Sensing Characteristics of Polarization-Dependent Helical Long-Period Grating-Based Interferometer

Abstract

We demonstrate an all-fiber Mach-Zehnder interferometer (MZI) by cascading a pair of helical long-period gratings (HLPGs) inscribed in a Panda-type polarization-maintaining fiber (PMF) using a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -laser. By cascading two grating with different lengths, a serial of polarization-dependent MZIs with different free spectral ranges have been demonstrated. The maximum polarization-dependence loss of the MZI was measured to be more than 35 dB. Two orthogonal interference spectra corresponding to fast axis mode and slow axis mode can be observed. The sensing characteristics of the polarization-dependent MZI we investigated experimentally. For liquid level sensing, the maximum wavelength shifts of the fast and slow axis interference dips were measured to be 11.03 and 3.25 nm when the surrounding refractive index is 1.424 and the interference length is 12.0 cm, respectively. The fast and slow axis interference dips have good linearity and opposite response with the temperature and strain change, the measured temperature and strain sensitivities of the fast and slow axis interference dips were 103.43 pm/°C, −31.26 pm/°C and −2.13 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , 1.68 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , respectively. Based on the high polarization extinction ratio, narrow resonant bandwidth, and low insertion loss, the demonstrated MZI has potential application in polarization-dependent multi-parameter sensing or multi-channel polarization filter.