Polarization-resolved sensing with tilted fiber Bragg gratings: theory and limits of detection

TL;DR

This paper compares two polarization-based methods for sensing with tilted fiber Bragg gratings, demonstrating that the polarization dependent loss approach yields lower detection limits and better stability in refractometric sensing.

Contribution

It introduces a correction method for eigenvalue crossing artifacts and compares two polarization analysis approaches, highlighting the superior sensitivity of the PDL method.

Findings

PDL method achieves lower detection limits.

Eigenvalue crossing artifacts can be corrected.

Principal axes stability varies with wavelength.

Abstract

Polarization based sensing with tilted fiber Bragg grating (TFBG) sensors is analysed theoretically by two alternative approaches. The first method is based on tracking the grating transmission for two orthogonal states of linear polarized light that are extracted from the measured Jones matrix or Stokes vectors of the TFBG transmission spectra. The second method is based on the measurements along the system principle axes and polarization dependent loss (PDL) parameter, also calculated from measured data. It is shown that the frequent crossing of the Jones matrix eigenvalues as a function of wavelength leads to a non-physical interchange of the calculated principal axes; a method to remove this unwanted mathematical artefact and to restore the order of the system eigenvalues and the corresponding principal axes is provided. A comparison of the two approaches reveals that the PDL method provides a smaller standard deviation and therefore lower limit of detection in refractometric sensing. Furthermore, the polarization analysis of the measured spectra allows for the identification of the principal states of polarization of the sensor system and consequentially for the calculation of the transmission spectrum for any incident polarization state. The stability of the orientation of the system principal axes is also investigated as a function of wavelength.