PM fiber based sensing tapes with automated 45° birefringence axis alignment for distributed force/pressure sensing.

Polarization maintaining (PM) fibers can be used for distributed force/pressure sensing in which the birefringence axis of the PM fiber should preferably be oriented 45° from the direction of the force/pressure for the maximum sensitivity. However, it is a challenge to achieve such 45° axis orientation for a long length of PM fiber in practice. In this paper, we report the development of what we believe the first equipment and process for making PM fiber based sensing tapes, capable of automatically adjusting the fiber axis orientation 45° with respect to the tape surface. In particular, we develop a machine vision system with the ability of continuously determining fiber axis orientation in real time as the fiber passes by and feeding back the orientation information to a fiber rotation apparatus to automatically adjust its orientation before fixing the fiber on a transparent PET tape with UV epoxy. We show the results of a successfully fabricated 70-m-long PM fiber sensing tape achieving an axis orientation accuracy of 45 ± 3° throughout the whole length of the tape, which is further validated with a distributed polarization crosstalk analyzer (DPXA). Finally, we demonstrate distributed transversal load sensing with 14 force applying weights randomly distributed along the sensing tape using the DPXA, with a polarization crosstalk measurement uniformity of 0.62 dB (standard deviation) using the same applied weight of 100 grams. The same sensing tape can also be used for pressure sensing with properly designed fixtures.

Accurate method for measuring the thermal coefficient of group birefringence of polarization-maintaining fibers.

We present a method to accurately measure the group birefringence variation with temperature in high-birefringence polarization-maintaining (PM) fibers using a distributed polarization analyzer. By analyzing polarization cross-talk peaks purposely induced at both ends of a PM fiber, the temperature coefficient of group birefringence can be accurately obtained. We confirm the theoretical prediction that the group birefringence of PANDA and TIGER PM fibers decrease linearly with temperature from -40 °C to 80 °C, and find that the temperature coefficients are -5.93 × 10(-7) °C(-1) and -5.29 × 10(-7) °C(-1) for two types of PANDA fibers, and -5.36 × 10(-7) °C(-1) for a TIGER fiber.