Investigation on refractive index sensing characteristics based on multimode fiber specklegram

Abstract In this study, the refractive index (RI) sensing characteristic of a single mode fiber (SMF)-multimode fiber (MMF) structure is investigated, and the two demodulation methods of specklegram and spectrum are compared experimentally. The results show that the RI resolution of the specklegram sensor is significantly higher than that of the optical spectrum analyzer. In addition, an SMF-no-core fiber (NCF)-MMF specklegram sensor is proposed. By extracting the eigenvalues of the gray-level co-occurrence matrix of the output specklegram, the eigenvalue of the angular second moment changes with the RI change, in which the addition of NCF increases the sensitivity from 0.25/RIU to 0.81/RIU. This specklegram sensor has good potential for high-resolution RI detection.