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Fabrication and temperature characteristics of erbium-doped fiber based on chemical vapor deposition

  • The 46th Research Institute of China Electronics Technology Group Corporation, Tianjin 300220, China

  • Received Date: 2022-10-26
    Accepted Date: 2022-12-22
  • In order to develop erbium-doped fibers with temperature stability to achieve the requirements of super-fluorescent light sources for medium and high precision fiber optic gyroscopes, two types of erbium-doped fibers, Al-Er co-doped and Al-Ge-Er co-doped were prepared by a chelate vapour deposition method. The absorption coefficients and background losses of the two fibers were measured, and the temperature stability of the Al-Ge-Er co-doped fibers was verified experimentally by building a test platform for the super fluorescent light source. The study shows that the Al-Er co-doped fibers have higher absorption coefficients but worse background losses when they are fabricated with an equal volume of erbium chelate. The absorption coefficients of the two fibers are 35.6 dB/m and 20.0 dB/m at 1530 nm, and the background losses are 31.7 dB/km and 6.3 dB/km at 1200 nm. In the temperature range of -45.0 ℃~ 70.0 ℃, the spontaneous emission spectrum mean wavelength variation of the Al-Ge-Er co-doped fiber fabricated by chelate vapor deposition method is about 6.52×10-7 nm/℃ at the central wavelength of 1560.84 nm with the bandwidth of 10.51 nm. This fiber can achieve the requirements of a super-fluorescent light source for high precision fiber optic gyroscope. And this study provides a reference for the development of erbium-doped optical fibers.
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    HAO Y Q, JIA R Y, DING B B, et al. Research of optimized wide-bandwidth optical source with Er3+-doped fiber amplified spontaneous emission[J]. Laser Technology, 2023, 47(4): 500-505(in Chin-ese).
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    OU P, CAO B, ZHANG Ch X, et al. Analysis of mean-wavelength stability of Er-doped super fluorescent fiber sources[J]. Laser & Optoelectronics Progress, 2008, 45(5): 26-30(in Chinese).
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    GUILLAUMOND D, MEUNIER J P. Comparison of two flattening techniques on a double-pass erbium-doped superfluorescent fiber source for fiber-optic gyroscope[J]. IEEE Journal of Selected Topics in Quantum Electronics, 2001, 7(1): 17-21. doi: 10.1109/2944.924004
    [7] 邱嘉荦, 王磊, 黄腾超, 等. 干涉式光纤陀螺技术发展综述[J]. 光学学报, 2022, 42(17): 1706004.

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    SODERLUND M, TAMMELA S, HOFFMAN H J, et al. Direct nano-particle deposition builds active fibers[J]. Laser Focus World, 2006, 42(1): 103-111.
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    TAMMELA S, KIIVERI P, SARKILAHTI S, et al. Direct nanoparticle deposition process for manufacturing very short high gain Er-doped silica glass fibers[C]//2002 28th European Conference on Optical Communication. New York, USA: IEEE, 2002: 1-2.
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    KIR'YANOV A V, BARMENKOV Y O, SANDOVAL-ROMERO G E, et al. Er3+ concentration effects in commercial erbium-doped silica fibers fabricated through the MCVD and DND technologies[J]. IEEE Journal of Quantum Electronics, 2013, 49(6): 511-521.
    [11] 傅永军, 简伟, 郑凯, 等. 掺铒光纤的纤芯折射率[J]. 中国激光, 2006, 33(3): 347-350.

    FU Y J, JIAN W, ZHENG K, et al. Refractive index control in fabrication of erbium doped fiber[J]. Chinese Journal of Lasers, 2006, 33(3): 347-350(in Chinese).
    [12] 程永师. 用于1.5 μm光纤激光器的掺铒及铒镱共掺光纤研究[D]. 武汉: 华中科技大学, 2020: 45-58.

    CHENG Y Sh. The research on erbium-doped and erbium-ytterbium co-doped fibers for 1.5 μm fiber laser[D]. Wuhan: Huazhong University of Science & Technology, 2020: 45-58(in Chinese).
    [13] 高亚明, 冯光, 刘永建, 等. 掺铒光纤的研制[J]. 红外与激光工程, 2009, 38(3): 515-519.

    GAO Y M, FENG G, LIU Y J, et al. Manufacture of erbium-doped optica fiber[J]. Infrared and Laser Engineering, 2009, 38(3): 515-519(in Chinese).
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    YANG Q, JIAO Y, YU C, et al. Gain and laser performance of heavily Er-doped silica fiber fabricated by MCVD combined with the sol-gel method[J]. Chinese Optics Letters, 2021, 19(11): 110603.
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    BISWAS A, MACIEL G S, KAPOOR R, et al. Er3+-doped multicomponent sol-gel-processed silica glass for optical signal amplification at 1.5 μm[J]. Applied Physics Letters, 2003, 82(15): 2389-2391.
    [16] 刘志明. 单模大模场直径高浓度掺铒光纤及相关器件的研制[D]. 北京: 北京交通大学, 2012: 59-91.

    LIU Zh M. Study and fabrication of single mode large-mode-diameter high concentration erbium doped fibers and related device[D]. Beijing: Beijing Jiaotong University, 2012: 59-91(in Chinese).
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    GU Zh M, CHU Y B, LI H Q, et al. Fabrication and amplification characteristics of multicore erbium-doped fiber[J]. Chinese Journal of Lasers, 2022, 49(9): 0906003(in Chinese).
    [18] 何乐, 褚应波, 戴能利, 等. 石英基L波段扩展掺铒光纤及其放大性能[J]. 物理学报, 2022, 71(15): 154204.

    HE L, CHU Y B, DAI N L, et al. Silicate-based erbium-doped fiber extended to L-band and its amplification performance[J]. Acta Physica Sinica, 2022, 71(15): 154204(in Chinese).
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    LENARDIC B, KVEDER M. Advanced vapor-phase doping method using chelate precursor for fabrication of rare earth-doped fibers[C]//2009 Optical Fiber Communication Conference. New York, USA: IEEE, 2009: 1538-1540.
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    ANUAR K, MUHD-YASIN S Z, ZULKIFLI M I, et al. Er2O3-Al2O3 doped silica preform prepared by MCVD-chelate vapor phase delivery technique[J]. Advanced Materials Research, 2014, 896: 219-224.
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    [22] 徐宏杰, 杜赛辉. 掺铒光纤吸收截面和发射截面温度特性研究[J]. 激光与光电子学进展, 2014, 51(10): 100601.

    XU H J, DU S H. Temperature dependence of absorption and emi-ssion cross sections in erbium-doped fibers[J]. Laser & Optoelectronics Progress, 2014, 51(10): 100601(in Chinese).
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Fabrication and temperature characteristics of erbium-doped fiber based on chemical vapor deposition

  • The 46th Research Institute of China Electronics Technology Group Corporation, Tianjin 300220, China
  • Received Date: 2022-10-26
  • Accepted Date: 2022-12-22
  • Available Online: 2023-11-25

Abstract: In order to develop erbium-doped fibers with temperature stability to achieve the requirements of super-fluorescent light sources for medium and high precision fiber optic gyroscopes, two types of erbium-doped fibers, Al-Er co-doped and Al-Ge-Er co-doped were prepared by a chelate vapour deposition method. The absorption coefficients and background losses of the two fibers were measured, and the temperature stability of the Al-Ge-Er co-doped fibers was verified experimentally by building a test platform for the super fluorescent light source. The study shows that the Al-Er co-doped fibers have higher absorption coefficients but worse background losses when they are fabricated with an equal volume of erbium chelate. The absorption coefficients of the two fibers are 35.6 dB/m and 20.0 dB/m at 1530 nm, and the background losses are 31.7 dB/km and 6.3 dB/km at 1200 nm. In the temperature range of -45.0 ℃~ 70.0 ℃, the spontaneous emission spectrum mean wavelength variation of the Al-Ge-Er co-doped fiber fabricated by chelate vapor deposition method is about 6.52×10-7 nm/℃ at the central wavelength of 1560.84 nm with the bandwidth of 10.51 nm. This fiber can achieve the requirements of a super-fluorescent light source for high precision fiber optic gyroscope. And this study provides a reference for the development of erbium-doped optical fibers.

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引言

  • 1985年, 英国南安普顿大学POOLE等人使用改进的化学汽相沉积法(modified chemical vapour deposition, MCVD)制备出低损耗的掺铒光纤[1]。经过近40年的发展，掺铒光纤因其具有光纤通信波段增益、抽运效率高、增益带宽大等优点，已在光纤通信、红外激光、光纤传感等领域得到了广泛的应用[2-3]。并且，利用铒离子放大自发辐射(amplification of spontaneous emission, ASE)效应制作的掺铒光纤超辐射光纤光源(super-fluorescent fiber sources, SFS)不仅兼具激光的高功率和发光二极管的宽光谱特点，更具有平均波长稳定性好的优点，现已成为高精度光纤陀螺光源的首选[4-7]。

    目前，掺铒光纤的制备技术主要有直接纳米粒子沉积法(direct nanoparticle deposition, DND)[8-10]和基于MCVD工艺的液相掺杂法[11-13]、溶胶-凝胶法[14-15]等。其中，DND技术可实现铒离子的高浓度掺杂，但因其制备工艺复杂、成本高、受专利限制等原因，仅有Liekki公司等少数企业仍在使用。基于MCVD工艺的液相掺杂法则应用广泛，2012年，北京交通大学LIU使用该技术制备出了1530 nm吸收系数为84.3 dB/m的单模掺铒光纤[16]; 2022年，华中科技大学LI团队在该技术基础上研制出了多芯掺铒光纤和L波段扩展掺铒光纤[17-18]。液相掺杂法和溶胶-凝胶法的制备工艺同样较为复杂，并且在预制棒制备过程中会产生大量的OH—，造成光纤本底损耗的恶化。2009年，LENARDIGˇ等人提出了基于MCVD的稀土螯合物气相沉积技术[19]，该技术制成的掺稀土光纤不仅OH—含量低，而且铒离子分布均匀; 2014年，ANUAR等人使用该制备技术实现了Er3+和Al3+的共同掺杂，并且完成4层芯层的沉积[20]，证明了该制备技术能够实现纤芯的多层沉积，可用于生产大芯径掺铒光纤。目前，国内还少有关于使用螯合物气相沉积法研制光纤陀螺光源用掺铒光纤的报道，本文作者使用该技术制备Al-Er共掺和Al-Ge-Er共掺两种掺铒光纤，并对光纤的温度特性开展研究。

1.   光纤制备

  • 掺铒光纤芯棒螯合物化学气相沉积系统如图 1所示[21]。该沉积系统由螯合物高温供料系统和改进型化学气相沉积设备组成。其中铒的螯合物Er(thmd)3经高温加热保温后，由氦气携带进入石英反应管内，并在反应管内与氧气等原料发生化学反应后沉积于石英管内壁，后在氢氧焰的高温加热下玻璃化，形成光纤芯棒的芯层。

    Figure 1.  Schematic diagram of chelate chemical vapor deposition equipment[21]

    反应由于Er3+在纯二氧化硅中的溶解度较低，实验中采用Al3+进行共同掺杂，提高预制棒中铒离子掺杂浓度，并降低因Er3+高浓度掺杂引起的团簇效应。其中，Al3+由AlCl3气体与O2反应后的Al2O3提供。为使Er(thmd)3和AlCl3能够充分气化，需将Er(thmd)3和AlCl3分别加热至190 ℃~200 ℃和130 ℃~140 ℃。同时因Er(thmd)3和AlCl3的气体浓度与其容器的温度正相关，为了稳定进入反应管中螯合物和AlCl3浓度，需在螯合物高温供料系统温度到达预设值后保温1 h以上。基于该工艺方案，本文中制备了Al-Er共掺和Al-Ge-Er共掺两种光纤，纤芯芯层制备参数设定值如表 1所示。反应管采用Heraeus的F300系列，并在芯层沉积前预沉积数层由SiO2-P2O5构成的隔离层，用于减少反应管和氢氧焰中羟基扩散进入芯层，防止由此引起的本底损耗升高。

    doping type deposition temperature/℃ Er(thmd)3 gas flow/
    (mL·min-1)    		 AlCl3 gas flow/
    (mL·min-1)    		 SiCl4 gas flow/
    (mL·min-1)    		 GeCl4 gas flow/
    (mL·min-1)    		 number of passes
    Al-Er 1940 120 225 110 0 2
    Al-Ge-Er 1940 120 160 110 200 2

    Table 1.  Parameters of erbium-doped fiber fabricating

    依照掺铒光纤纤芯直径与包层直径的设计比例，选定合适的石英管套管，将掺铒光纤芯棒经加套处理后制作成掺铒光纤预制棒，最后使用光纤拉丝塔将其拉制成直径为125.0 μm±1.0 μm的掺铒光纤。

2.   预制棒及掺铒光纤特性分析

  • 使用光纤预制棒分析仪对实验制备的预制棒进行测试分析，Al-Er共掺和Al-Ge-Er共掺两种光纤预制棒的径向折射率分布如图 2所示。其中y轴为纤芯与石英包层的折射率差值。

    Figure 2.  Radial refractive index profile of optical fiber preform

    由图 2可以看出，当纤芯中掺入Ge元素时，预制棒的折射率呈现“倒刺”状，而Al-Er共掺预制棒的折射率分布曲线则较为平滑，这是因为在高温下，相较于Ge4+，Al3+在二氧化硅基底中极易发生扩散，Al3+在纤芯中的分布更加均匀，从而造成芯层折射率趋于平坦。同时Al-Ge-Er共同掺杂的预制棒中心存在折射率凹陷的现象，该问题是因为在石英管塌缩成石英棒过程中，石英管的温度会达到2200 ℃以上，而GeO2在高温下极易挥发，从而造成纤芯中心折射率的降低。

    在对掺铒光纤预制棒拉丝过程中发现，Al-Er共掺的预制棒在拉丝后，剩余预制棒的芯部出现了大量的白色不透明物质，如图 3所示，且在预制棒芯棒制备过程中并未出现该白色物质。随后通过分别改变Er(thmd)3和AlCl3的载气流量进行了多次实验，实验中发现，白色物质的产生并不随Er(thmd)3载气流量的增减发生变化，但当AlCl3的载气流量较低时，该白色物质并未出现，而当流量较高时，可重复出现此白色物质，由此证明该白色物质为Al2O3的析晶。

    Figure 3.  Erbium doped fiber preform after drawing

    对两种拉制后的光纤进行了相关测试，Al-Er共掺光纤的1530.0 nm吸收系数为45.8 dB/m，而Al-Ge-Er共掺光纤的吸收系数仅为20.0 dB/m。但是，Al-Er共掺光纤在1200.0 nm处的本底损耗为31.7 dB/km，远高于Al-Ge-Er共掺光纤的6.3 dB/km。在两种光纤的制备过程中，Er(thmd)3的载气流量均为120 mL/min，且温度相同，由此可以看出，Al3+的沉积能够提高Er3+的沉积效率，提升Er3+掺杂浓度，同时随着Al3+掺杂量的升高，掺铒光纤的本底损耗恶化明显。该损耗可能是由拉丝过程Al2O3的析晶所导致。所以，在使用Al3+离子掺杂降低Er3+的团簇和提升Er3+的沉积浓度的同时，应设计合适的Al3+掺杂量，减少Al3+掺杂的掺杂浓度，降低Al3+对光纤本底损耗的影响。而对于光纤相对折射率的调制，可在纤芯中共同掺杂Ge4+，通过调整Ge4+的浓度，实现相对折射率的调控。

    由于Al-Er共掺光纤的本底损耗较高，不满足正常使用的要求，故只对Al-Ge-Er共掺光纤的吸收与自发辐射光谱同温度的关系进行了测试研究，其测试结果如图 4和图 5所示。

    Figure 4.  Temperature dependence of absorption spectrum of Al-Ge-Er co-doped fibers

    Figure 5.  Temperature dependence of ASE spectrum of Al-Ge-Er co-doped fibers

    由图 4可以看出，与25.0 ℃室温情况相比，在-44.9 ℃时掺铒光纤980.0 nm附近的吸收峰峰值变化较大，增大1.0 dB以上，而在70.0 ℃时980.0 nm吸收峰峰值较25.0 ℃时的变化较小。同时观测到相较于室温25.0 ℃，1200.0 nm处的本底损耗在-44.9 ℃和70.0 ℃时均有所增加，最大变化约为0.15 dB。在图 5中，Al-Ge-Er共掺光纤自发辐射各波长的功率密度跟随温度相继变化，并且各波长的变化规律不完全一致。这是由于掺铒光纤的自发辐射光谱不仅与光纤纤芯直径、模场直径有关，同时受光纤本底损耗变化的影响[22]，这些参数均会受到光纤温度变化的影响，在多个参数的相互作用下，造成了掺铒光纤自发辐射光谱中各波长功率密度变化不相同的现象。

    掺铒光纤作为掺铒光纤超荧光光源的重要部件，其平均波长的温度稳定性决定了光源系统的性能。为了测试基于螯合物气相沉积法制备的Al-Ge-Er共掺光纤平均波长的温度稳定性，搭建了如图 6所示的掺铒光纤超荧光光源测试平台，使用光谱分析仪采集光源光谱。其中Al-Ge-Er共掺光纤的长度为3.2 m，抽运源中心波长为974.3 nm，实验中抽运源电压电流保持恒定。

    Figure 6.  SFS test platform diagram of Al-Ge-Er co-doped fiber

    如图 7所示，在常温状态下，通过调整抽运源功率，使光源出射功率达到12.02 mW，此时，Al-Ge-Er共掺光纤的荧光光谱中心波长为1560.84 nm，激发峰的3 dB谱宽为10.51 nm。

    Figure 7.  ASE peak of Al-Ge-Er co-doped fiber at room temperature

    将掺铒光纤放置于温箱中，在-45.0 ℃~70.0 ℃的变化范围内，对光源的出射光谱进行连续监测，并每间隔1 min对光源光谱的平均波长进行采样计算，Al-Ge-Er共掺光纤的平均波长与在不同温度下的变化如图 8所示。实验结果表明，在该变温区间内，基于螯合物气相沉积法制备的Al-Ge-Er共掺光纤的荧光光谱平均波长变化约为6.52×10-7 nm/℃。

    Figure 8.  Mean wavelength of ASE peak in Al-Ge-Er co-doped fiber at full temperature

3.   结论

  • 本文中介绍了使用螯合物气相沉积法制备掺铒光纤的工艺技术，并在此基础上制备了Al-Er共掺和Al-Ge-Er共掺两种掺铒光纤，通过实验证明了Al-Er共掺光纤中Al3+掺杂会导致光纤预制棒纤芯析晶失透和光纤本底损耗的增加。对Al-Ge-Er共掺光纤在45.0 ℃~70.0 ℃范围内吸收光谱和发射光谱的温度特性进行了研究，并搭建了超荧光光源测试平台对Al-Ge-Er共掺光纤的平均波长温度稳定性进行测试研究。实验证明，在-45.0 ℃~70.0 ℃变温区间内，使用螯合物气相沉积法制备的Al-Ge-Er共掺光纤平均波长变化约为6.52×10-7 nm/℃，可满足中高精度光纤陀螺中超荧光光源的使用要求。

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