Research On A Reflective Fiber Refractometer Based On Multimode Interference In A Multimode Fiber

Multimode interference in a coreless fiber has been studied numerically and experimentally. Its application in a reflective fiber refractometer has been proposed.A wide-angle beam propagation method in cylindrical coordinate has been employed to simulate the multimode interference and self-imaging effect in a multimode fiber. The influence of MMF length, MMF diameter and input light wavelength on sensitivity of the refractometer is simulated. Due to the mode orthogonality and the on-axis excitation, only the first seventeen LP modes with different coupling efficiencies and mode angles are found to be strongly excited when the light is launched into coreless fiber from the single mode fiber at the input end. The phase differences between the excited modes and the peak mode have been calculated when the self-imaging effect occurs. For a given free space wavelength and a given refractive index profile, the length of the coreless fiber is extremely important for obtaining a one-to-one relationship between the refractive index of the surrounding liquid and the well-defined loss peaks. To determine the optimal length of the coreless fiber, a WABPM is used to investigate the transmission loss as a function of the length of the coreless fiber and the refractive indices under test. Given a specific wavelength scanning range, the output spectra versus refractive index for the selected length have been calculated using the WABPM models. The corresponding relationships between the characteristic wavelengths and the surrounding refractive indices have been established.Based on the numerical simulation results, the sensing head is a17mm coreless fiber, spliced to a single mode fiber at the input end, interrogated in reflection and characteristic wavelength shift. Experimental results show that the experimental output spectra fit very well with theoretical output spectra obtained by WABPM models. The measurement range of the refractometer is from1.33to1.45. When the refractive index of the sample is lower than1.38, the interference valley value has an approximately linear relationship with the refractive indices of samples, the sensitivity is141.75nm/RIU. When the refractive index of the sample is higher than1.38, the sensitivity of the refractometer is highly increased, the maximum sensitivity is1473.70nm/RIU, the corresponding maximum resolution is2.71×10-6.