Use of Polarization-Optical Time Domain Reflectometry for Observation of the Faraday Effect in Single-Mode Fibers

Magnetooptic Faraday rotation in a strongly twisted singlemode fiber is measured using polarization-optical time domain reflectometry (POTDR) in which a light pulse of 904 nm wavelength makes a roundtiip along the fiber. The experimental results are in good agreement ,with the calculations, and the Verdet constant thus obtained is 0.86 X 10" min/G. cm at h = 904 nm. T HE Faraday rotation of polarization in single-mode fibers is of great interest because of its applicability to current sensing on high-voltage transmission lines [ 11 [4] . The quenching of the Faraday effect due to the linear birefringence, intrinsic or induced by bending, is greatly reduced by twisting the low birefringent fiber [3]. Yet, as the Verdet constant of silica fiber is very small, the single-mode fiber for a current sensor should be uniform and long enough for adequate sensitivity, which is somewhat troublesome. So, it is desirable to magnify the Faraday effect within the given length of the fiber. Usually, the magnetooptic current sensing has been accomplished by coupling CW light into a fiber and measuring the polarization state of transmitted light at the output. Recently, a new method for measuring the birefringence of single-mode fibers has been reported [5] -[7]. This is polarization-optical time domain reflectometry (POTDR), which is very promising in .various fields of single-mode fiber applications. We report here that POTDR is successfully applied for the first time to measure the Faraday rotation in a strongly twisted singlemode fiber, where the optical path is doubled along the fiber of fixed length. A strongly twisted single-mode fiber with low intrinsic birefringence is the most suitable one for current sensing because the unwanted linear birefringence is quenched. The twist induced circular birefringence acts as a constant bias for the Faraday rotation and does not affect the sensing [3]. In such a fiber, the Faraday effect rotates the polarization direction of linearly polarized light propagating along the fiber through an angle