Optomechanical microbubble Fabry-Pérot interferometer for ultrasensitive curvature measurement

We present an ultrasensitive fiber-optic curvature sensor based on a micro-air-bubble Fabry-Pérot interferometer (MFPI) that incorporates a micro-optomechanical coupling (MOC) mechanism via two specially designed fiber tapers. During bending, the upstream non-adiabatic taper creates a curvature-dependent refractive index wedge, which steers the guided mode off-axis and shifts its entry point into the spherical MFPI cavity. This mechanism greatly enhances optical-path-difference (OPD) modulation efficiency and wavelength sensitivity. These effects are governed purely by bend-induced radial beam deflection through MOC, rather than by conventional cavity-length change. Despite an overall length of only ∼740 µm, the sensor achieves a remarkable curvature sensitivity (-15.24 nm/m^-1 over 0.42-1.13 m^-1) and intensity sensitivity (-15.31 dB/m^-1 for curvatures <0.42 m^-1), which are one to two orders of magnitude higher than those of conventional single-cavity Fabry-Pérot sensors. Furthermore, the sensor exhibits negligible temperature cross-sensitivity and good repeatability, making it well-suited for high-precision, real-time curvature monitoring in confined or thermally variable environments (e.g., soft-robotic joints or steerable catheters).