Offset-tailored multimode interference in an SMF-HCF-SMF interferometer for axial strain and orientation-dependent...

A controlled lateral offset is employed as a symmetry-breaking parameter to tailor multimode interference in an SMF-HCF-SMF interferometer for axial-strain and orientation-dependent curvature measurements. Numerical field-propagation and wavenumber-domain FFT analyses show that the lateral offset redistributes the relative spectral contributions of different optical-path-difference components. At the same time, the HCF length modifies accumulated phase and the effective-index-difference distribution. Under axial loading, selected spectral features exhibit linear blue shifts over a model-estimated HCF strain range of 0-4800 µε, with R² values of 0.974-0.994 and responsivities of -0.49 to -0.38 pm/µε. Opposite bending orientations produce reversed wavelength-shift polarities. The positive and negative orientations show monotonic nonlinear responses over 0.003-0.17 mm^-1 and 0.01-0.10 mm^-1, with range-averaged responsivities of 7.44 and -20.61 nm/(mm^-1), respectively. A full 360° rotation further yields sinusoidal wavelength modulation, demonstrating azimuth-dependent spectral encoding. These results identify the collinear double offset as a controllable parameter for multimode spectral tailoring and orientation-dependent mechanical encoding in a compact fusion-spliced all-fiber platform.