# A Novel Dual-Wavelength Method for Evaluating Temperature Effect in Fiber-Optic SPR Sensors
Previous studies have proven that the temperature of the detecting situation has multiple influences on the performance of the SPR sensor and may lead to incorrect measurements [11; 12; 13; 14]. For a fiber-based SPR sensor, the temperature influence on the measurement generally includes two main aspects: the property changes from detecting the analyte and the sensor structure parameters. Therefore, the spectrum resonance shift could be caused by the changes of the RI and temperature simultaneously [15; 16], leading to the major technological problem of discriminating between the RI-induced and temperature-induced SPR changes. As a result, it poses a substantial challenge for the applications of fiber-optic SPR sensors. In order to eliminate the uncertainty of RI measurement of fiber-optic SPR sensor caused by temperature perturbation, an alternative method is to measure RI and temperature simultaneously. Several methods have been proposed to extract two different parameters from the output signal of the conventional fiber SPR sensing probe. These methods can be summarized into four categories: (i) measurements at two different fluid channels (one of the channels is taken as the reference for the measurement); (ii) angular spectroscopy at two different wavelengths [17] corresponding to the angle intensity interrogation mode; (iii) wavelength spectroscopy at two different angles [18] corresponding to the wavelength interrogation mode; (iv) multi-mode SPR spectroscopy [19]. However, it is not easy to apply these methods in practice. To this end, a common approach is to design the sensing structure by a pair of cascaded or embedded fiber devices. Typically, a double-channel SPR with one of them taking as a reference is proposed and applied. However, this method is not suitable for fabricating compact sensors or, in general, requiring a complicated fabrication process and specialized equipment [20]. In addition, [PERSON] et al. proposed a gold-coated MM-SM-MM fiber structure to measure the RI and temperature simultaneously [21]. With the design of the single-mode fiber section inserted between two multimode fibers in sensor probes, the sensitive section was partially covered with an analyte to generate two SPR resonance dips in the transmission spectrum. Therefore, a complex fiber optic sensor probe structure is expected to carry out in the experiment. To this end, we present and demonstrate a novel dual-wavelength method for simultaneous measurement of the RI and temperature variation in this paper. Compared to other types of fiber-optic SPR sensors with temperature self-compensation [22; 23; 24], our proposed method can be directly applied to the traditional fiber SPR probe. In the dual-wavelength technique, the dependence of the defined output signals at two dual-wavelengths on RI and temperature are linear and independent with each other. Therefore, there are two main advantages of the dual-wavelength technique. Firstly, the probe of the fiber-optic SPR sensor is a single channel rather than a dual-channel. Secondly, the structure is just the same as the traditional fiber SPR sensor. Therefore, this technique can be applied to the traditional fiber SPR sensor, and no extra structure changes is needed in the sensor probe section. The only requirement is to launch the sensor probe with two suitable wavelengths of lights. This makes the dual-wavelength technique more suitable for practical application. In this paper, a systematic theoretical analysis is conducted for the temperature effect in fiber-optic SPR sensors. Based on the analysis, a theoretical calculation and the corresponding numerical analysis are conducted to validate the change of light reflectance signals with a pair of specific wavelengths due to the resonance wavelength shift. In some specific dual wavelengths, the dependence of the defined output signals on the RI and temperature is linear and independent of them. An experimental platform is constructed to verify the dual-wavelength technique.