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.
The analysis of the temperature property of sensors based on symmetrical metal-cladding optical waveguide (SMCOW) is focused on analyzing the temperature property of reflectivity of SMCOW sensors, which is theoretically studied with single-factor investigation under spectral and angular interrogation scheme. There are mainly four factors influencing the temperature dependence of reflectivity, it is the temperature dependence of refractive index and thickness of guiding layer, along with the temperature dependence of the metal film thickness and metal-dielectric function. The simulation result shows that the effect of temperature on the reflectivity of SMCOW is mainly attributed to the temperature dependence of refractive index and thickness of guiding layer;on the contrary, the temperature properties of metal film hardly contributes to the influence of temperature on the reflectivity. Based on the analysis, the sensitivities of SMCOW with guiding layer of different optical glasses are computed under both spectral and angular interrogation. This paper is supposed to provide direction in designing temperature-sensitive SMCOW structure sensors.
The temperature-dependent reflection spectrums of ultrahigh order guided modes of symmetrical metal-cladding waveguide (SMCW) are measured in angular interrogation. The results show that the reflection spectrum is characterized by extremely narrow resonance dip, which agrees well with numerical calculation. Based on the falling or rising edge of resonance dip of ultrahigh order guided modes of SMCW, a temperature sensor with characteristic of high sensitivity is proposed. Meanwhile, by experimentally determine the linear character between the shift of resonance angle and the variation of temperature, a method of angle compensation is put forward to extend the sensing range. Owing to its characteristics of high sensitivity, low cost and easy fabrication, the temperature sensor based on SMCW will be a promising sensor in many fields.
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