Determination of refractive index by a U-shaped multimode fiber sensor

The U-shaped multimode fiber structure is proposed to measure the refractive index of liquid. In the previous work, the U-shaped structure by single-mode fiber has been widely investigated. However, the large bending loss restricted its practical applications, especially for the reflection-type fiber probe. To reduce the bending losses, the multimode fiber is used as the bending section of the fiber sensor in this study. The structures with different bending radiuses are fabricated to verify this design. Moreover, to simplify the interrogation method, the differential intensity interrogation method is used by cascading two fiber Bragg gratings after the U-shaped fiber structure. The low bending loss of the proposed structure is helpful to measure the reflected light power by fiber Bragg gratings. The influence of incident light power fluctuation was largely suppressed by this differential method. In addition, the sensitivity is also increased by a factor of 2 compared with the intensity interrogation method using one-cascaded fiber Bragg grating. The experimental results show that the refractive index sensitivities are −115.09 dB/RIU in the refractive index range of 1.3405–1.3874 and −260.24 dB/RIU in the refractive index range of 1.3874–1.4223, respectively.