基于双面金属包覆光波导的传感器温度特性研究及实验验证

双面金属包覆波导结构(SMCW)是由一层介质波导层被两层金属膜层上下包覆的一种新型波导结构; 本文基于金属层和介质层材料的热-光效应和热膨胀作用, 研究了双面金属包覆波导结构的温度特性. 计算分析的结果表明, 金属膜层的厚度、金属的介电系数、波导层的厚度及其介电常数几乎都与温度变化成比例, 同时, 对双面金属包覆波导结构的波导功能起主要影响的是介质层的厚度值随温度的变化. 本文分别在光谱模式和角度模式下研究双面金属包覆波导结构的反射特性, 并将其应用于基于双面金属包覆波导结构的传感器, 其灵敏度约为21.89 pm/K(光谱模式)和1.449×10^-3 rad/K(角度模式). 最后, 本文对角度模式的模拟分析进行了实验验证, 实验验证结果与模拟分析结果基本一致, 实验所用SMCW样品的平均灵敏度约为0.517×10^-3 rad/K, 与模拟分析的灵敏度结果同一量级. 双面金属包覆波导结构的传感器对温度非常敏感, 且该结构的物理构造简单, 成本低, 具有非常大的潜在应用价值.

Analysis and experimental investigation of the temperature property of sensors based on symmetrical metal-cladding optical waveguide

Symmetrical metal-cladding waveguide (SMCW) is a kind of new waveguide construction, and it consists of a planar glass slab sandwiched in two metal films with different thicknesses. The metal in this structure is usually a noble metal, such as Au, Ag and Cu etc. One of the characteristics of the glass is the sub-millimeter thickness, which is useful for exciting the ultrahigh order mode. Since the SMCW structure was proposed, it has received much attention from the researchers for its excellent characteristics of free-space coupling technique and ultrahigh order mode excitation. This free-space coupling technology has a higher sensitivity compared with the end-face coupling, prism coupling and grating coupling techniques. The ultrahigh order mode is very sensitive to the incident light wavelength, the thickness of guiding layer and the refractive index, but not sensitive to polarization. Based on the thermal-optical effect and thermal expansion effect of metal film and guiding layer materials, we research the temperature property of the SMCW structure. Researching methods include simulation analysis and experimental demonstration. First, we calculate the relation of the thickness and dielectric property of metal films, and the thickness and refractive index of the guiding layer with the temperature. Results show that these four factors are nearly proportional to the temperature difference. Then, we simulate the relationship of the reflectivity of the SMCW structure with those four factors by means of single-factor investigation under spectral and angular interrogation mode of operation, and find that the temperature-dependence of thickness of the guiding layer makes the chief contribution to the waveguide function of SMCW. Meanwhile, we analyze the sensitivity of the sensors based on SMCW structure, and the result shows that the sensitivity of this kind of sensor can be up to 21.89 pm/K (spectral mode) and 1.449×10^-3 rad/K (angular mode). Finally, we demonstrate the simulation results by experiment. In our experiment, a series of reflectivity is measured at temperatures varying from 320 to 380 K, and the value is expressed in the form of voltage output of PSD (position sensitive diode). The sensor shows a good linearity and a high average resolution of 0.517×10^-3 rad/K; furthermore, we fit the experimental data and get the linear function between angle shifts and temperature difference of Δθ = 0.02965×ΔT. So, once the temperature has any minute variation, it will easily give a change in the resonance incident angle and show the effect of sensor. Owing to the advantages of high sensitivity, low cast and easy fabrication, the temperature sensor based on SMCW will be a promising sensor in many fields.