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HFI结构如图 1所示。图中,d为第1段无源单模光纤(single-mode fiber, SMF)与EDF的光纤错位量,L为中间段EDF长度。入射光从SMF1左端(8 μm/125 μm)进入,光场沿着光纤纤芯(LP01模式)传输至中间段有源EDF光纤(5.5 μm/125 μm)处。LP(line-ar polarization)表示线偏振。光传输到SMF1和EDF交界面处,由于纤芯失配而激发包层模(LP1m模式),部分光场进入包层传播从而形成包层模,另一部分则继续在纤芯中传输。在SMF1和EDF接触处引入适当的错位量d,激发更多的包层模与纤芯模产生模式干涉。由于纤芯与包层具有不同的折射率,光在EDF中传输一段距离之后,纤芯模与包层模之间会产生一定的相位差。当光传播到EDF和第2段单模光纤SMF2(8.2 μm/125 μm)交界面处,包层模和纤芯模将汇聚并同时进入SMF2中进行传输,从而形成模间干涉[10]。
图 1 混合介质光纤干涉仪结构及光场模式传输示意图
Figure 1. Schematic diagram of mixed medium optical fiber interferometer structure and optical field mode transmission
根据光场干涉叠加原理,干涉光谱强度为[20]:
$ I=I_{\text {core }}+I_{\text {clad }}+2 \sqrt{I_{\text {core }} I_{\text {clad }}} \cos (2 {\rm{\mathsf{π}}} \delta / \lambda+\Delta \phi) $
(1) 式中,λ为入射光波长,Icore和Iclad分别为LP01模式和LP1m模式的传输光强度,δ为LP01模式和LP1m模式的光程差,Δϕ为初始的相位差,这里认为Δϕ=0,Δφ是LP01模式和LP1m模式之间的相位差,可表示为:
$ \Delta \varphi=\frac{2 {\rm{\mathsf{π}}} \delta}{\lambda}+\Delta \phi=\frac{2 {\rm{\mathsf{π}}} \Delta n_{\text {eff }} L_{\text {eff }}}{\lambda} $
(2) 式中,Δneff是光纤纤芯和包层两种传输介质的有效折射率差,Leff为有效干涉长度,此处等于中间段错位光纤的长度L。干涉的极大值即为干涉谱的峰值处,干涉的极小值即为干涉谱的谷底处。
当LP01模式和LP1m模式的相位差为π的奇数倍(2N+1)时,即Δφ=2πΔneffLeff/λ=(2N+1)π,有cos(2πδ/λ+Δϕ)=-1,I取最小值Imin,干涉相消,对应的干涉波谷中心波长λmin可表示为:
$ I_{\min }=I_{\text {core }}+I_{\text {clad }}-2 \sqrt{I_{\text {core }} I_{\text {clad }}} $
(3) $ \lambda_{\text {min }}=2 \Delta n_{\text {eff }} L_{\text {eff }} /(2 N+1) $
(4) 当LP01模式和LP1m模式的相位差为π的偶数倍2N时,即Δφ=2πΔneffLeff/λ=2Nπ,有cos(2πδ/λ+Δϕ)=1,I取最大值Imax,干涉相长,对应的干涉波峰中心波长λmax可表示为:
$ I_{\max }= I_{\text {core }}+I_{\text {clad }}-2 \sqrt{I_{\text {core }} I_{\text {clad }}} $
(5) $ \lambda_{\text {max }}=\Delta n_{\text {eff }} L_{\text {eff }} / N $
(6) 由此可以得出两个相邻波谷(波峰)之间的波长差为:
$ \Delta \lambda=\lambda^2 /\left(2 \Delta n_{\text {eff }} L_{\text {eff }}\right) $
(7) 相邻两个波峰(波谷)的波长间距为自由光谱范围(free spectral range, FSR)RFSR由干涉区域的长度Leff来控制,当EDF长度越长,RFSR越小,可表示为:
$ R_{\mathrm{FSR}}=\lambda^2 / \delta $
(8) $ \delta=2 \Delta n_{\text {eff }} L_{\text {eff }} $
(9) 干涉光谱的消光比Re可表示为:
$ R_{\mathrm{e}}=\frac{2 \sqrt{I_{\text {core }} I_{\text {clad }}}}{1+I_{\text {core }} / I_{\text {clad }}} $
(10) 由(10)式可知,Re的大小主要由干涉光的光强决定,当通过LP01模式和LP1m模式的光强相等时,干涉光谱的Re最高。利用相关软件仿真计算得到模式干涉光谱的归一化谱线, 如图 2所示。图 2a为EDF长度(L=15 mm)固定不变,改变错位量(d变化范围为0 μm~16 μm)得到的HFI干涉光谱变化规律; 图 2b为错位量固定不变(d=14 μm),改变中间段EDF长度(L变化范围为10 mm~25 mm)得到的HFI干涉光谱变化规律。由图 2a可知,当L=15 mm时,存在最佳错位量(d=14 μm),干涉光谱Re最大可达34 dB;由图 2b可知,当d=14 μm时,干涉光谱的RFSR随着L的增加而变小,与(8)式相一致。
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预制棒为Er3+、Al3+离子掺杂SiO2玻璃材料,经过高温熔缩、沉积纤芯,再浸泡于Er3+、Al3+离子溶液等流程制备。通过拉丝装置[21]将EDF预制棒拉制成光纤,具体过程为:首先将预制棒放置在送棒装置上,置于加热炉口中心处且垂直于地面,加热炉中的电阻丝被用来加热预制棒;然后通过拉丝塔控制台和热电偶将温度范围升高至1900 ℃~2000 ℃,步长为20 ℃~30 ℃,保温10 min使拉丝温度稳定;最后调整送棒速率和最大送棒长度的同时控制拉丝滚轮转速调整拉丝速率(1.5 m/min~20 m/min)以保证制备光纤的直径大小,得到的EDF光包层约125 μm, 模场直径在1550 nm时为5 μm~6 μm,抽运功率吸收系数在980 nm时大于3 dB/m,在1530 nm时为5 dB/m~7 dB/m。
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图 3为使用错位熔接法制备的HFI中间段EDF与SMF1的实物照片图。中间段EDF长度为15 mm, 两端无源SMF长度之和小于50 cm; 光纤对准错位d变化范围为0 μ~10 μm,错位控制精度为0.1 μm。图 3a展示了用于控制两端光纤之间的错位量的程序;改变错位量之后两光纤位置如图 3b所示;通过放电将两端光纤错位熔接,熔接后的图片即为图 3c。为研究并获得最佳HFI光谱,向SMF1通入宽带光源,改变d以及中间段EDF的长度,通过光谱仪(optical spectrum analyser, OSA)实时观察干涉仪光谱的变化情况。
图 3 a—用于控制光纤错位量程序图b—光纤错位图c—错位熔接后的实物图照片
Figure 3. a—program diagram for controlling optical fiber misalignment b—optical fiber dislocation diagram c—picture of the real object after dislocation welding
图 4为L=15 mm、不同错位量d时测得的HFI干涉光谱变化规律。
图 4 L=15 mm、不同错位量d时的HFI干涉光谱图
Figure 4. HFI interference spectrum measured experimentally with L=15 mm and different dislocation d
表 1中列出了在L=15 mm时、不同错位量d时的RFSR和Re实验数值特性。实验测试结果表明,干涉光谱的RFSR随着错位量d的增加而减小,损耗随着错位量d的增加而增加。可以看出存在一个最佳值,当d=14 mm时,使得干涉光谱Re最大可达34.66 dB。实验变化规律与上述理论仿真结果相一致。
表 1 L=15 mm、不同错位量d时的RFSR和Re
Table 1. RFSR and Re under different dislocation d obtained with L=15 mm
d/μm Re/dB RFSR/nm loss/dB 0~6 — — — 7 3.3 16.23 -12.36~-9.03 8 4.3 16.5 -15.41~-11.13 9 5.4 16.23 -18.66~-13.23 10 6.6 16.23 -21.98~-15.43 11 7.7 16.1 -24.82~-17.12 12 14 16.91 -33.99~-19.95 13 27.5 14.81 -49.81~-22.28 14 34.66 14.84 -62.11~-27.45 15 26.9 14.6 -55.64~-28.75 16 21.8 14 -52.04~-30.26 当L=20 mm、错位量d变化范围为10 μm~16 μm时, 对应的HFI干涉光谱实验测试结果如图 5所示。可以看出, 当d=13 μm时,Re最大为24.35 dB。与L=15 mm相比较,L=20 mm时损耗更高,RFSR和Re都更低, 即当EDF的长度较长时,HFI的光场传输损耗更高。
图 5 L=20 mm、不同错位量d时的HFI干涉光谱图
Figure 5. HFI interference spectrum measured experimentally with L=20 mm and different dislocation d
当L分别为7 mm和10 mm时,实验测得的干涉光谱分别如图 6a和图 6b所示。当L < 15 mm时, HFI的损耗降低,但是获得的Re最大值(30.02 dB)小于L=15 mm时的Re最大值(34.66 dB),并且可以看出, RFSR随着L的减小而增加,与上述理论仿真结果相一致。
图 6 当L=7 mm和L=10 mm时, 实验测得的干涉光谱图
Figure 6. Interference spectrum of experimental test results when L=7 mm and L=10 mm
当d=12 μm时,EDF长度L变化规律如图 7所示。实验测试结果表明,干涉光谱的Re随着L的增加呈现出先增加再减小的趋势,当L=15 mm时达到最大,处于最佳。
基于混合介质光纤干涉仪的单波长光纤激光器
Single wavelength fiber laser based on hybrid fiber interferometer
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摘要: 为了实现单波长输出, 提出了一种基于单模石英-掺铒-单模石英结构混合介质光纤干涉仪(HFI)的单波长光纤激光器。基于混合介质光纤波导内的光场干涉原理及光波导理论, 分析了HFI的模式干涉理论, 研究了HFI内干涉光特性随中间段铒掺杂光纤(EDF)长度以及错位量的变化规律; 采用熔融错位法制备HFI, 优化了EDF的长度和单模光纤与EDF之间的偏移量。结果表明, 最佳EDF长度为15 mm, 单模光纤与EDF之间的最佳偏移量为7.9 μm; 选择长度为15 mm的EDF HFI作为环形光纤激光器谐振腔的选模器件, 可获得较高稳定性的单波长光纤激光器输出。这一结果对该激光器在光纤传感和光纤通信系统的应用是有帮助的。Abstract: To achieve single wavelength output, a single wavelength fiber laser based on a single-mode quartz-erbium-doped single-mode quartz hybrid fiber interferometer (HFI) was proposed. Based on the optical field interference principle and optical waveguide theory in hybrid dielectric fiber waveguides, the mode interference theory of HFI was analyzed, and the variation of the interference optical characteristics in HFI with the length and dislocation amount of the intermediate erbium-doped fiber (EDF) was studied. HFI was prepared using a melt dislocation method, which optimized the length of the EDF and the offset between the single-mode fiber and the EDF. The results show that, the optimal EDF length is 15 mm, and the optimal offset between the single-mode fiber and the EDF is 7.9 μm. Selecting an EDF HFI with a length of 15 mm as a mode selector for the resonator of a ring fiber laser can achieve a relatively stable single wavelength fiber laser output. This result is helpful for the application of the laser in optical fiber sensing and communication systems.
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表 1 L=15 mm、不同错位量d时的RFSR和Re
Table 1. RFSR and Re under different dislocation d obtained with L=15 mm
d/μm Re/dB RFSR/nm loss/dB 0~6 — — — 7 3.3 16.23 -12.36~-9.03 8 4.3 16.5 -15.41~-11.13 9 5.4 16.23 -18.66~-13.23 10 6.6 16.23 -21.98~-15.43 11 7.7 16.1 -24.82~-17.12 12 14 16.91 -33.99~-19.95 13 27.5 14.81 -49.81~-22.28 14 34.66 14.84 -62.11~-27.45 15 26.9 14.6 -55.64~-28.75 16 21.8 14 -52.04~-30.26 -
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