A novel directional curvature sensor based on a hybrid method: FBG and V-grooved polymer fiber

Conventional fiber Bragg grating (FBG) sensors can measure curvature magnitude but cannot detect bending direction. Conversely, polymer optical fiber (POF) sensors with structured surfaces can detect direction but lack spectral resolution. Thus, there is a need for a compact sensing system that can measure both curvature magnitude and direction simultaneously. This study presents a hybrid optical sensor that combines a spiral-wound FBG with a V-grooved POF to achieve dual-parameter sensing. A single-mode photosensitive FBG was inscribed using the phase mask method and wrapped around a PMMA-based POF with pre-formed V-grooves. Curvature was applied bi-directionally over a range of 0-14.5 m-1, and four sensor configurations were analyzed using spectral shift and analog voltage readings. The hybrid sensor demonstrated stable curvature sensitivity of approximately 86.6 pm/m-1, enabling reliable direction discrimination. Concave bending produced analog voltages consistently above 2 V, while convex curvature yielded values below this threshold. Unlike FBG-only structures, the proposed design provides dual-mode output with high repeatability and minimal spectral noise under repeated loading. This compact sensor simultaneously detects curvature and direction without requiring multilayer alignment or metal coatings. Its analog compatibility and structural simplicity make it ideal for realtime, multidimensional monitoring in structural health, wearable technologies, and robotics applications.