Optical fiber sensing technology based on Mach-Zehnder interferometer and orbital angular momentum beam

An optical fiber sensing and signal demodulation technology based on Mach-Zehnder interferometer (MZI) and orbital angular momentum (OAM) beam was proposed in this work. One path of the MZI containing only single mode fiber (SMF) emits Gaussian beam, and the other path which contains a segment of two-mode fiber (TMF) generates OAM beam. The OAM beam was generated by offset splicing a segment of TMF with SMF, and then rotating the TMF by using a customized fiber rotator. The interference between the Gaussian beam and the OAM beam generates spiral interference pattern, which can rotate with the change of phase difference between these two beams. A segment of SMF in the Gaussian beam path was used as a sensing fiber, which was placed on a thermostat. When the temperature changes, the phase difference between the Gaussian beam and the OAM beam was changed owing to the thermo-optic effect and thermal expansion effect of the sensing fiber, resulting in the rotation of spiral interference pattern. An optical phase difference demodulation method based on spiral interference pattern feature extraction technology was proposed to demodulate optical phase difference between these two beams. Experimental result shows that the sensing system has a linear temperature measurement sensitivity of 12.67 rad/°C. The temperature measurement resolution is 0.0000122 °C in theory and about 0.005 °C achieved in experiment.