Temperature and Strain Insensitive Refractometer Based on a Multimode Fiber Embedded in a Few-Mode Fiber Structure

A temperature and strain insensitive refractive index (RI) sensor based on a multimode fiber (MMF) embedded in a few-mode fiber (FMF) structure constructing a long-period fiber grating (LPFG) is proposed and investigated. Due to the RI difference between the MMF and the FMF and the mismatch of their fiber core diameters, an RI modulation region is provided in the combined region of the MMF and the FMF. The periodic splicing can be regarded as the introduction of an LPFG, which generates a strong evanescent field, and the RI sensing can be achieved due to the LPFG-excited cladding modes. The experimental results show that the RI sensitivity of the proposed MFM-LPFG fiber structure is 779.00 nm/RIU, while it exhibits low strain response and temperature response with an overall size of 3.2 mm. The MFM-LPFG sensor with resistance to interference from other parameters is expected to have a wide range of roles in chemical and biochemical sensing.