Ultra-compact directional curvature sensor using an asymmetric fiber-optic microbubble Fabry-Perot interferometer

We present a fiber-optic directional curvature sensor utilizing an asymmetric microbubble Fabry-Perot interferometer (AMFPI). The device features a monolithically integrated sealed cavity embedded in a tapered optical fiber, with a waist diameter of 52 µm. Directional sensitivity originates from the asymmetrical displacement of reflection points on the internal spherical cavity surface, induced by asymmetric light incidence during bending. This displacement effectively modulates the optical path difference (OPD), resulting in distinct spectral shifts (redshift or blueshift) corresponding to a bending direction. Further enhancing directional sensitivity, radial wall thickness asymmetry in the microbubble waist is precisely engineered. The fabricated sensor, with an ultra-compact cavity length under 50 µm, achieves a high directional curvature sensitivity of 1.06 nm/m^-¹ over a curvature range of 0-4.35 m^-¹. Notably, the sensor exhibits negligible sensitivity to ambient refractive index variations and an exceptionally low thermal sensitivity of 1.09 pm/°C. These attributes render the sensor highly suitable for precise directional curvature detection under challenging environmental conditions, such as biological fluids and temperature fluctuations.