Compact fiber curvature and temperature sensor inscribed by femtosecond laser through the coating

An integrated structure in the original single mode fiber (SMF) which can measure curvature and temperature simultaneously is proposed and demonstrated. The key components of the device include an in-fiber Mach-Zehnder interferometer (MZI) and a fiber Bragg grating (FBG), which are integrally fabricated in a coating-unstripped SMF by femtosecond laser direct writing. The curvature sensing of the device is based on the wavelength shift of the MZI interference fringe dips. It is found that the bending response is closely related to the bending orientation. The measured curvature sensitivities are 2.324 nm/m $^{-1}$ and 2.491 nm/m $^{-1}$ for the 0° and 180° orientation in the range of 0 - 10 m $^{-1}$ , respectively. Under different bending states, the temperature response of the FBG resonant wavelength is almost the same and the mean temperature sensitivity is 9.72 pm/°C. The advantages of compact, simple fabrication, low insertion loss, no assembly, wide operating range of curvature measurement and real-time temperature sensing make our device very attractive in practical engineering monitoring and somatosensory posture recognition.