Wavelength Dependence of the Sensitivity of All-Fiber Refractometers Based on the Singlemode–Multimode–Singlemode Structure

In this paper, the wavelength dependence of the sensitivity of all-fiber refractometers based on the singlemode-multimode-singlemode fiber structure, in which a piece of no-core fiber (NCF) is used to serve as the multimode section, is investigated via numerical simulations and experiments. We found that the sensitivity of a refractometer is linearly proportional to the wavelength of the dip/peak in the transmission spectrum that is chosen to measure the refractive index (RI) change. Because the wavelength shift is larger for a dip/peak at a longer wavelength than at a shorter wavelength, a linear spectral chirp will occur in the transmission spectrum when the surrounding RI changes. We also found that the sensitivity significantly increases with the decrease in the diameter of the NCF and is independent of its length. The experimental results agree well with the numerical predictions.