Simultaneous measurement of magnetic field and temperature based on photonic crystal fiber with eliminating cross-sensitivity

Abstract

Measurement of magnetic field is very important in many fields, such as industrial manufacture, marine environmental monitoring, medical testing, etc. However, there is a cross sensitivity between the measurement of magnetic field and the fluctuation temperature in the environment. So how to accurately measure the magnetic field and the temperature simultaneously by eliminating the cross-sensitivity has been an urgent problem. In recent years, photonic crystal fiber (PCF) sensor has been widely used due to its particular advantages, such as high sensitivity, small size and its flexibility of filling various sensitive media into the air hole. So the PCF provides a new idea for designing the high-sensitivity magnetic sensor. In this paper, a new PCF sensing structure based on the mixed effects of directional resonance coupling and surface plasmon resonance (SPR) is proposed. In the cladding of the PCF, one air hole infiltrated with the magnetic fluid (MF) forms a defect core and is used as a directional coupling channel. When the wave vector matching condition is satisfied in the directional coupling channel, the power is transferred from the fiber core region to the clad defect core at a particular wavelength, and a loss peak is generated in the transmission spectrum. The MF has its unique magneto-optical effect. This is because its refractive index changes with external magnetic field. So the loss peak can be shifted with the magnetic field at a fixed temperature. Another air hole coated with a gold nano film and infiltrated with the methylbenzene is used as the SPR channel. So plasmon modes are excited, and the resonance peak occurs when the real part of the effective index of the core mode is equal to that of the SPR mode at a particular wavelength. The resonance peak can also be shifted with the index of the methylbenzene at changed temperature. The simulation and numerical analysis of the magnetic field and temperature sensing characteristics of the structure are carried out, and the structure parameters of PCF are optimized by COMSOL Multiphysics through using the finite element method under the boundary condition of perfectly matched layer. In a magnetic field range of 90-270 Oe and in a temperature range of 25-60 ℃, the highest magnetic field sensitivity and temperature sensitivity are respectively 1.16 nm/Oe and -9.07 nm/℃, each with a good linearity in the sensing structure. To eliminate the cross sensitivity between the temperature and magnetic field, a sensitivity coefficient matrix is established. As a result, the highly sensitive double-parameter detection of magnetic field and temperature is realized. Moreover, this sensing structure can be used in an extensive range, which has a certain potential value and practical significance.