Dispersion Turning Point Shift Based on Spatial-frequency Downconversion of Sensing Signal

Ultra-high sensitivity can be obtained near the dispersion turning point (DTP), which is intrinsically determined by the dispersion characteristics of two arms in a Mach-Zehnder interferometer (MZI) and usually far from the observation window. The shift of DTP towards the observation window was previously realized by physically adjusting the waveguide structure of MZI's arms, which is complex and hard to control. In this paper, it is proposed that the DTP of an MZI can be conveniently shifted to any specified wavelength just by simply down-converting the spatial frequency of its interference fringe. A common microfiber model interferometer with waist diameter of 7.98 μm for refractive index (RI) and temperature sensing is demonstrated. The DTP of sensing signal is successfully shifted into the observation window based on the proposed scheme, and an ultra-high RI sensitivity of 171000 nm/RIU was obtained near the DTP. The proposed scheme is simple and feasible, providing a totally new idea for improving the sensitivity of any form of MZI with DTP.