Femtosecond Laser Inscribed In-Fiber Mach–Zehnder Interferometer of Cladding Waveguide and Fiber Bragg Grating for Simultaneous Measurement of Twist Angle and Temperature

To realize twist angle sensing with resistance to electromagnetic interference, ease of fabrication, and temperature measurement simultaneously applying in engineering and medical fields, a miniature in-fiber cladding waveguide Mach-Zehnder interferometer (CWMZI)-fiber Bragg grating (FBG) sensor for twist angle and temperature measurement was proposed and fabricated. Using femtosecond (fs) laser point-to-point inscribing technology, a waveguide coupling with the fiber core is inscribed in the single-mode fiber (SMF) cladding to form the MZI, and an FBG is introduced at the fiber core for temperature detection. When the transmission intensity of the MZI interference dip is monitored, the CWMZI-FBG structure is temperature-insensitive. A twist angle sensitivity of 0.1702 dB/(rad/m) with the linearity of 0.994 was achieved in the experiment. Meanwhile, if wavelengths of the transmission dips of the CWMZI and FBG are monitored, both twist angle and temperature can be measured simultaneously.