A new measurement technique for polarization-based optical-fiber accelerometers

Several studies report the use of optical fibers for vibration measurements. Among the different light properties, the polarization state is interesting as it leads to simple sensor. The principle of such a sensor is the following: Polarized light is sent into an optical fiber. A mechanical transducer submitted to the vibration deforms the fiber,inducing birefringence variation which modifies the state of polarization. This polarization variation gives a measure of the acceleration. However it is difficult to have stable measurements with such a system. The sensor sensitivity depends on the one hand on the intrinsic fiber birefringence, which is unknown, on the other hand on the initial light polarization state launched into the fiber. The purpose of this article is to propose a measurement technique where the sensitivity does not depend anymore on the intrinsic birefringence nor on the initial polarization state. Three different polarization states are successively sent into the fiber and for each of them the output state is measured. This allows to compute the vibration induced birefringence variation and then the acceleration. The first part of this article presents the theoretical developments at the basis of the measurement. In the second part experimental results are presented. A polarization controller modifies the light polarization state before the sensor. Different measurements are performed. With the previous technique the sensitivity goes from nearly zero to a maximal value while with our technique the difference between the lowest and highest sensitivities is smaller than 4%.