2-D Vector Microdisplacement Measurement Based on Fiber Mode Demodulation Technology

This paper presents a 2-D vector microdisplacement sensor based on mode coupling between single-mode fiber (SMF) and two-mode fiber (TMF). The offset alignment between SMF and TMF along the radial direction of optical fibers can influence the mode coefficients of LP₀₁ mode, LP_11o mode, and LP_11e mode in TMF, including mode power proportion coefficients and mode relative phase coefficients. Modal decomposition based on digital holography is used for obtaining these mode coefficients. The variations of mode coefficients of all modes in TMF with the change of the offset between SMF and TMF (i.e., microdisplacement) in two vertical directions are simulated. Experimental results are consistent with simulation results, displaying that the microdisplacement in the range from −5 to <inline-formula> <tex-math notation="LaTeX">$5~\mu \text{m}$ </tex-math></inline-formula> along the positive or negative <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula>-axis or <inline-formula> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula>-axis direction can be judged by using the proposed scheme based on mode demodulation technology.