于3.77 μm的拉曼激光输出,其平均功率为9 mW、峰值功率为112 mW[20]。与上述两种光纤相比,碲酸盐玻璃光纤具有较大的拉曼频移、高的拉曼增益系数和较强的抗激光损伤能力,利用其作为拉曼增益介质有望实现平均功率十瓦量级的3~5 μm拉曼光纤激光器和波长调谐范围覆盖2.8~4.8 μm的中红外拉曼孤子激光光源[21-22]。受当前红外玻璃光纤和超短脉冲泵浦激光光源性能参数的限制,实验上获得的中红外拉曼激光光源的工作波长和调谐范围同理论计算结果还存在较大差距。探索新型中红外玻璃光纤材料,突破低损耗中红外玻璃光纤制备技术,是提高中红外拉曼激光光源输出功率、拓展其工作波长的关键。
近年来,面向高功率中红外激光光源的应用需求,笔者研究团队研制出一种具有宽红外透过窗口、高稳定性、高热机械品质的氟碲酸盐玻璃光纤。利用该光纤作为增益介质,实现了光谱范围覆盖1.5~3.7 μm的级联拉曼散射并搭建了级联拉曼光纤放大器;实现了波长调谐范围覆盖1.98~2.82 μm的拉曼孤子激光光源和工作波长为~4 μm的红移色散波。文中主要介绍了几种中红外玻璃光纤材料的特点及相应的拉曼激光光源的研究进展情况,并对其发展趋势进行了展望。
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与氟化物玻璃相比,硫系玻璃具有更宽的红外透过窗口(1~25 μm)。硫系玻璃是以硫(S)、硒(Se)、碲(Te)等硫系元素为主体,结合一种或几种其他类金属元素(如锗、砷、锑等)形成的一种中红外玻璃材料,其具有极低的声子能量(300~450 cm−1)、高的折射率(2.0~3.5)、高的非线性折射率((2~20)
$ \times $ 10−18 m2/W,比石英玻璃高两三个数量级)、超短的非线性响应时间(<200 fs)[51]。目前,商品化的硫系玻璃光纤有As2S3和As2Se3光纤,其最低损耗分别为~0.05 dB/m和0.20 dB/m,其性能参数如表2所示[52-54]。中国科学院西安光学精密机械研究所、宁波大学、江苏师范大学等多个国内研究单位围绕低损耗硫系玻璃光纤开展了系列研究工作[55-60]。其中,中国科学院西安光学精密机械研究所郭海涛研究团队采用反复蒸馏提纯和开放式动态蒸馏相结合的工艺成功制备出高纯As-S和高纯Ge-Sb-Se等多种硫系玻璃,并发展了高温真空高速旋转预制棒成型和超窄光纤拉丝温控等关键技术,成功制备出As2S3、As2Se3、Ge-Sb-Se、Ge-Ga-S-Cds(CdI2)等多种硫化物玻璃光纤, As2S3光纤的最低损耗为0.2 dB/m@3.8 μm,Ge-Sb-Se光纤的最低损耗为2.2 dB/m@7.0 μm[59-60]。Index CoreActive IRFlex Art Photonics P/N IRT-SU IRT-SE IRF-S IRF-Se IRF-SeG CIR Core/clad As2S3 As2Se3 As2S3 As2Se3 As2Se3/GeAs2S3 As2S3 Transmission range/μm 2-6 2-9 1.5-6.5 1.5-10 1.5-9.3 1.1-6.5 Core refractibe index 2.4 2.7 2.4 2.7 2.7 2.42 Numerical aperture 0.25 0.26 0.28-0.30 0.275-0.350 0.76 0.25-0.30 Typical loss/dB·m−1 0.15@2.7 μm
0.70@4.0 μm0.20@6 μm
0.50@4.55 μm0.05@2.8 μm 0.21@2.59 μm 0.32@2 μm 0.20@2.5-4 μm Tensile strength/kpsi >15 >15 >15 >15 - >70 另外,硫系玻璃还具有极大的拉曼增益系数,其拉曼增益系数比石英玻璃高两三个数量级。典型As2S3玻璃的最大拉曼增益系数为3.75
$ \times $ 10−12 m/W@2 μm,峰值拉曼频移为350 cm−1,对应于[AsS3/2]正棱锥的对称伸缩振动。As2Se3玻璃的最大拉曼增益系数为2.55$ \times $ 10−11 m/W@2 μm,峰值拉曼频移为250 cm−1,对应于As-Se键的振动[61]。在光纤中的级联拉曼散射现象研究方面: 2011年,法国宇航院Duhant等利用一段长度为1.7 m的As-Se微结构光纤作为非线性介质,观察到了四级级联拉曼散射[62]。实验中泵浦光源为工作波长位于1 995 nm的纳秒光纤激光器,当其峰值功率达到11 W时,产生了波长位于2450 nm的四级斯托克斯光。2018年,日本丰田工业大学Cheng等首次报道了基于硫化物玻璃光纤的八级级联拉曼散射,这是在非石英光纤中获得的级联数目最多的拉曼散射现象[63]。实验中所选用的增益光纤为一段基于As38S62和As36S64玻璃的As-S光纤,其长度为16 m,泵浦光源为工作波长为1545 nm的纳秒光纤激光器,其脉冲宽度为~4.1 ns,重复频率为~25 kHz。当平均泵浦功率为100 mW时,获得了波长位于~2698 nm的八级斯托克斯光。2021年,东北大学Wang等采用脉冲宽度为~20 ns、重复频率为~10 kHz的2 μm纳秒激光泵浦一段长度为8 m、纤芯直径为~5.0 μm的自制As2S5玻璃光纤,获得了六级级联拉曼散射,其实验结果如图8所示,最长输出波长位于3.43 μm[64]。
在中红外拉曼光纤激光器研究方面:2006年,澳大利亚悉尼大学Jackson等首次报道了基于As2S3玻璃光纤的拉曼激光器,其输出波长依次为2062、2102、2166 nm,对应的输出功率分别为0.64、0.2、0.016 W[65]。该研究工作中的谐振腔由一个宽带反射镜和反射率为22%的光纤端面构成。随着中红外FBG制备技术和~3 μm波段泵浦源(例如掺铒氟化物光纤激光器)的发展,中红外拉曼光纤激光器的性能也取得了较大提升。2012年,加拿大拉瓦尔大学的Bernier等通过相位掩膜法成功制备出硫化物玻璃光纤光栅[66]。次年,该团队报道了基于硫化物玻璃光纤的第一台工作波长>3 μm的拉曼激光器[67]。实验中所选用的泵浦光源为一台输出波长位于3.005 μm的准连续掺Er3+氟化物玻璃光纤激光器,其脉冲宽度为5 ms,重复频率为20 Hz,所选用的增益介质为一段长度为3 m、芯径为4 μm的As2S3玻璃光纤。实验中,谐振腔由直接刻写在As2S3玻璃光纤两端的FBG构成,其反射率分别为>99%和63%,获得的拉曼激光工作波长为3.34 μm,平均输出功率为47 mW,相应的峰值功率为0.6 W,斜率效率为39%。2014年,Bernier等进一步实现了工作波长位于3.77 μm的拉曼激光输出,这是迄今为止在拉曼光纤激光器中获得的最长波长,其实验装置如图9所示[20],泵浦源为输出波长位于3.005 μm的准连续掺Er3+氟化物玻璃光纤激光器,增益介质为一段2.8 m长As2S3玻璃光纤,谐振腔由两对嵌套刻写在平均功率为9 mW、峰值功率为112 mW的增益光纤两端的FBG构成。当泵浦光功率为3.9 W、输出端FBG反射率为80%时,获得了3.766 μm拉曼激光输出,激光斜率效率为8.3%,如图10所示。为了获得更长波长的拉曼光纤激光器,2019年,宁波大学Peng等理论计算了基于硫化物玻璃光纤的4.3 μm拉曼激光器性能,结果表明,通过优化光纤长度和FBG反射率有望获得输出功率为0.269 W的4.3 μm拉曼激光[68],但目前作为泵浦光源的~3.9 μm掺钬氟化物光纤激光器研制尚不成熟,>4.0 μm的拉曼激光器尚未见报道。
在中红外拉曼孤子激光光源的研究方面:由于传统的阶跃型硫系玻璃光纤的零色散波长一般大于2 μm (As2S3和As2Se3玻璃材料的零色散波长分别为4.89和7.22 μm),为了获得基于硫系玻璃光纤的中红外拉曼孤子激光光源,研究者通常采用零色散波长位于2 μm附近的硫系微结构光纤作为增益介质。2014年,日本丰田工业大学Cheng等利用拉锥As2S5微结构光纤(零色散波长由2.02 μm逐渐移动至1.61 μm)作为增益介质,利用脉冲宽度为200 fs、重复频率为80 MHz的1 900 nm激光作为泵浦光源,获得了调谐波长范围覆盖2.206~2.800 μm的拉曼孤子激光光源[69]。随后,该研究组利用AsSe2-As2S5微结构光纤作为增益介质,利用脉冲宽度为~200 fs、重复频率为80 MHz的2.8 μm激光作为泵浦光源,获得了调谐波长范围覆盖2.916~3.489 μm的中红外拉曼孤子激光光源,实验结果如图11所示[70]。实验中所使用的AsSe2-As2S5微结构光纤的截面如图12(a)所示,色散曲线如图12(b)所示,其零色散波长位于2.759 μm。最终获得的中红外拉曼孤子激光光源功率较低,仅为毫瓦量级。2022年,电子科技大学Hou等分别对4.1 μm和5.2 μm激光泵浦条件下AsSe2-As2S5光纤产生宽调谐拉曼孤子激光进行了数值模拟[23]。计算结果显示,利用硫系玻璃光纤作为非线性介质,有望获得工作波长>8 μm的拉曼孤子激光光源,如图13所示。
目前已报道基于硫系玻璃光纤的拉曼激光光源均采用As2S3或As2Se3玻璃光纤作为增益介质。由于As元素毒性较高,在光纤制备、测试及使用等过程中,该类玻璃光纤存在一定安全隐患。因此,新型无As环保型硫化物玻璃光纤材料(如Ge-Sb-Se系统硫化物玻璃等)的探索及其在中红外拉曼激光光源方面的应用成为该领域的重要研究方向之一。此外,硫系玻璃光纤的激光损伤阈值较低,严重限制了相关激光输出功率的进一步提升。探索稳定性好、抗激光损伤能力强的新型红外玻璃光纤材料,对推进高功率中红外拉曼激光光源的发展及应用具有重要意义。
Progress on mid-infrared glass optical fiber materials and Raman laser source (invited)
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摘要: 高功率中红外光纤激光光源在前沿科学研究、空间光通信、医学诊断与治疗、环境污染监测和光电对抗等领域有着重要应用。拉曼光纤激光光源输出波长灵活,原则上可以在光纤材料透过窗口范围内获得任意波长激光,是实现中红外激光输出的一种重要手段。目前,基于硫系玻璃光纤、氟化物玻璃光纤、碲酸盐玻璃光纤等中红外玻璃光纤材料,已实现工作波长位于3.77 μm的拉曼光纤激光器、平均输出功率为3.7 W的2231 nm拉曼光纤激光器和波长调谐范围覆盖2~4.3 μm的拉曼孤子激光光源。近期,笔者研究组制备出一种具有高热学和化学稳定性、高激光损伤阈值、大拉曼频移和高拉曼增益系数的氟碲酸盐玻璃光纤,并利用其作为非线性介质,先后实现了级联拉曼散射、级联拉曼光纤放大器、波长调谐范围覆盖1.96~2.82 μm的拉曼孤子激光以及波长为~4 μm的红移色散波,验证了氟碲酸盐玻璃光纤在中红外拉曼光纤激光光源研制方面的应用潜力。主要介绍了氟化物、硫化物及碲酸盐玻璃光纤材料的特点及相应的拉曼激光光源的相关研究进展,并对其未来发展趋势进行了展望。Abstract:
Significance High-power mid-infrared fiber laser sources have important applications in molecular spectroscopy, optical communications, biomedical, remote sensing, environment monitoring, and national defense security. Currently, mid-infrared laser sources mainly include rare ion doped fiber lasers, Raman fiber lasers and broadband supercontinuum light sources. At present, 3-4 µm fiber lasers have been demonstrated based on rare ions (such as holmium ions, erbium ions, dysprosium ions and so on) doped fluoride glass fiber. However, limited by the inherent energy levels of rare earth ions and large quantum defects, rare earth ion-doped fiber lasers are difficult to achieve lasing at any wavelength in mid-infrared band, and the laser output power decreases significantly with the increase of wavelength. Raman fiber lasers based on the stimulated Raman scattering (SRS) effects have the characteristic of low quantum loss and flexible output wavelength. SRS is an important nonlinear optical process in optical fibers, and it is an inelastic scattering with stimulated radiation properties. Raman fiber laser has a wide gain spectral bandwidth and can realize the cascade operation. So, with an appropriate pump source and a low loss gain fiber, Raman fiber lasers operating at any wavelength within the transmission window of the fiber glass matrix can be achieved, which is inaccessible for rare earth ions doped fiber laser. In addition, the Raman soliton lasers achieved by using soliton self-frequency-shift effect is also one important way to obtain widely tunable mid-infrared laser sources. Researchers are focus on developing fiber materials with wide mid-infrared transmission window, high laser damage threshold, big Raman shift, large Raman gain coefficients, and corresponding high power mid-infrared Raman laser sources. Progress This paper introduces the progress of several mid-infrared glass optical fiber materials and the corresponding Raman laser sources. At present, the nonlinear medium used in the development of mid-infrared Raman laser source is mainly based on glass fibers with low loss in the mid-infrared region, including fluoride, chalcogenide and tellurite glass fibers. Fluoride glass fibers have a low transmission loss. By using fluoride glass fiber as Raman gain media, researchers have reported a 3.7 W Raman fiber laser at 2231 nm and a Raman soliton laser source with a tunable wavelength rang covering 2-4.3 µm. Chalcogenide glasses have the widest mid-infrared transmission window and the largest Raman gain coefficients among mid-infrared glasses. By using chalcogenide glass fiber as Raman gain media, researchers reported a second-order cascaded Raman laser operating at 3.77 µm, which is the longest wavelength for the Raman fiber lasers obtained in mid-infrared glass fibers. However, its output power is quite low (several milliwatts). Compared with the fluoride and chalcogenide glass, tellurite glasses have a larger Raman frequency shift and stronger laser damage resistance. Theoretical studies show that using tellurite glass fibers as Raman gain media, a Raman fiber laser with an average output power of tens of watts and a Raman soliton laser source with a tunable wavelength range covering 2.8-4.8 µm could be achieved. Very recently, to further improve the performances of tellurite fiber-based laser sources, fluorotellurite fibers with a broadband transmission window (0.4-6.0 µm), high laser damage threshold, big Raman shift (~785 cm−1), and large Raman gain coefficient (1.28×10−12 m/W@2 µm) have been developed by the authors. By using them as Raman gain medium, the authors achieved fifth-order cascaded Raman shift at ~3.75 µm and build cascaded Raman amplifiers. Besides, the authors also obtained Raman soliton laser sources with wavelength tuning rang covering 1.98-2.82 µm, and dispersive wave at ~4 µm. Conclusions and Prospects As one of the important technologies to obtain mid-infrared laser sources, Raman fiber lasers have received extensive attention. At present, by using fluoride, chalcogenide or tellurite glass fibers as gain media, the Raman fiber laser operating at 3.77 µm and Raman soliton laser source with a tunable wavelength range of 2-4.3 µm have been developed. The authors developed fluorotellurite fibers with good stabilities and high laser damage threshold, and preliminarily verified their potential for constructing high power mid-infrared Raman laser sources. It is believed that, in the near future, by further improving the quality of fluorotellurite glass fibers, mid-infrared Raman fiber lasers with output power up to tens of Watts or even hundreds of Watts and the mid-infrared Raman soliton laser source with a tunable wavelength range covering 2-5 µm can be realized. -
Key words:
- Raman laser /
- infrared and far-infrared lasers /
- fiber lasers /
- fiber materials
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图 7 2.39~3.17 μm拉曼孤子光谱图。(a)实验测得和理论模拟结果对比;(b)理论模拟孤子光谱沿40 cm长石英光纤和3.5 m长ZBLAN光纤的演化[48]
Figure 7. Spectra of Raman soliton from 2.39 to 3.17 μm. (a) Comparison between measured and computed spectral profiles; (b) Numerical simulation of the evolution of the spectrum along the 40 cm silica fiber followed by the 3.5 m ZBLAN fiber[48]
图 25 中红外色散波产生实验结果[92]。(a)测量得1 m长氟碲酸盐玻璃光纤输出光谱随1.98 μm飞秒激光器平均泵浦功率的演化;(b)平均泵浦功率1 W下的实验测量光谱与数值模拟光谱;(c)数值模拟1 m长氟碲酸盐玻璃光纤输出光谱随1.98 μm飞秒激光器平均泵浦功率的演化
Figure 25. Experimental results[92]. (a) Measured spectral evolution of output signals from a 1 m long fluorotellurite fiber with the average power of the 1.98 μm femtosecond laser; (b) Simulated and measured spectra output from the fluorotellurite fiber for a same average pump power of ~1 W; (c) Simulated spectral evolution of output signals from a 1 m long fluorotellurite fiber with the average power of the 1.98 μm femtosecond laser
表 1 几种典型近红外和中红外玻璃光纤材料的透过窗口及拉曼增益特性[9,11-12]
Table 1. Transparency windows and Raman gain characteristics of a few key near-infrared and mid-infrared glass optical fiber materials[9,11-12]
Glass Transparency window/
μmPeak Raman gain at 2.0 μm/
×10−13 m·W−1Peak Raman shift/
cm−1Nominal Raman gain bandwidth/
cm−1Silica 0.35-2 0.5 442 200 Fluoride 0.22-4.5 0.57-2.1 570 50 Arsenic sulfide (As2S3) 1.5-6.5 21.5-28.5 345 90 Arsenic selenide (As2Se3) 1.5-9.5 100-255 226 60 Tellurite 0.5-4.5 4.5-26 750 140 Index CoreActive IRFlex Art Photonics P/N IRT-SU IRT-SE IRF-S IRF-Se IRF-SeG CIR Core/clad As2S3 As2Se3 As2S3 As2Se3 As2Se3/GeAs2S3 As2S3 Transmission range/μm 2-6 2-9 1.5-6.5 1.5-10 1.5-9.3 1.1-6.5 Core refractibe index 2.4 2.7 2.4 2.7 2.7 2.42 Numerical aperture 0.25 0.26 0.28-0.30 0.275-0.350 0.76 0.25-0.30 Typical loss/dB·m−1 0.15@2.7 μm
0.70@4.0 μm0.20@6 μm
0.50@4.55 μm0.05@2.8 μm 0.21@2.59 μm 0.32@2 μm 0.20@2.5-4 μm Tensile strength/kpsi >15 >15 >15 >15 - >70 -
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