Sagnac interferometer embedded with fiber Bragg grating for relative humidity and temperature sensing

In this paper, we first propose and demonstrate an ultra-compact fiber sensor consisting of fiber Bragg grating (FBG) and Sagnac loop interferometer with a specific taper-based coupling region. It is utilized for the relative humidity and temperature monitoring at the same time. The taper-based coupling region in the Sagnac loop can enhance the interaction between high-order propagating modes and the surrounding perturbations, therefore improving the fiber-optic sensitivity. The Sagnac interferometer demonstrates the humidity sensitivity of 89.4 p. m./%RH and the temperature sensitivity of -8.7 p.m./oC. FBG, the structure embedded in the Sagnac loop, shows the temperature sensitivity of 11.6 p.m./oC, while along with no feedback on the ambient relative humidity. It could be used as the reference component to demodulate the humidity information from the total reflection spectrum, finally realizing the simultaneous response of temperature and relative humidity.S