  In this paper, we report a capillary-based M-Z interferometer that could be
used for precise detection of variations in refractive indices of gaseous
samples. This sensing mechanism is quite straightforward. Cladding and core
modes of a capillary are simultaneously excited by coupling coherent laser
beams to the capillary cladding and core, respectively. Interferogram would be
generated as the light transmitted from the core interferes with the light
transmitted from the cladding. Variations in refractive index of the air
filling the core lead to variations in phase difference between the core and
cladding modes, thus shifting the interference fringes. Using a photodiode
together with a narrow slit, we could analyze the fringe shifts. The resolution
of the sensor was found to be 1*10-8 RIU, that is comparable to the highest
resolution obtained by other interferometric sensors reported in previous
literatures. Finally, we also analyze the temperature cross sensitivity of the
sensor. The advantages of our sensor include very low cost, high sensitivity,
straightforward sensing mechanism, and ease of fabrication.
