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传感器如实验装置如图1所示。传感器由SMF、FCF和TCF组成。首先将各类使用的光纤去除涂覆层,四芯光纤长度选择为0.5 cm,薄芯光纤选择合适的长度再用切割刀将需要连接部分的端面切平,最后使用熔接机(S178C, Furukawa Electric, Japan)将各部分用熔接机自带的熔接程序进行熔接。由于一般光纤的端面反射率只有4%,因此利用敏化镀银的化学方法在薄芯光纤末端镀了一层银膜作为反射镜增强传播光的反射率。再在银膜外涂一层紫外胶并用紫外灯照射20 min使紫外胶凝固,紫外胶对端面涂覆的银膜起保护作用。
传感结构所用SMF的包层直径为125 μm,纤芯直径为9 μm。FCF的三个边缘纤芯呈正三角形分布,还有一个中间纤芯。FCF的包层直径为125 μm,四个纤芯的直径都为7.6 μm。薄芯光纤的包层直径为125 μm,纤芯直径为2.5 μm。由于各光纤的纤芯直径不匹配,传播的光在光纤熔接处会产生激发耦合现象。首先光源发出的光从SMF传播到FCF中,在经过SMF熔接处时会被激发为FCF的包层模式和纤芯模式,再从FCF中激发为TCF的纤芯模式和包层模式,在TCF中传播后被端面银膜反射回最后耦合进SMF中。
在该结构中,四芯光纤充当耦合器。当光从SMF传入FCF时,光被激发为FCF的纤芯模式和包层模式,传输到TCF时被二次激发为TCF的包层模式和纤芯模式。最后由端面的银膜反射,经FCF耦合进SMF中,输出到光谱仪中。因此TCF的长度、各光纤之间的熔接方式和光纤端面的处理方法会影响传感器的灵敏度。
由于包层模与纤芯模的干涉形成干涉图谱,其强度可表示为[14]:
$$I = {I_{{{core}}}} + \sum {I_{clad}^m + \sum\limits_m {2\sqrt {{I_{core}}I_{clad}^m} } } \cos {\varPhi ^m}$$ (1) 式中:Icore和
$I_{clad}^m$ 分别为四个纤芯中纤芯模式的总光强和第m包层模式的光强;Φm为纤芯和第m包层模式的相位差。相位差与TCF的长度以及纤芯模式和第m包层模式之间的有效折射率差成正比,可表示为[15]:$${\varPhi ^{{m}}} = \frac{{4\pi \left( {n_{eff}^{core} - n_{eff}^{clad,m}} \right)L}}{\lambda } = \frac{{4\pi \Delta n_{eff}^mL}}{\lambda }$$ (2) 式中:
$n_{eff}^{core}$ 为核心模的有效折射率;$n_{eff}^{clad,m}$ 为第m包层模的有效折射率;$\Delta n_{eff}^m$ 为纤芯模式与第m包层模式之间的有效折射率差;L为TCF的长度;λ为传播光的波长。干涉波谷的波长λn可表示为:$${\lambda _n} = \frac{{4\Delta {n_{eff}}L}}{{2n + 1}}$$ (3) 干涉波谷的波长漂移Δλn可表示为[16]:
$$\Delta {\lambda _n} = \frac{{4(\Delta {n_{eff}} + \Delta n)L}}{{2n + 1}} - \frac{{4\Delta {n_{eff}}L}}{{2n + 1}}$$ (4) 当光纤所处外界环境的折射率发生变化时,会引起光纤中包层模式的改变,从而导致传感光纤中包层模式和纤芯模式的有效折射率差发生改变,使波长发生漂移。
Optical fiber Michelson interference sensor for measuring refractive index
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摘要: 提出了一种基于单模光纤-四芯光纤-薄芯光纤(SMF-FCF-TCF)迈克尔逊干涉结构的折射率传感器。采用直接熔接的方式将各光纤进行熔接,由于各光纤之间纤芯的直径不匹配,因此在光纤的熔接处会发生光的激发和耦合。薄芯光纤端面涂覆有一层银面反射膜并用紫外固化胶进行保护来增强光在端面的反射率。四芯光纤作为传感结构中的耦合器,激发了更多的光进入薄芯光纤的包层中,提升了传感器的灵敏度。对传感器的折射率和温度传感特性分别进行了实验探究,实验结果表明,在折射率1.3333~1.3794范围内的灵敏度为137.317 nm/RIU,线性度为0.999,并且温度对传感器的影响较小。该传感结构熔接方式简单,在折射率测量领域具有一定的应用前景。Abstract: A refractive index sensor based on single-mode fiber (SMF) four-core fiber (FCF) and thin-core fiber (TCF) is proposed, forming a SMF-FCF-TCF Michelson interference structure. The optical fibers are spliced by direct splicing. Because of the mismatch of the diameter of the optical fibers’ cores, light excitation and coupling will be induced at the splicing part. The end face of the TCF is coated with a layer of silver film and protected with ultraviolet curing glue to enhance the reflectivity of the light at the end face. The four-core fiber is used as a coupler in the sensing structure, which excites more light into the cladding of the TCF, improving the sensitivity of the sensor. The refractive index and temperature sensing characteristics of the sensor were investigated experimentally. The experimental results show that the sensitivity in the refractive index range of 1.3333 to 1.3794 is 137.317 nm/RIU, the linearity is 0.999, and the temperature has little effect on the sensor. The sensing structure has a simple welding method and has certain application prospects in the field of refractive index measurement.
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