Improved design of slow light interferometer and its application in FBG displacement sensor

• An optical fiber Mach–Zehnder interferometer based on slow light in polymer-infiltrated PCW was proposed for an FBG displacement sensor.
• By optimizing the structure of PI-PCW, slow light with high group index of 110 was realized.
• Differential and orthogonal method was used to demodulate the interference fringes of MZI with high stability and good linearity.
• The proposed method demonstrated a high sensitivity of 1.035 rad/mm with good linearity and wide measurement range of 55.6 mm.

An optical fiber Mach–Zehnder interferometer (MZI) based on slow light in polymer-infiltrated PCW (PI-PCW) was proposed to enhance the demodulation sensitivity of fiber Bragg grating (FBG) displacement sensor. By optimizing the structure of PI-PCW, slow light with high group index of 110 was realized, which is advantageous to develop high-sensitive MZI. And thanks to electro-optic effect of the infiltrated polymer, the working wavelength of flat band slow light, as well as the demodulation range of MZI, could be flexibility changed and enlarged by tuning external driving voltage. At last, differential and orthogonal method was used to demodulate the interference spectrum of MZI with high stability and good linearity. The FBG was pasted on an Omega-like beam, thus the displacement variation of the free end of the Omega-like beam could be measured by monitoring the output phase of MZI. Finally, the proposed displacement sensor demonstrated a high sensitivity of 1.035 rad/mm with good linearity and wide measurement range of 55.6 mm.