在纤式回音壁模式微球谐振腔及其传感特性 下载: 1009次
The whispering-gallery-mode (WGM) microcavity sensor has the advantages of a small mode volume and a high quality (Q) factor, and thus it can be applied in high-sensitivity sensing of various physical quantities. Now, common coupling methods for exciting WGMs include prism coupling, tapered fiber coupling, and fiber end coupling. The main disadvantage of prism coupling is that the system is bulky and not easy to be applied to sensing. Tapered fiber coupling is the most common method, whose coupling efficiency can reach 99%. However, the waist diameter of the tapered fiber is too small, and the effective waist diameter should be less than 2 μm to effectively excite WGMs, which makes the overall structure fragile. The fiber end coupling features low efficiency and poor stability, and the control of the coupling angle is difficult. In this paper, an in-fiber WGM microsphere resonator is proposed, which is composed of single-mode fiber (SMF) and hollow-core fiber (HCF). The inner diameter of HCF is small, and the light intensity reflected by the fiber end after corrosion is relatively large, which can effectively improve the stability of the reflection spectrum and play a role in temperature and refractive index sensing.
First, we use simulations to analyze the phase matching of the coupling between microsphere cavities of different sizes and fiber structure and obtain the influencing factors of the spectral shape. It is concluded that the phase difference δ can be changed by the control over the distance between HCF etching end and coupling region to obtain a better Fano profile and increase the slope. Second, in device preparation, the phases of SMF and HCF are fused, and the HCF is cut into a segment of about 2 mm by a fixed-length cutting device. The segmented HCF is then vertically immersed in a hydrofluoric acid (HF) solution with a volume fraction of 40% for etching. Third, a tapered fiber is used as a probe to pick up and move the barium titanate microspheres, which are embedded in the HCF to form a fiber-type resonator structure. In the experiment, it is found that the WGM excited in the microsphere cavity interacts with the reflected light at the HCF end, which results in Fano resonance. The resonator has both temperature and refractive index sensing capabilities. The conclusions obtained by calculation and simulation are consistent with the experimental results.
The optical fiber simulation model is built by the beam propagation method. When the fiber length is fixed, a smaller inner diameter of HCF means stronger light intensity reflected by the fiber end (Fig. 2). In addition, the appropriate size of microspheres is selected by simulation to excite WGMs (Fig. 3). The simulation shows that the phase difference δ is the main factor affecting the spectral shape, and δ can be changed by the control over the distance between HCF etching end and coupling region to obtain a better Fano profile and increase the slope (Fig. 4). During the sensing experiment, the WGM excited in the microsphere cavity participates in the Fano resonance with a slope of -99.3 dB/nm (Fig. 9), and the cavity can sense the temperature and refractive index. In the temperature sensing experiment, the temperature sensitivity of Fano line of the resonator is 26.8 pm/℃ (Fig. 10), which is consistent with the simulation results obtained in the previous section (Fig. 5) and is higher than the sensitivity of the Lorentz line (Fig. 11). In the refractive index sensing experiment, the Fano line is degraded to the Lorentz line, and the refractive index sensitivity is -244.97 dB/RIU (Fig. 12). The calculation method of the optical path difference can be used to confirm that WGM is excited inside the microsphere cavity (Fig. 13).
In this paper, an in-fiber WGM microsphere resonator is fabricated and investigated, and the temperature and refractive index sensing characteristics are studied. The influence of different parameters on the shape of the Fano resonance spectrum is explored. Through simulation, the formation of the Fano profile is researched by the matching of the fiber structure and microsphere diameter with the help of the propagation constant. Moreover, the interval of the theoretical value L that can lead to a better Fano profile is calculated, which is of guiding significance for subsequent experimental operations. The experiments demonstrate the temperature and refractive index sensing characteristics of the designed structure, with temperature sensitivity of 26.8 pm/℃ and reflective index sensibility of -244.97 dB/RIU. The resonator is stable, compact, and simple to process, and this in-fiber structure is expected to be applied in complex sensing environments.
1 引言
2002年,英国学者Vollmer等[1]第一次研究了回音壁模式(WGM)微腔传感器。WGM传感器由于具有微小的模式体积和较高的品质因子(Q值)等优点,在多种物理量的高灵敏传感领域[2-3]有所应用。目前,WGM微腔已经在窄线宽滤波器[4-5]、传感器[6-7]、低阈值激光器[8-10]、无源器件[11-12]和非线性效应[13-14]等方面展现出良好的应用前景。WGM微腔的工作原理是:通过波导的光在倏逝波作用下耦合进入微腔,在腔内以全面反射形式传播,当满足相位匹配条件时,微腔内形成稳态的谐振模式[15-16]。目前常见的激发WGM的耦合方法有棱镜[17]耦合、锥形光纤[18]耦合和光纤端面耦合[19]。棱镜耦合的缺点主要是系统体积庞大,如果想减小系统的体积,就需要搭建较为复杂的系统结构,因此不容易为传感领域所用。锥形光纤耦合的方式最常见,耦合效率可以达到99%,但是锥形光纤的纤腰直径太小,有效纤腰直径要小于2 μm[20]才会有效激发WGM,这就使得光纤的整体结构易折易碎。利用锥形光纤来激发WGM时,在耦合过程中会出现结构不稳定、材料易磨损等情况。光纤端面耦合效率较低,稳定性较差,对于耦合角度的控制较为困难。与此同时,上述常见方法都有一个共同的缺点,那就是需要精确地对齐微球与相关组件,这在许多实际传感应用中是很难实现的。
目前,为了实现简单稳定的耦合,学者们提出一种在纤式光纤耦合方法,即将微球固定在光纤端部的楔形腔内或光纤内部的微孔结构中,例如:在制作光纤结构时,使用嵌入式双芯空心光纤(EDCHF)来将单模光纤(SMF)中的光耦合到嵌入式微球[21],该结构中的两根锥形光纤能够将微球完全包覆在纤维中;还可以采用多模光纤(MMF)与空心光纤(HCF)相熔接的方法[22],然而,MMF对于环境扰动十分敏感,这就会导致反射光谱不稳定。与此同时,光从MMF耦合到HCF的多个模式之间可能会存在一定的干扰,从而加剧光谱的不稳定性。针对光子晶体光纤(PCF)的多孔结构呈几何分布的特点,PCF的孔直径有所不同,因此可以将合适大小的微球嵌入尺寸相匹配的孔内,即将聚苯乙烯小球插入悬浮芯光纤孔内,以实现在纤式WGM耦合和输出[23];或者在光纤微结构的末端粘结微球[24-25],当微球与光纤结构之间的角度合适时,就会激发WGM。利用这些方法不仅可以缩小传感器结构的体积,还可以增强结构的鲁棒性。
本文提出一种在纤式WGM微球谐振腔,由SMF和HCF熔接而成,其中HCF的内径较小,在腐蚀之后光纤端面反射的光强相对较大,能够有效地提高反射光谱的稳定性。首先,理论分析了法诺共振形态的产生及其影响因素,并在实验中得到与理论基本相符的法诺线型,其斜率可达-99.3 dB/nm。其次,实验探究了此谐振腔的温度和折射率传感特性,得到的温度传感灵敏度和折射率灵敏度分别为26.8 pm/℃和-244.97 dB/RIU,线性度良好。
2 工作原理与共振特性分析
2.1 器件结构与原理
图 1. 在纤式WGM微球谐振腔结构和光路示意图。(a)结构示意图;(b)光路示意图
Fig. 1. Structural diagram of in-fiber WGM microsphere resonator and lightpath schematic. (a) Structural diagram; (b) lightpath schematic
该谐振器为反射式结构,在HCF端面反射回来的光与重新耦合进HCF的WGM相互作用。微球中的WGM为离散束缚态,光纤端面反射回来的光为连续态,满足法诺共振产生的理论条件[26],在适当的结构参数下,即可发生法诺共振。
2.2 理论与仿真分析
对于微球嵌入光纤内部的在纤式结构来说,光在传播和耦合过程中的损耗较大,因此,选取合适的光纤参数来降低损耗非常重要。另外,当光纤结构与微球之间的相位匹配时才会产生WGM,即光纤中的光传播常数
图 2. 光纤结构的光场路径传播模拟和空心光纤端面M1处反射光强与光纤内径d的关系。(a)光路模拟仿真;(b) M1端面的反射光强度与HCF内径d的关系
Fig. 2. Lightpath simulation of optical fiber structures and the relationship between reflected light intensity of hollow core fiber end face M1 and inner diameter d of the fiber. (a) Lightpath simulation; (b) reflected light intensity on the end face M1 versus inner diameter d of the HCF
在仿真过程中,HCF的腐蚀深度设置为550 μm,剩余端面的壁厚设置为3 μm,之后计算前九阶模式的传播常数,如
图 3. 光纤结构在不同径向模数下的传播常数和光纤结构与微球的传播常数匹配关系。(a)光纤结构在不同径向模数下的传播常数;(b)光纤结构与微球的传播常数匹配关系
Fig. 3. Propagation constants of optical fiber structure with different radial mode numbers and the relationship between propagation constants of optical fiber structure and microsphere resonator. (a) Propagation constants of optical fiber structure with different radial mode numbers; (b) relationship between propagation constants of optical fiber structure and microsphere resonator
对于微球结构,在微腔中激发WGM的传播常数[27]
式中:
式中:
由式(
在确定HCF内径和微球直径后,要进一步计算法诺共振的实现条件。为了将此结构中产生的法诺共振形象化,利用传输矩阵方法(TMM)[28]计算法诺共振光谱,公式为
式中:PR为归一化反射值;
在计算过程中,将透射系数t和往返传输系数
图 4. δ在不同范围内 与θ的函数关系图像。(a) ;(b) ;(c) ;(d)
Fig. 4. Images of the functional relationship between and θ in different ranges of δ. (a) 0-0.25 ; (b) 0.25 -0.50 ; (c) 0.50 -0.75 ; (d) 0.75 -1.00
图 5. 不同温度下法诺线型移动的仿真结果和法诺共振峰波长的线性拟合结果。(a)仿真结果;(b)线性拟合结果
Fig. 5. Simulation results of Fano line shape movement at different temperatures and linear fitting results of Fano resonance peak wavelengths. (a) Simulation results; (b) linear fitting results
3 在纤式谐振腔的制备与测试
在纤式WGM微球谐振腔的制备过程如
图 6. 传感结构制备过程。(a)(b) SMF与HCF熔接;(c)腐蚀HCF;(d)微球置入HCF内部
Fig. 6. Sensing structure preparation process. (a)(b) Splicing of SMF and HCF; (c) etching of HCF; (d) placing the microsphere resonator into HCF
在熔接过程中,调整熔接时间和放电功率,使SMF和HCF的熔接处塌陷,形成锥形空腔,如
图 7. 在纤式微球谐振腔的显微镜图像。(a) SMF与HCF熔接处形成塌陷锥角;(b) HCF腐蚀后形成圆锥体;(c)微球嵌入后的谐振腔结构
Fig. 7. Microscope images of in-fiber microsphere resonator. (a) Collapse angle between SMF and HCF at splicing; (b) cone after etching the HCF; (c) microsphere embedded in resonator structure
在制备好耦合结构之后,将该结构与一根锥形光纤探针分别固定在两个三维位移平台上,通过控制三维位移台来进行微球的拾取和移动工作。将准备好的钛酸钡微球(Cospheric BTGMS-4.25)吸附在锥形光纤尖端,通过调整三维位移平台来移动钛酸钡微球,使其嵌入腐蚀后的HCF内部,嵌入后的结构如
在实验过程中,以锥形光纤探针为拾取和移动微球的工具,将直径为50 μm、折射率为1.93的钛酸钡微球(Cospheric BTGMS-4.25)嵌入HCF腐蚀的前端开口中。 C+L波段的ASE宽谱光源发出的光经过环形器入射到微腔中,反射回来的光再经过环形器被光谱分析仪(AQ6370D YOKOGAWA)接收,实时观察并采集反射光谱,如
图 9. 在纤式WGM微球谐振腔的反射光谱和放大的法诺共振及其参数定义。(a)反射光谱;(b)放大的法诺共振及其参数定义
Fig. 9. Reflection spectra of in-fiber WGM microsphere resonator and the enlarged Fano resonance and relevant parameter definitions. (a) Reflection spectra; (b) the enlarged Fano resonance and relevant parameter definitions
4 传感实验结果与分析
所提出的内嵌式微球腔结构中使用的微球是由钛酸钡玻璃制成的,当外界温度发生变化时,由于热光效应和热膨胀效应,微球的折射率和半径会发生变化,进而引起共振峰的移动。温度变化时WGM波长移动[29]可以表示为
式中:T表示恒温恒湿箱中的温度;
由于此谐振腔结构开放,微球仍能与外界接触,因此当外界环境的折射率改变时,反射光谱也将发生变化。下面对此微腔的温度和折射率传感特性进行实验测试与分析。
4.1 温度传感实验
测试微腔的温度特性时,使用恒温恒湿箱(WHTH-150-20-880)来改变光纤结构周围的环境温度,温度变化范围为20~110 ℃,温度每变化10 ℃,记录一次数据。在实验过程中,随着温度升高,利用光谱仪探测的法诺共振线型显示出红移的趋势,如
图 10. 温度传感实验得到的法诺线型反射光谱和线性拟合结果。(a)反射光谱;(b)线性拟合结果
Fig. 10. Fano reflection spectra and linear fitting results in the temperature sensing experiments. (a) Reflection spectra; (b) linear fitting results
对法诺共振峰的波长进行拟合,结果如
图 11. 温度传感实验得到的洛伦兹线型反射光谱和线性拟合结果。(a)反射光谱;(b)线性拟合结果
Fig. 11. Lorentz reflection spectra and linear fitting results in the temperature sensing experiments. (a) Reflection spectra; (b) linear fitting results
根据温度变化时WGM波长移动的原理可以得到某一波长λ处波长变化量
式中:α和
4.2 折射率传感实验
当所设计的微腔结构被用作折射率传感器时,将传感结构浸入配置好的氯化钠溶液中,并对反射光谱进行采集和分析。氯化钠溶液的折射率在1.33534~1.35190范围内,按照折射率将氯化钠溶液分为6组。在不同折射率溶液中采集到的反射光谱如
图 12. 折射率传感实验的反射光谱和线性拟合结果。(a)反射光谱;(b)线性拟合结果
Fig. 12. Reflection spectra and linear fitting results in the refractive index sensing experiments. (a) Reflection spectra; (b) linear fitting results
对1577 nm波长处的峰值强度变化进行拟合,发现折射率传感灵敏度为-244.97 dB/RIU,线性度为0.9724,如
对反射光谱进行快速傅里叶变换(FFT),获得的空间频谱如
图 13. 通过FFT获得的空间频谱和在纤式WGM微球谐振腔的耦合光路示意图。(a)空间频谱;(b)耦合光路示意图
Fig. 13. Spatial frequency spectrum obtained by FFT and schematic of coupled lightpath of in-fiber WGM microsphere resonator. (a) Spatial frequency spectrum; (b) schematic of coupled lightpath
FSR满足公式
式中:n1和n2分别表示HCF的包层折射率和微球腔的折射率;
5 结论
提出一种能够激发微球谐振器中的WGM模式的在纤式光纤耦合器,并对其温度与折射率的传感特性进行了探究。研究了不同参数对法诺共振光谱形态的影响,通过仿真研究了光纤结构与微球直径之间借助传播常数的匹配来形成法诺线型的情况,计算出能得到较好法诺线型的理论值L的区间,这对于后续的实验操作具有指导意义。通过实验证明了所设计结构具备的传感特性,其温度灵敏度为26.8 pm/℃,折射率灵敏度为-244.97 dB/RIU。该器件结构紧凑、加工简单、不需要精确对准,在传感应用和光开关领域具有一定的潜力。
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Article Outline
殷琦寓, 蔡露, 李尚文, 赵勇. 在纤式回音壁模式微球谐振腔及其传感特性[J]. 光学学报, 2023, 43(1): 0106002. Qiyu Yin, Lu Cai, Shangwen Li, Yong Zhao. An In-Fiber Whispering-Gallery-Mode Microsphere Resonator and Its Sensing Characteristics[J]. Acta Optica Sinica, 2023, 43(1): 0106002.