Stress Direction Measurement Based on Polarization State in Optical Fibers Using the Quaternion Method

The state of polarization (SOP) in a polarization-maintaining fiber subjected to external stress is studied using the quaternion method. The stress principal axis on the Poincare sphere is independent of intrinsic birefringence, but has a linear relationship with the direction of external stress. The above linear relationship, in turn, enables a measurement of the external stress's direction by calculating the consecutive trajectory of SOP on the Poincare sphere. This method is proposed and experimentally validated by measuring different types of fibers with maximum angle measurement errors of less than 2.24%.