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掺铒光纤通过强的泵浦光将Er的基态电子(4I15/2)激发至激发态(4I11/2),然后快速下落至亚稳态4I13/2能级。由于受激辐射的原理,4I13/2能级上的电子可以为1550 nm光进行增益[9]。在掺铒光纤中,过短的光纤长度会使得泵浦光吸收不充分,光的转化率较低。而过长的光纤长度会使得泵浦光能量在光纤的后半段不足以实现粒子数反转,不能实现放大作用。由于Er离子自身在1550 nm的吸收作用,光纤中的1550 nm信号光会在掺铒光纤后半段被快速吸收,造成光纤输出功率下降的情况。在辐照前后光纤使用长度基本相同的情况下,辐照后光纤单位长度内980 nm处的吸收系数急剧增大,这会导致辐照后光纤在较短的光纤长度上完成泵浦光的吸收,使得光纤在一定长度内只进行信号光的吸收而不具备光信号放大的作用。
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含有Al的掺杂光纤在高能粒子辐照下,内部会形成Al相关的色心缺陷,在辐照过程中石英网格结构内部会因核外电子电离过程产生大量的载流子。[AlO4/2]−和[AlO3/2]0基团分别是Al-OHC和Al-E’的前身,[AlO4/2]-基团通过俘获空穴而成为Al-OHC色心,而[AlO3/2]0基团通过俘获电子而成为Al-E’色心。辐射诱发的Al-OHC和Al-E’色心缺陷是造成空间辐射环境中应用的掺铒光纤性能衰退的主要损伤机制。Al-OHC的形成如公式(1)所示[10]:
$$ {\left\{{\mathrm{A}\mathrm{l}\mathrm{O}}_{4/2}\right\}}^-+{\mathrm{h}}^+\to {\{{\mathrm{A}\mathrm{l}\mathrm{O}}_{4/2}+{\mathrm{h}}^+\}}^{0}(\equiv \mathrm{A}\mathrm{l}-{\mathrm{O}}^{\circ }) $$ (1) 为了补偿Al-OHC的电荷,需要形成受困电子中心(电子型缺陷)。对于单掺Al的二氧化硅玻璃,
$ \mathrm{A}\mathrm{l}-{\mathrm{E}}^{\mathrm{\text{'}}}(\equiv {\mathrm{A}\mathrm{l}}^{\boldsymbol\cdot}) $ 中心作为捕获电子的色心缺陷,它们的形成过程如公式(2)所示:$$ {\left\{{\mathrm{A}\mathrm{l}\mathrm{O}}_{3/2}\right\}}^{0}+{{e}}^-\to {\{{\mathrm{A}\mathrm{l}\mathrm{O}}_{3/2}+{{e}}^-\}}^-(\equiv {\mathrm{A}\mathrm{l}}^{ {{\boldsymbol\cdot}} }) $$ (2) -
Ge的原有缺陷有两种类型,分别为Ge-ODC(I)和Ge-ODC(II),在辐照下,预先存在的Ge-ODC(I)和Ge-ODC(II)通过结合空穴成为Ge-E′色心,转化过程如公式(3)所示[11]:
$$ 2[={\mathrm{G}\mathrm{e}}^{\boldsymbol\cdot }]\begin{array}{c}^{hv}\\ \leftrightarrows \end{array}\equiv \mathrm{G}\mathrm{e}-\mathrm{G}\mathrm{e}\equiv +{\mathrm{h}}^+ \stackrel{h v}{\longrightarrow} \equiv {\mathrm{G}\mathrm{e}}^{\boldsymbol\cdot \circ }\mathrm{G}\mathrm{e}\equiv $$ (3) 式中:
$ 2[={\mathrm{G}\mathrm{e}}^{\boldsymbol\cdot }] $ 为$ (\mathrm{G}\mathrm{e}-\mathrm{O}\mathrm{D}\mathrm{C}(\mathrm{I}\mathrm{I}\left)\right) $ ;$ \equiv \mathrm{G}\mathrm{e}-\mathrm{G}\mathrm{e}\equiv $ 为$(\mathrm{G}\mathrm{e}- \mathrm{O}\mathrm{D}\mathrm{C}(\mathrm I\left)\right)$ ;$ \equiv {\mathrm{G}\mathrm{e}}^{\boldsymbol\cdot \circ }\mathrm{G}\mathrm{e}\equiv $ 为$ (\mathrm{G}\mathrm{e}-{\mathrm{E}}^{\mathrm{\text{'}}}) $ 。[GeO4/2]0基团是Ge(1)和Ge(2)的前驱体缺陷,在辐照下,[GeO4/2]0基团通过捕获电子成为Ge(1)、Ge(2),转化过程如公式(4)所示:
$$ \equiv \mathrm{G}\mathrm{e}-\mathrm{O}-\mathrm{M}\equiv {+{e}}^- \stackrel{h v}{\longrightarrow} \equiv {\mathrm{G}\mathrm{e}}^{ \cdot }-\mathrm{O}-\mathrm{M}\equiv (\mathrm M=\mathrm{S}\mathrm{i}/\mathrm{A}\mathrm{l}/\mathrm{G}\mathrm{e}) $$ (4) 式中:
$ \equiv \mathrm{G}\mathrm{e}-\mathrm{O}-\mathrm{M}\equiv $ 为$ \left({\left[{\mathrm{G}\mathrm{e}\mathrm{O}}_{4/2}\right]}^{0}\right) $ ;${\mathrm{G}\mathrm{e}}^{ \cdot }-\mathrm{O}-\mathrm{M}$ 为Ge(1)或Ge(2)。 -
对于Al/Ge/Ce共掺杂的光纤,被困的空穴中心(Al-OHC)和被困的电子中心(
$ {\mathrm{S}\mathrm{i}-\mathrm{E}}{{{'}}}/{\mathrm{A}\mathrm{l}}^{-}{\mathrm{E}}{{{'}}}) $ 都减少。这是由于色心缺陷的形成需要载流子的参与,而Ce掺杂可以有效降低辐照环境下光纤中的载流子数量。$ {\mathrm{C}\mathrm{e}}^{3+} $ 通过与捕获的空穴结合来抑制Al-OHC、Ge-E′的形成。$ {\mathrm{C}\mathrm{e}}^{4+} $ 通过与被困的电子中心结合来抑制$ {\mathrm{S}\mathrm{i}-\mathrm{E}}{{{'}}}/{\mathrm{A}\mathrm{l}}^{-}{\mathrm{E}}{{{'}}} $ 、Ge(1)、Ge(2)的形成[12]。$$ {\mathrm{C}\mathrm{e}}^{3+}+{\mathrm{h}}^+\to {\mathrm{C}\mathrm{e}}^{4+} $$ (5) $$ {\mathrm{C}\mathrm{e}}^{4+}+{{e}}^-\to {\mathrm{C}\mathrm{e}}^{3+} $$ (6) 因此,色心的产生可以被Ce3+/4+的价态变化有效抑制,在辐照过程中,Ce3+/4+的价态诱导的相反变化倾向于保持玻璃中Ce3+和Ce4+离子的比例平衡[1, 12] ,辐射诱导的色心缺陷的吸收可以通过Ce掺杂得到抑制。
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实验所用的光纤预制棒及掺铒光纤均采用螯合物MCVD沉积法制备,纤芯直径3.5 µm,内包层直径 125 µm。光纤预制棒样品的厚度为 3 mm,选取三种掺铒光纤,其中高掺Ce掺铒光纤为,低掺Ce掺铒光纤为LCe,非掺Ce掺铒光纤为NCe,对应的预制棒切片编号为-g,LCe-g,NCe-g,样品名-ra代表辐照后的样品。为了阐明光纤中的各物质组分,利用电子探针对光纤的组分进行了表征。其中电子探针测试使用西安地质科技创新创新中心的JXA-8230设备,点扫描使用电流10 nA。表1 为三种光纤的电子探针测试结果。
表 1 三种光纤电子探针测试的组分表
Table 1. Component table for electronic probe testing of three optical fibers
Components HCe LCe NCe Al2O3 1.89 wt% 1.93 wt% 1.98 wt% SiO2 85.7 wt% 83.876 wt% 84.376 wt% GeO2 12.303 wt% 14.12 wt% 13.6 wt% Ce2O3 0.055 wt% 0.027 wt% 0 wt% Er2O3 0.052 wt% 0.047 wt% 0.044 wt% 损耗吸收谱测试,使用光纤综合参数测试仪(Photonic Kenetics 2500),测试中选取辐照前后的适宜光纤长度,利用截断法实现对光纤损耗谱的测试,在光纤综合参数测试仪上实现对光纤吸收及损耗值的读取。光谱范围为90~1600 nm,步进为5 nm。
透过率测试,使用Jasco V5750 紫外-可见-红外透过率测试仪测试了200~850 nm波段的预制棒切片纤芯透过率,测试步进为1 nm。
荧光寿命测试,使用MDL-Ⅲ激光器产生980 nm连续光,通过调制器将光调制为脉冲光,对预制棒切片进行测试,使用光谱仪和示波器测试荧光的强度与时间的对应关系。脉冲光频率为10 Hz,占空比为0.05%。
电子顺磁共振波谱测试,使用BRUKER公司的ELEXSYS-II E500设备,测试中将预制棒切片芯层研磨成粉,装入测试样品用的顺磁管中,在100 K下对光纤的纤芯材料开展EPR测试。微波功率20 mW,磁场范围为3200~3600 Gauss。
增益性能测试示意图如图1所示,使用的信号光为1550 nm,功率为−20 dBm,泵浦光波长为980 nm,功率为100 mW。测试中使用的光纤长度为10 m。光纤长度从10 m开始裁剪,每次裁剪长度为0.5 m。
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利用甘肃兰州天辰辐照站的60Co辐照源对光纤及预制棒开展离线辐照试验,在辐照前、后分别对光纤的性能参数开展测试。根据空间辐射效应评估软件SEREAT预测,地球同步轨道,在10 mm铝球屏蔽下,空间辐射环境在硅中产生的剂量每年不大于10 krad[13],可以作为航天器内部辐照剂量的参考;辐射剂量率决定了单位时间辐射到光纤的能量,在相同总剂量条件下,辐照剂量率越低,色心缺陷越少,辐致增益衰减越小[14],随着剂量率的增大,对应的辐致缺陷数量对应增多[15]。文中主要研究Ce掺杂对辐照光纤内部缺陷的影响,选用较高的剂量率有助于光纤内部色心缺陷的产生与测试分析。因此,结合实际试验条件选取的辐照累积剂量为100 krad,剂量率为6.17 rad/s,辐照时间为4.5 h。
Effect of Ce doping on radiation resistance of erbium-doped fiber for space laser communication
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摘要: 辐照环境下掺铒光纤性能下降严重影响了其在空间环境中的应用,而Ce可以凭借其变价能力抑制光纤的辐致损伤效应。利用螯合物气相沉积法制备了不同Ce掺杂量的掺铒光纤,在常温下使用60Co辐照源对光纤进行了累积剂量100 krad、剂量率6.17 rad/s的辐照实验。通过吸收损耗谱的测试发现Ce掺杂含量高的光纤在辐照后损耗为419.185 dB/km@1200 nm,且荧光寿命变化量减小了0.578 ms。通过切片芯层透过率及电子顺磁共振测试发现Ce掺杂可以有效降低光纤中Al和Ge相关的色心缺陷数量。最后通过增益测试验证了Ce掺杂对掺铒光纤抗辐照能力的改善,辐照后高Ce掺杂的光纤比未掺杂Ce光纤的增益高出4.15 dB。实验结果表明,Ce掺杂可以有效增强掺铒光纤抗辐照性能,这一结论对掺铒光纤在太空中的应用具有重要意义,该研究结果能够为后续掺铒光纤的耐辐照加固及其在空间中的应用提供参考。Abstract:
Objective Space laser communication has the advantages of fast transmission speed, large bandwidth and good confidentiality, and is one of the key development directions of future interplanetary communication. Laser communication requires fast enough transmission rate and high enough transmission power, and erbium-doped fiber amplifier with erbium-doped fiber as the core device is widely used as a signal amplifier in the transmitter and receiver of space laser communication. However, erbium-doped fibers are inevitably affected by the irradiation of space particles in space, which can cause a large number of color-centered defects inside the erbium-doped fiber, resulting in a dramatic decrease in the gain capability and slope efficiency of the device, and then affect the smooth implementation of space laser communication missions. Cerium (Ce) doping is considered as an option to suppress the irradiation loss in optical fibers. Ce can be easily doped into SiO2 glass together with Al, and Ce can suppress the formation of color-centered defects in optical fibers by trapping carriers. Further understanding of the radiation-induced absorption mechanism of Ce doped erbium-doped fibers and enhancing the gain performance of fibers in irradiated environments is essential for the development of space laser communications. Methods Three kinds of erbium-doped optical fibers, namely, high Ce doped(HCe), low Ce doped(LCe) and non-Ce doped(NCe) fibers were prepared by chelate vapor deposition. The fibers were irradiated at a cumulative dose of 100 krad and a dose rate of 6.17 rad/s using a 60Co irradiation source at room temperature. The effect of Ce doping on the performance of the erbium-doped fibers under 100 krad gamma irradiation was investigated. The changes of the fiber color center defects were analyzed by absorption coefficient, loss, and Electron Paramagnetic Resonance (EPR) spectra before and after irradiation of the fiber. By testing the fluorescence lifetime and gain coefficient of the fiber, verification of Ce doping enhances the irradiation resistance of erbium-doped fibers. Results and Discussions The fiber loss and absorption spectra were tested and found that the loss values of all three fibers decreased gradually with the increase of wavelength after irradiation, and the loss changes in the range of 900-1600 nm showed the characteristics of short wavelength and high loss, and it was speculated that there might be higher absorption peaks before 900 nm. Through the EPR test, The paramagnetic defects are mainly Al-OHC, Ge(1), Ge(2) and other Ge/Si related defects, and the EPR test verified that the irradiation loss in the operating band of the fiber is mainly due to Al-OHC, and Ce3+/Ce4+ can effectively reduce the number of Al-OHC and Ge(1)/Ge(2) related defects number. Thus making the absorption of radiation-induced color-centered defects suppressed. The fluorescence lifetime and gain performance tests showed that the fluorescence lifetime was reduced by 1.099 ms for the NCe and 0.578 ms for the HCe, and the gain of the HCe was 4.15 dB higher than that of the NCe after irradiation. This is due to the fact that Ce doping reduces the AL-OHC defects, decreases the irradiation loss in the working band of the fiber, makes the pump light of the fiber more absorbed by rare-earth ions rather than by color-center defects, and improves the irradiation resistance of the erbium-doped fiber. Conclusions Ce doping can reduce the number of carriers during fiber irradiation and thus suppress the formation of color-centered defects during fiber irradiation. Three types of erbium-doped fibers containing different ratios of Ce ions were selected to study the radiation damage from both macroscopic gain performance and microstructural changes. The loss spectra and absorption spectra before and after irradiation were tested, and it was assumed that the main cause of irradiation loss was the trailing of the color-centered absorption peak before 900 nm in the infrared band. Through the EPR test, it was found that the irradiation loss of fibers with high Ce content is smaller and less color-centered defects appear. The analysis is due to the opposite change induced by the valence state of Ce3+/4+ which tends to keep the balance of the ratio of Ce3+ and Ce4+ ions in the glass, The fluorescence lifetime test before and after fiber irradiation shows that the samples with less change in fluorescence lifetime have stronger irradiation resistance, and Ce doping can suppress the shortening of fluorescence lifetime of erbium-doped fibers, which verifies that Ce doping can effectively improve the irradiation resistance of erbium-doped fibers. The gain performance of the fiber before and after irradiation shows that Ce doping can effectively reduce the number of color center defects in the fiber due to irradiation, which can improve the gain performance of the fiber after irradiation. The results of this study can be used as a reference for the subsequent spatially irradiation-resistant reinforcement technology and space applications of erbium-doped fibers. -
表 1 三种光纤电子探针测试的组分表
Table 1. Component table for electronic probe testing of three optical fibers
Components HCe LCe NCe Al2O3 1.89 wt% 1.93 wt% 1.98 wt% SiO2 85.7 wt% 83.876 wt% 84.376 wt% GeO2 12.303 wt% 14.12 wt% 13.6 wt% Ce2O3 0.055 wt% 0.027 wt% 0 wt% Er2O3 0.052 wt% 0.047 wt% 0.044 wt% -
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