Abstract: The internal mechanism about the effect of evanescent wave in tapered modal interferometer was investigated theoretically. The relationships between the power of evanescent wave and the effective RI of cladding modes were calculated， and the simulated interference spectrum shifted with external RI was ohtained.It can be theoretically proofed that the generation of evanescent wave provides a vital role in improving the sensing sensitivity of tapered optical fiber RI sensor. In addition， effects of the taper shape， such as outline， waist radius and taper length， on the power of evanescent wave attenuated in taper region were investigated based on the evanescent wave absorption spectroscopy， which is helpful to optimize the taper shape and improve the sensitivity of RI measurement. Finally， the experiment of RI measurement by tapered modal interferometer has been carried out to verify the accuracy of the theory.

Key words: modal interferometer, evanescent wave, modal effective refractive index, refractive index measurement, optical fiber