A fiber Mach-Zehnder interferometer based on waist-enlarged fiber taper and graded-index multimode fiber

An in-fiber Mach-Zehnder interferometer (MZI) based on waist-enlarged fiber taper and graded-indexmultimode fiber is proposed and demonstrated in this paper. The sensor head is fabricated by fusion splicing a 30 mm graded-index multimode fiber with a fiber taper, and the distance between them is 25 mm, thus forming SMF-taper-SMF-GI MMF-SMF fiber structure. The optical fiber taper can increase cladding mode energy, gradient multimode optical fiber serves as sensing arm, and the changes of the ambient temperature, refractive index and the axial stress will make the sensor core mode and cladding mode of optical path difference varied, causing interference sensor spectral change. Monitoring the spectra variation can determine the ambient physical parameters. The experimental results show that the temperature sensitivity of the sensor is 78.6 pm/℃ with a linearity of 0.997 in the water temperature range of 30-89℃; the refractive index sensitivity is 81.48 dB/RIU with a linearity of 0.989 in the refractive index range of 1.333-1.394 RIU; the strain sensitivity is 0.33×10^-2 dB/με with a linearity of 0.998 in the strain range of 0-933.3 με. The sensor has advantages of simple fabrication, low cost, simultaneous measurement of multiple physical parameters.