Fiber curvature sensor based on spherical-shape structures and long-period grating

• A curvature sensor based on optical fiber Mach–Zehnder interferometer is demonstrated.
• A dip in the spectrum presents the curvature response with larger scale.
• The best curvature sensitivity is −28.225 nm/m−1 in the range of 6.93–8.43 m−1.
• Temperature cross sensitivity is so small that it can be ignored.
• This curvature sensor has the merits of good robustness, high sensitivity.

A novel curvature sensor based on optical fiber Mach–Zehnder interferometer (MZI) is demonstrated. It consists of two spherical-shape structures and a long-period grating (LPG) in between. The experimental results show that the shift of the dip wavelength is almost linearly proportional to the change of curvature, and the curvature sensitivity are −22.144 nm/m−1 in the measurement range of 5.33–6.93 m−1, −28.225 nm/m−1 in the range of 6.93–8.43 m− and −15.68 nm/m−1 in the range of 8.43–9.43 m−1, respectively. And the maximum curvature error caused by temperature is only −0.003 m−1/°C. The sensor exhibits the advantages of all-fiber structure, high mechanical strength, high curvature sensitivity and large measurement scales.