Theoretical analysis of the bending effect in SMF–FMF–SMF structures and its application in interferometry sensors, the optical Vernier effect, and its harmonics

The effect of bending on single-mode-few-mode-single-mode optical fiber structures is analyzed theoretically and compared experimentally. The few-mode fiber considered in this work supports two fundamental modes, LP 01 and LP 11 , with different symmetries that do not couple under normal conditions. Breaking the fiber’s mode symmetry due to bending allows mode coupling, resulting in a bend-dependent wavelength interference pattern. Bending modifies the spectral interference pattern, enabling various sensing applications and signal enhancement techniques, such as the optical Vernier effect and its harmonics.