Two-Parameter Optical Fiber Sensor Using Photopolymer Microtips

We propose and develop a two-parameter optical fiber sensor based on photopolymer microtips, which may measure both refractive index (RI) and temperature simultaneously. We fabricate the sensor by growing two microtips on the end face of a twin-core fiber (TCF) and launch two beams of supercontinuous laser source with a wide spectral range into two cores independently. One microtips works as an RI sensing unit. The laser entering into the microtip will produce multimode interference (MMI), and the interference spectrum will shift along with the environmental RI. The other microtip works as a temperature-sensing unit. To improve the temperature sensitivity, we coat a layer of polydimethylsiloxane (PDMS) on the surface of the microtip. We receive the reflective spectrum with two overlapped MMIs, separate the spectrum by using the fast Fourier transform (FFT) method, and finally, realize the independent detection of the two parameters. Experimental results show that the RI sensitivity is −274 nm/RIU with the range of $1.33\sim 1.413$ RIU, while the temperature sensitivity is 1.188 nm/°C with the range of 10 °C–90 °C. In practical application, we construct a matrix equation with the testing sensitivity to solve the crosstalk between two parameters. The proposed two-parameter optical fiber sensor, featuring the benefits of low cost, easy manufacturing, and simplicity of structure, can find potential application prospects in various areas such as industry, environment, and medicine.

I. Introduction

With the progress of science and technology, the accurate monitoring of temperature and refractive index (RI) plays an important role in various fields such as oilfield development, aerospace, biochemical medicine, and power systems [1]. Compared with traditional electronic sensors, optical fiber sensors possess clear advantages, including but not limited to compactness, lightweight, high accuracy, and robustness to electromagnetic interference [2], [3], [4]. Therefore, researchers have invented and manufactured a variety of optical fiber sensors, such as: fiber surface plasmon resonance (SPR) sensor [5], [6], [7], [8], fiber grating sensor [3], [9], [10], [11], [12], fiber Fabry-Perot (FP) sensor [3], [12], [13], [14], [15], [16], [17], [18], fiber multimode interference (MMI) sensor [1], [2], [19], [20], [21], [22], [23], and so on. In some studies, some people strive for the integration of sensing functionalities, aiming to minimize the usage of optical fibers while achieving multiparameter detection. For example, Liu et al. [9] proposed a sensor based on a large bendy fiber grating structure that can measure temperature and RI, but the sensitivity is only 165.9276 nm/RIU and 31.7 pm/°C. Hu et al. [8] proposed an optical fiber SPR sensor based on a multimode fiber-fiber Bragg grating-multimode fiber structure, which is used for two-parameter measurement of liquid RI and temperature. The sensor has an RI sensitivity of 2556.8 nm/RIU and a temperature sensitivity of 172 pm/°C. Wang et al. [12] designed a multiparameter fiber optic sensor with a multifiber structure cascade, which can measure temperature, RI, stress, and humidity. The maximum sensitivity of the sensor is 0.797 nm/°C and 450.97 nm/RIU, respectively. According to these experimental results, most of the structures are excessively complex and the sensitivity to some parameters is relatively low, although the sensor can measure various parameters. Therefore, it is imperative to develop multiparameter optical fiber sensors with structural simplicity, high sensitivity, and a wide range of measurements.