In this paper, a novel in-line modal interferometer for refractive index (RI) sensing is proposed and experimentally fabricated by cascading single-taper and multimode-double- cladding-multimode (MDM) fiber structure. Owing to evanescent field in taper area, the ultra-sensitive and linear intensity-responses to the varied surrounding RI are gained in both single- and double-pass structures. Moreover, the crosstalk from temperature can be effectively discriminated and compensated by means of the RI-free nature of MDM. The experimental results show that the RI sensitivities in single- and double-pass structures, respectively, reach 516.02 and 965.46 dB/RIU (RIU: refractive index unit), both with the slight wavelength shift (~0.2 nm). The temperature responses with respect to wavelength and intensity are 68.9 pm°C-1/0.103 dB°C-1 (single-pass structure) and 103 pm°C-1/0.082 dB·°C-1 (double-pass structure). So the calculated cross-sensitivity of intensity is constrained within 8.49 × 10-5 RIU/°C. In addition, our sensor presents high measurement-stability (~0.99) and low repeatability error (<4.8‱). On account of the ~620 μm size of taper, this compact sensor is cost-efficient, easy to fabricate, and very promising for the applications of biochemistry and biomedicine.
Keywords: intensity demodulation; modal interferometer; refractive index sensor; temperature compensation.