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Chen Jie, Zhou Shengming. Review of magneto-optic materials for high power laser isolators (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201072. doi: 10.3788/IRLA20201072
Citation: Chen Jie, Zhou Shengming. Review of magneto-optic materials for high power laser isolators (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201072. doi: 10.3788/IRLA20201072

Review of magneto-optic materials for high power laser isolators (Invited

doi: 10.3788/IRLA20201072
  • Received Date: 2020-10-15
  • Rev Recd Date: 2020-11-19
  • Available Online: 2021-01-14
  • Publish Date: 2020-12-25
  • As the key component of optical isolators, magneto-optic(MO) materials play an important role especially in high power laser system to ensure one-way light propagation, protect the laser sources and stabilize the laser output. The recent research progress of the MO materials used in near-infrared high power optical isolators was introduced. The key thermal-optic characteristics of MO materials under high power laser conditions and their effects on device performance were illustrated in detail. The studies and high power performance of several newly developed MO material candidates like TSAG crystal, TAG ceramic, and TGG ceramic were reviewed and compared with the commonly used TGG single crystal and Tb-doped glasses on aspects of Verdet constant, thermal conductivity, magneto-optic figure of merit and so on. Among them, TAG ceramics were discussed emphatically including the effects of ions doping and synthesis technology on its magneto-optic and thermal-optic properties. At last, the newest progress on the study of TAG ceramics was introduced, as well as the application prospect and research trend of MO materials used in the 3-5 μm mid-infrared region.
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Review of magneto-optic materials for high power laser isolators (Invited

doi: 10.3788/IRLA20201072
  • 1. Laboratory of Micro-Nano Optoelectronic Materials and Devices, Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
  • 2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: As the key component of optical isolators, magneto-optic(MO) materials play an important role especially in high power laser system to ensure one-way light propagation, protect the laser sources and stabilize the laser output. The recent research progress of the MO materials used in near-infrared high power optical isolators was introduced. The key thermal-optic characteristics of MO materials under high power laser conditions and their effects on device performance were illustrated in detail. The studies and high power performance of several newly developed MO material candidates like TSAG crystal, TAG ceramic, and TGG ceramic were reviewed and compared with the commonly used TGG single crystal and Tb-doped glasses on aspects of Verdet constant, thermal conductivity, magneto-optic figure of merit and so on. Among them, TAG ceramics were discussed emphatically including the effects of ions doping and synthesis technology on its magneto-optic and thermal-optic properties. At last, the newest progress on the study of TAG ceramics was introduced, as well as the application prospect and research trend of MO materials used in the 3-5 μm mid-infrared region.

    • 近年来,高功率全固态激光器和光纤激光器在激光医疗、信息产业、激光加工、空间探测、聚变能源和国防工业等领域的技术应用突飞猛进。而伴随着激光输出功率的快速增长,在激光器使用过程中强烈的反射光对脆弱的前端系统如谐振腔和种子源的破坏风险大大增加,对激光系统的稳定性和可靠性造成严重威胁,迫切需要能够有效隔离反射光的手段。

      基于磁光材料的非互易性法拉第效应,法拉第隔离器是一种在激光系统中确保入射光单向传输并隔离反向光的光无源器件[1]。不同种类激光器对隔离器结构的设计要求有所差别,其原理如图1所示,对于全固态激光器,一般采用自由空间(free space)型隔离器,其原理是通过两个透光轴成45°的偏振片使正向线偏振光经旋光器旋转45°后通过,反向光继续沿同一方向旋转45°被偏振片隔离;对于光纤隔离器,一般采用在线(in-line)型隔离器,是将入射光经由分光晶体分束为o光和e光,使正向光合束后通过,而反向光则被分开无法耦合回光纤,从而实现隔离。

      Figure 1.  Working schemes of free space polarization dependent and in-line polarization independent optical isolators, in which P1, P2 are birefringent crystals, FR is the Faraday rotator, and HW is a half wave plate

      然而,相对于激光器输出功率的快速增长和应用场景的多样化,高功率法拉第隔离器的发展远远滞后,无法满足实际需求。目前,商用高功率法拉第隔离器采用的磁光材料基本为铽镓石榴石(TGG)单晶,其两端带尾纤的光纤隔离器持续工作功率一般不超过200 W,难以满足动辄达上千瓦的高功率宽带光纤激光加工需求。其原因主要是TGG晶体带核心的生长方式以及晶体的热力学各向异性在高功率激光辐照下加剧了材料的热透镜效应和光弹效应,引起光束退偏、隔离度下降,从而导致隔离器性能恶化甚至失效,更有可能对前端系统造成破坏。此外,为了减小高功率激光带来的寄生热效应,避免激光损伤和热损伤,在国防工业和聚变能源等国家战略领域需要的高重频大功率固体激光系统要求元件具有更大通光口径以减小单位面积的功率密度,避免对材料和器件产生破坏[2-3],但目前大尺寸高均匀性TGG单晶的生长仍存在很大难度。目前大口径磁光隔离器仍只能采用低热导率(<1 W·m−1·K−1)和低Verdet常数(~0.3 min/Oe/cm)的铽玻璃作为磁光材料,限制了器件的性能上限[4-5]

      作为隔离器的核心,对新一代高功率磁光材料的研究是突破目前高功率隔离器发展困境的重要途径。近年来,以TGG陶瓷,铽钪铝石榴石(TSAG)单晶,Tb2O3陶瓷、铽铝石榴石(TAG)陶瓷等为代表的新型磁光材料研究在国内外得到了广泛开展[6-11]。其中,TAG透明陶瓷在磁光性能、热光性能和大尺寸制备等方面显示出较大的优势,是重要的下一代高功率磁光候选材料[12-15]。TAG陶瓷在2011年首次由上海光机所制备,发现其Verdet常数在632 nm达到173 rad·T−1·m−1,高于TGG晶体25%,在1 064 nm达到~48 rad·T−1·m−1,高于TGG晶体~21%,且热导率达到6.5 W·m−1·K−1,同样高于TGG晶体,但此时的TAG陶瓷限于制备工艺问题,散射损耗较大,尚无法实用[8]。后续工作中,研究人员通过Y3+、Ce3+、Si4+、Zr4+等离子掺杂改性和TEOS加MgO的复合烧结助剂对TAG陶瓷的磁光性能和光学质量实现大幅度提高[16-20]。2014年,在俄罗斯应用物理研究所的高功率激光测试中,证实光机所制备的TAG陶瓷热退偏达1.5×10−4,低于TGG陶瓷约5倍,在300 W运行时能保持38 dB的高隔离度[21]。2017年,日本Akio Ikesue博士通过Y3+掺杂和HIP预处理的方法,制备出在1064 nm的光学损耗达到1‰ cm−1以下,插入损耗达0.05 dB,消光比达40.3 dB的极高质量TAG陶瓷,其各项器件指标均已优于TGG晶体[14]

      另一方面,随着现代卫星遥感、激光制导、环境监测、生物医疗和红外光谱等领域的发展,对中红外激光的研究正受到前所未有的重视,尤其是3~5 μm波段的“大气窗口”中红外激光[22]。目前,对中红外激光材料的研究较多,如Cr2+、Fe2+等过渡金属离子掺杂的II-VI族如ZnSe、CdMnTe等晶体[23-25]以及具有非线性效应的PPLN、ZnGeP2、AgGaSe2等晶体的研究等[26-29]。但对应的激光隔离器和磁旋光材料的研究报道较少,在国内外都处于起步阶段。在商用器件方面,仅美国Thorlabs、EOT等公司可提供某些特定波段的数瓦级低功率中红外隔离器,难以满足未来高功率需求,而国内尚且没有类似产品的报道。可以预见,随着中红外激光工作功率的提高,在提高激光系统稳定性、保护前端器件方面,对相应磁光材料的前瞻性研究有着重要的战略意义。

      文中介绍了目前高功率磁光材料的研究现状,阐述了材料的高功率热效应对光束退偏和隔离度下降的影响,对比了TGG单晶、TAG磁光陶瓷以及其他高功率磁光候选材料的磁光品质因子等综合性能,说明TAG陶瓷的优势和问题,详细介绍了TAG磁光陶瓷在磁光性能、热光性能和光学质量上的研究进展和机理,指出了TAG陶瓷的发展前景和方向。同时也为3~5 μm的中红外磁光材料研究尝试做出总结,为其材料体系的开发提出了一些想法和观点。

    • 隔离器在高功率下性能退化的根本原因是由于磁光材料在高功率激光入射下热吸收使得其内部产生温度梯度,导致了热退偏和热透镜效应的产生。其中,热透镜效应(Thermal lensing effect)是指不均匀的温度场导致的折射率分布梯度,对入射光形成了类似透镜的会聚效果,造成激光的波前辐射畸变。热透镜效应可以通过外加透镜补偿,较容易消除。而热退偏效应主要来自温度梯度导致的机械应力使传输光在材料中产生的线性双折射现象,又称为光弹效应(Photo-elastic effect),补偿难度较大,是影响器件隔离度的决定性因素。热退偏效应大小可以由退偏度γ确定,其定义为出射光中垂直于正常偏振方向的光功率与正常偏振方向的光功率比值,测试装置如图2所示[21]

      Figure 2.  Diagrammatic sketch of test device of high power magneto--optic thermal effects. (a) Thermal depolarization test; (b) Thermal lensing test, in which 1-Yb fiber laser, 2-focusing lense, 3-calcite wedge, 4-magneto-optic material, 5-NdFeB permanent magnet, 6-quartz wedge, 7-light absorber, 8-Glan prism, 9-focusing lens, 10-CCD camera[21]

      由隔离器工作原理不难看出,隔离器的隔离度指标在高功率下完全可以由退偏度表示,即

      E. Khazanov提出,当旋光角度为π/4时,退偏度理论值可以通过以下公式计算,即[30]

      式中:A是取决于光束空间分布的参数;α为吸收系数;Q为热光常数;L为材料长度;Plas为入射激光功率;λ为激光波长;κ为材料热导率。对于高斯光束A=0.137,X为晶轴取向和光学各向异性参数ξ的函数,对于TGG单晶[001]取向,X=1,对于陶瓷材料,X=(2+3ξ)/5,而ξ满足:

      式中:$ {p}_{ij} $为材料光弹系数张量中的对应值。

      显然,材料的退偏度会随着工作功率的上升而增加,因而,在高功率时造成器件的隔离度下降,这是目前隔离器工作功率受限的根本原因。

      由公式(2)可以看出:热退偏效应的大小由多方面参数决定,是一个综合指标。由于磁光材料长度在磁场和旋光角度大小一定的情况下可以由Verdet常数确定。在高功率情况下,磁光材料的综合性能可以由磁光品质因子(Magneto-optic figure of merit)的大小评判,即

      式中:V为费尔德常数,其他参数与上式中一致。相比于传统的磁光品质因子定义,即品质因子M*=V/α,此种定义更能显示高功率下热效应对材料性能的影响,突出了与热效应相关的热导率、热光系数和光学各向异性的重要性。通过磁光品质因子的定义可以得出,高功率磁光材料应该具有更高的Verdet常数和热导率,更低的吸收系数,更小的热光常数和更小的光学各向异性参数。

    • 表1给出了美国Kigre公司M24磁光铽玻璃、TGG晶体与新兴的如TSAG晶体、Tb2O3陶瓷、TGG陶瓷和一系列TAG基陶瓷等高功率磁光材料室温下的磁光常数、热导率、吸收系数、磁光品质因子等参数。从中可以看出,在所有材料中Tb玻璃的Verdet常数和热导率最低,其磁光品质因子低于TGG单晶一个数量级。相比于TGG晶体,TSAG晶体632 nm的Verdet常数高出~20%,但热导率略低。TSAG单晶的一个特点是具有达到ξ=−101的光学各向异性参数(TGG仅为2.2),导致了其退偏值计算需要考虑与ξ有关的高阶项,不能简单用公式(2)计算磁光品质因子。值得注意的是,2019年,Starobor等采用TSAG晶体作为磁光介质,在1440 W的高功率激光隔离系统中得到了35.4 dB的高隔离度,是目前高功率激光隔离的最高纪录,证明了TSAG的优异磁光性能[31]。尽管TSAG单晶的生长避免了易在高温下挥发的Ga2O3的添加,降低了生长难度和成本,但其生长中的晶核和较大的应力问题是目前限制其生长尺寸的瓶颈,仍有待解决[41]

      MO materialsV/rad·T−1·m−1
      @632 nm
      κ/W·m−1·K−1α/cm−1
      @1070 nm
      M/rad·W·(T·m)−1
      @1070 nm
      M24-Tb glass88.2[5]0.68[32]2×10−3[32]1×108[33]
      TGG crystal[001]138.2[34]4.2[35]1×10−3[33]11×108[33]
      TSAG crystal165.8[33]3.6[33]
      Tb2O3 ceramic447.5[9]2.9×10−3[36]
      TGG ceramic139.6[37]4.9[38]1.4×10−3[38]4.4×108[38]
      TAG ceramic173[8]5[35]1.5×10−3[21]8.4×108[39-40]

      Table 1.  Comparison of Verdet constant(V), thermal conductivity(κ), absorption coefficient(α), and magneto-optic figure of merit(M) of several high power MO candidates at room temperature

      随着透明陶瓷技术的发展和日趋成熟,其在大尺寸制备、材料均匀性和成本控制方面相对于晶体展现出显著优势,且陶瓷烧结一般采用高温固相反应,对一些熔点极高或具有不一致熔融特性而难以生长高质量单晶的材料制备具有重要意义。在磁光陶瓷方面,由日本神岛化学制备的TGG陶瓷达到了和TGG晶体几乎完全一致的Verdet常数,且已有10×10 cm2的大尺寸陶瓷制备报道,如图3所示,证明了磁光陶瓷在大尺寸制备上相比于单晶的巨大优势[40, 42]。尽管TGG陶瓷的散射损耗仍相对晶体较高,磁光品质因子要差于[001]晶向的TGG晶体,限制了其在高功率隔离器中进一步的发展,但其可大尺寸制备和成本较低等优点仍具有相当的吸引力。

      Figure 3.  TGG ceramic with a size over 10 cm×10 cm prepared by Konoshima Chemical, Japan[42]

      相比于TGG,TAG具有更高的Verdet常数,但其面临的问题在于TAG不一致熔融的特性导致了其无法生长大尺寸单晶[43-45]。2011年,中国科学院上海光学精密机械研究所通过高温固相反应克服了TAG不一致熔融的缺点,首次制备出TAG磁光陶瓷,其1064 nm的Verdet常数高于TGG晶体~21%,且热导率也高于TGG晶体。从表1中可以看到,中国科学院上海光学精密机械研究所制备的TAG陶瓷测得的磁光品质因子高于日本神岛化学TGG陶瓷近两倍,达到8.4×108 rad·W/(T·m)[40, 46]。与之对应地,如图4所示,在300 W的高功率激光隔离实验中,TAG陶瓷保持了1.5×10−4的低退偏度,相比TGG陶瓷低5倍,提供的隔离度为38 dB,证明其具有优异的高功率性能[21]。必须提到的是,散射损耗对TAG陶瓷的磁光品质因子影响极大,如参考文献[47]中所制备的TAG陶瓷,由于光学质量问题,其热退偏值仅在~100 W激光系统中已达到3×10−4,导致其磁光品质因子仅为0.58×108 rad·W/(T·m),数值相差达一个数量级以上,说明了陶瓷光学质量对其高功率性能的决定性作用。

      Figure 4.  Results of high power thermal depolarization test based on TAG ceramic(circle) and TGG ceramic(square)[21]

      2017年,日本A. Ikesue等人通过HIP后处理和Y3+掺杂的方法,成功制备出α<1‰ cm−1的极低散射Y-TAG陶瓷[14],其散射损耗可以降低到了和晶体同等的水平,证明随着陶瓷制备水平的提高TAG陶瓷磁光品质因子进一步提高的空间,且尺寸达到φ45 mm,如图5所示。可以预见的是,在未来高功率磁光材料选择中,TAG陶瓷显然是优于TGG陶瓷的选项,并且随着TAG陶瓷制备工艺的改善,其在高磁光品质因子、大尺寸制备和成本控制方面的优势将意味着TAG陶瓷在替代TGG晶体方面有着巨大潜力。

      Figure 5.  TAG ceramics with large aperture and extreme low scattering loss prepared by A. Ikesue et al[14]

      另一方面,由于近红外波段磁光材料的Verdet常数和其Tb3+离子浓度直接相关,Tb2O3的Tb3+相对含量在所有含铽氧化物中最高,是理论上最理想的高功率磁光材料[48]。但实际上,由于此类倍半氧化物熔点极高,且随温度变化存在数种相变和体积变化过程,极易碎裂,在现有技术下难以生长单晶,而透明陶瓷的制备也存在很大难度[49-50]。2017年,日本A. Ikesue等人通过优化工艺,成功制备出632 nm处Verdet常数高于TGG晶体3.2倍,达447.5 rad·T−1·m−1的完整Tb2O3陶瓷[36]。虽然由于其散射损耗仍相对较高,暂不适用于高功率激光系统,但可以预见,由于极高的Verdet常数对磁场要求和材料长度要求的大幅降低,Tb2O3磁光陶瓷在元件小型化上的巨大优势,如空间激光应用等方面仍相当具有吸引力。且随着制备技术的改进,更低散射损耗的Tb2O3陶瓷制备技术一旦实现,将在磁光性能上远优于其他磁光材料,很可能是未来高功率磁光隔离器的最佳选择。

    • 从反映热退偏效应大小的公式(2)和磁光品质因子大小的公式(4)可以看出,对于特定材料,降低退偏度、提高磁光品质因子的方法之一是提高其Verdet常数。因此,通过离子掺杂提高TAG陶瓷的Verdet常数是一种进一步改善其高功率热退偏效应,提高磁光品质因子的思路。

      表2给出了几种稀土离子如Ce3+、Tm3+、Ho3+、Pr3+、Y3+以及非稀土离子Zr3+在Tb3+位的掺杂和Si4+、Ti4+在Al3+位的掺杂对TAG陶瓷Verdet常数、吸收系数和磁光品质因子的影响。

      MO materialsDoping concentrationV/rad·T−1·m−1
      @632 nm
      α/cm−1
      @1070 nm
      M/rad·W·(T·m)−1
      @1070 nm
      Ce:TAG0.5 at%199[16]3.7×10−3[39]6.43×108 [46]
      Si:TAG0.2 at%192[46]6.8×108 [46]
      Ti:TAG0.3 at%192[46]0.97×108 [46]
      Tm:TAG0.16 at%189.5
      Ho/Pr:TAG0.5/1 at%183/187[51-53]~0.17×108 [47]
      Y:TAG0-50 at%91-194[14]<1×10−3[14]
      Zr:TAG0.1 wt%173[18]0.99×108

      Table 2.  Effects of several ions doping on the Verdet constant(V), absorption coefficient(α), and magneto-optic quality factor (M) of TAG ceramics

      可以看出,顺磁性的稀土离子Ce3+、Tm3+、Ho3+、Pr3+掺杂能将TAG的Verdet常数提高6~15%左右,且掺杂浓度在一定范围内的变化并不会对Verdet常数的提高量产生明显影响,而仅取决于掺杂离子的种类[47]。然而,尽管理论上Verdet常数的提高可提高磁光品质因子,实际测试结果却与之相反。在高功率激光实验中,发现TAG陶瓷的磁光品质因子随稀土离子的掺杂出现了不同程度的降低。其中,Ce3+离子掺杂退偏度高于纯TAG陶瓷,使M值降低了23%,如图6所示。而Ho3+、Pr3+离子的掺杂甚至使M值降低至仅~0.17×107 rad·W/(T·m)。需要指出,由于陶瓷制备方的不同,表2中Ho3+、Pr3+掺杂的TAG陶瓷异常低的磁光品质因子有很大因素是来源于工艺不足导致的散射损耗,考虑到其同组的纯TAG的M值仅0.58×108 rad·W/(T·m),预计Ho3+、Pr3+掺杂的TAG陶瓷M值可以通过改善工艺提高10倍左右。然而即使如此,Ho3+、Pr3+离子掺杂仍然会降低TAG陶瓷的磁光品质因子。

      Figure 6.  Pictures of TGG, TAG, and Ce:TAG ceramics and their thermal depolarization test result[40]

      正如上节所提到的,TAG陶瓷在高功率下的磁光品质因子对散射和吸收损耗极为敏感,同样对于纯TAG陶瓷,其由制备工艺造成的光学质量的不同就可以导致10倍以上的磁光品质因子差值。而对于Ce3+等稀土离子,其对不同波段激光存在大量的吸收峰,因此会严重影响TAG的退偏效应,相比而言,由于Ce离子的吸收峰较少且位于较短波段,而Tm3+、Ho3+、Pr3+的吸收峰更加庞杂,分布更广泛,因此造成了磁光品质因子更严重的下降。

      另一方面,由于Tb本身存在两种常见价态,即Tb3+和Tb4+,而Tb2O3在室温下并不稳定,商业用于合成磁光材料的原料粉体一般为Tb4O7,是三价与四价的混合。因此,在TAG中对Tb价态的控制理论上有益于提高其Verdet常数。基于此,由中国科学院上海光学精密机械研究所制备的Al位掺杂Si4+/Ti4+的TAG陶瓷于2016年分别得到了报道。如图7所示[46, 54],发现Si4+/Ti4+掺杂通过抑制Tb4+的存在,可以提高TAG陶瓷Verdet常数~10%左右,并且仅0.2 at%左右的低掺杂浓度就可以达到提高Verdet常数的效果,而更高浓度的掺杂没有进一步提高的效果。这说明低浓度的掺杂即可以达到控制体系中所有Tb离子到三价的效果。但与稀土离子掺杂引起的吸收效应类似的,由于Ti3+的形成本身带来的宽吸收带,其测得的高功率热退偏值较大,如图8所示,对应的磁光品质因子仅为0.97×108 rad·W/(T·m),而Si4+由于价态稳定,且在可见及近红外波段无吸收峰,因此磁光品质因子达到了6.8×108 rad·W/(T·m),是目前除纯TAG陶瓷外的最高值。

      Figure 7.  Pictures of Si4+/Ti4+-doped TAG ceramics and their transmittance curves[11, 17, 46]

      Figure 8.  High power thermal depolarization test results of (a) Si4+ and (b) Ti4+ doped TAG ceramics[46]

      由通过离子掺杂提高TAG的磁光性能的系列实验可以看出,尽管理论上提高TAG陶瓷的Verdet常数可以提高其磁光品质因子,但实际实验中,伴随掺杂离子而来的吸收损耗往往会造成更严重的热退偏效应,反而导致磁光品质因子不同程度的下降。因此,对掺杂离子的选择必须满足不能增加吸收峰的前提,否则将会得不偿失,损害TAG的高功率性能。

    • 从掺杂离子导致的吸收和制备工艺的差异对TAG陶瓷磁光品质因子的显著影响可以看出,降低吸收系数,提高光学质量才是改善TAG陶瓷高功率性能的最根本途径。一般来说,透明陶瓷的吸收和散射损耗主要来自于第二相、气孔、晶界相、杂质等各类散射源,而随着原料纯度和制备手段的提高,目前气孔和晶界相已经成为造成散射的主要来源。

      由于YAG和TAG可以以任意比例固溶,而YAG陶瓷的烧结难度要显著低于TAG,2012年,陈冲等首次通过在Tb3+位的20 at% Y3+替代,制备了Y-TAG陶瓷,其光学质量大幅提升,但是,由于Y3+为抗磁性稀土离子,其大量掺杂导致TAG的Verdet常数下降至108.8 rad/T/m,甚至低于TGG[16]。2015年,他们又通过采用TEOS加MgO的复合烧结助剂,进一步改善了TAG陶瓷的光学质量,其1064 nm透过率达到~80%,接近单晶的透过率水平[20]

    • 引入掺杂和添加剂其本身在陶瓷中的存在和残留会在晶界引入杂相而导致散射的增加。为了解决这一问题,陈杰等在TAG陶瓷制备中进一步开发了ZrO2作为烧结助剂,发现其可以在烧结前期起到抑制晶粒过快生长,促进陶瓷致密化的作用,而在高温烧结后期,排出到陶瓷表面,同时引起晶粒在此时无气孔环境下的显著长大,大大降低了晶界散射,得到的Zr:TAG陶瓷1064 nm透过率达到~82%,如图9所示[18, 55]。但是,本身在可见及近红外无吸收峰的Zr4+在真空弱还原气氛下容易被还原为Zr3+,导致掺杂的TAG在400~700 nm引入了宽吸收带。利用与前文中所示的测试装置,在图10所示的高功率退偏实验结果中,发现ZrO2的引入将导致TAG陶瓷严重的热退偏效应,通过计算发现,其磁光品质因子仅为~1×108 rad·W/(T·m),远低于纯TAG陶瓷。

      Figure 9.  (a),(b) Pictures of Zr:TAG ceramics before and after 1400 ℃ annealing; (c) Change of transmittance curves ; (d) SEM micromorphology of polished S1-S3 after thermal etching, an exaggerated grain growth was observed due to the introduction of ZrO2[18]

      Figure 10.  High power thermal depolarization test result of Zr:TAG ceramics, the depolarization degree was greatly enhanced by the introduction of Zr

      2017年,同样是采用Y掺杂,激光陶瓷先驱A. Ikesue课题组通过HIP后处理,仅使用0.01 wt% SiO2作为烧结助剂,得到了1064 nm达到82.4%理论透过率、散射损耗在1‰ cm−1以下的极高质量Y-TAG陶瓷,其40 at% Y3+掺杂的TAG陶瓷搭建的隔离度插入损耗达到0.01 dB,隔离度达到39.5 dB,无掺杂的纯TAG隔离器插损达到0.05 dB,隔离度达到40.3 dB,优于商用TGG晶体的插损(0.05 dB)和隔离度(35 dB),且口径最大达到φ45 mm[14]

      从以上内容可以看出,通过离子掺杂提高TAG陶瓷的Verdet常数往往会由于离子带来的吸收,反而导致TAG磁光品质因子的降低。提高制备工艺水平,降低陶瓷的吸收和散射损耗是保证TAG陶瓷高功率性能的决定性因素。

    • 对于工作波长更长的波段,如1.3 μm和1.5 μm的通讯波段,研究较多的磁光材料是具有巨磁光法拉第效应的YIG基磁光薄膜或晶体,其在此波段不仅透过率好,且Verdet常数可达~300 rad·T−1·m−1, 并且通过Y3+位的Bi3+、Ce3+等离子掺杂,可以进一步增强法拉第效应[56-57],因此,在集成化、小型化器件方面已经得到广泛应用。而由于CO2激光器的普遍使用,在9~10 μm波段的中红外波段磁光材料同样报道较多,如n型掺杂的Ⅲ-V族InSb、InAs等半导体材料[58-61]。与近红外磁光材料不同,半导体法拉第磁光效应来自于掺杂后带内自由载流子在分立的量子化能级间跃迁,其Verdet常数极高,一般可以使用薄片或薄膜就能达到大旋光角度实现隔离[62-63]

      但是,相对于应用较为成熟的可见及近红外波段以及通讯波段磁光材料,对于具有重要战略意义的3~5 μm波段的磁光材料的研究在国内外几乎为空白,仅有很少的如n型InAs在~5.4 μm波段的隔离器制备报道[64]。但由于以上报道的半导体材料吸收系数较大,工作功率很低。更高质量的III-IV族磁光半导体薄膜或薄片的制备,是满足未来中红外波段磁光材料要求的前提。

      以EuF2等为代表的中红外磁光晶体,在2 μm波段显示出良好的高功率性能,Verdet常数保持到12.3 rad·T−1·m−1, 并且本身在3~5 μm波段也具有较好的透过率,如图11所示,是有潜力的高功率中红外磁光材料,但如何消除在1.7~7 μm 具有多个吸收峰的Eu3+离子是目前需要解决的问题[65]

      Figure 11.  Transmittance curve of EuF2.11 MO crystal with 1.5 mm thickness (the absorption band at 1.7-7 μm was caused by Eu3+)[65]

    • 文中介绍了近红外波段高功率磁光材料的研究现状,指出了磁光材料高功率热效应对其性能指标的决定性作用,基于此对比了新型TSAG单晶、TGG陶瓷、TAG陶瓷等磁光材料的高功率性能,重点阐述了具有优异综合性能的TAG磁光陶瓷的研究历程,包括近年来在提高TAG磁光性能和改善TAG光学质量方面的研究。对提高TAG陶瓷高功率磁光品质因子的研究做出了总结和提炼,提出通过优化制备工艺和离子掺杂筛选降低陶瓷散射及吸收损耗是实现TAG陶瓷高功率性能提升的根本途径。此外,介绍了3~5 μm中红外磁光材料的研究现状,指出了未来此波段的磁光材料发展方向。

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