Sub-millimeter in-fiber Michelson interferometer with intrinsic Vernier amplification enabled by a non-adiabatic...

Achieving both high sensitivity and a sub-millimeter footprint in fiber-optic curvature and strain sensors remains challenging, particularly for single-structure interferometric devices. Here, we propose and experimentally demonstrate a sub-millimeter in-fiber dual-tapered Michelson interferometer (DTMI) incorporating a non-adiabatic dual-tapered region and a solid microsphere. Within this single continuous fiber structure, an intrinsic (non-cascaded) Vernier effect is generated by two Michelson interference components with slightly mismatched optical path differences, arising from non-adiabatic-taper-induced modal oscillations and microsphere-assisted spatial mode separation. Owing to the distinct relative spectral responses of these two internal interference components, the device exhibits an enhanced Vernier effect under curvature and an attenuated Vernier effect under axial strain. As a result, a high curvature sensitivity of -52.18 nm/m^-1 over a range of 0.189-0.801 m^-1 and a strain sensitivity of -128 pm/µε over 0-120 µε are achieved in a device with an overall length of approximately 0.8 mm. In addition, a low curvature-temperature cross-sensitivity of 2.82 × 10^-4m^-1/°C and a strain-temperature cross-sensitivity of 0.115µε/°C are experimentally obtained. The proposed DTMI provides an effective single-structure route for realizing intrinsic Vernier amplification in ultra-compact fiber-interferometric sensors.