Femtosecond laser-inscribed parallel cladding and core fiber Bragg gratings for the dual-parameter measurement of twist and temperature

To enable simultaneous miniature dual-parameter sensing of twist and temperature at the same position, we propose and demonstrate an in-fiber parallel cladding and core fiber Bragg grating (PCCFBG) structure. The sensor was fabricated using femtosecond laser point-to-point inscription technology into a single-mode fiber: first, a cladding waveguide coupled to the core was inscribed and followed by the parallel inscription of two Bragg gratings at the same axial position-one in the fiber core and the other in the cladding waveguide. By simultaneously monitoring the reflection intensity of the cladding waveguide FBG and the central wavelength shift of the core FBG, twist and temperature can be measured concurrently. The sensor achieved twist rate sensitivities of 0.467 and -0.481dB/(rad/m) over a range of ±14rad/m, along with a temperature sensitivity of 0.012 nm/°C at 30-60°C.