0733-8724 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JLT.2014.2387291, Journal of Lightwave Technology Abstract— We propose a relatively simple technique to infer the birefringence on single mode low-birefringence optical fibers based on the use of the Faraday effect. The theoretical model for Faraday rotation in the presence of non-negligible fiber birefringence and a suitable measurement technique offer a fast and efficient way to determine low values of linear birefringence employing short fiber lengths. Alternative known techniques are not sensitive enough or they are of more difficult implementation. The method is used in only ~1 m-long single mode optical fibers to obtain the upper-limit birefringence that can be tolerated in order to retain good current sensing sensitivity. The temperature dependence of the Faraday rotation and its causes are also investigated. Index Terms— Low Birefringence Fibers, Current sensors, Faraday effect, Optical fiber sensors, Spun fibers. I. INTRODUCTION INGLE mode optical fibers with ultra low intrinsic birefringence are good candidates for current sensing applications [1], and high-speed optical networks [2] due to the low polarization mode dispersion. Reducing fibre birefringence and polarization mode dispersion relies on introducing controlled polarization mode coupling by fibre spinning [2, 3]. By spinning the fiber preform during fiber drawing, fiber nonuniformities become more homogenized along the fiber, resulting in a reduction of the unwanted polarization effects [3]. The reported average intrinsic birefringence of spun fibers is of the order of ∼10 -9 [3, 4]. Nevertheless, despite such low intrinsic birefringence of spun fibers, linear birefringence due to bend- and coating- induced stresses still exist in those fibers. In particular, such linear birefringence quenches the Faraday effect in current sensing applications [3-5]. To minimize this non-negligible induced birefringence, the fibers are usually annealed or twisted [6-8]. Notably, over the years the Faraday-effect-based current Manuscript received September 12, 2014; revised October 23, 2014 and December 3, 2014; accepted December 22, 2014. Date of publication January xx, 2015; date of current version January xx, 2015. We gratefully acknowledge support for this work from the Engineering and Physical Sciences Research Council (EPSRC) through funding for the EPSRC Centre for Innovative Manufacturing in Photonics. The authors are with the Optoelectronics Research Center of the University of Southampton, Southampton, SO17 1BJ, United Kingdom (e-mail: myss1d12@soton.ac.uk; ntv@orc.soton.ac.uk; nmw@orc.soton.ac.uk; tcms@soton.ac.uk; whl@orc.soton.ac.uk; frap@orc.soton.ac.uk; m.n.zervas@soton.ac.uk). sensors have become commonly used with spun high- birefringent fibers, when performance, accuracy and thermal stability is critical. Despite the fact that the current sensors industry standard is based on the spun elliptically birefringent fibers, there is still interest in the development of low linear birefringent fibers, mainly due to the ease and low cost of fabrication and low polarization mode dispersion [9]. However, birefringence characterization of such spun low birefringence fibers is an important challenge. Up to date, numerous techniques have been demonstrated including the twist method [10], the Lyot-Sagnac interferometer [11], the polarization sensitive optical time domain reflectometry (OTDR) [12] and the Brillouin OTDR (BOTDR) [13]. However, majority of measurement techniques lack sensitivity and usually need long fiber lengths. For example, in Ref. [10], a birefringence value of 3.8×10 -7 was determined at a wavelength of 0.725 μm with 10% accuracy and meter-length fibers. The technique based on the Lyot-Sagnac interferometer [11] is limited by the birefringence – length product of the fiber in the 10 -3 – 10 -6 m range. Both techniques require longer lengths to determine ultra-low birefringence values. When OTDR based techniques are considered, the main limitation is that it requires km-long fibers and the spatial resolution is limited to 0.5 – 0.6 m [12, 13]. Recently, we have proposed a new technique to provide an insight into the birefringence of the fibers, including spun low birefringence fibers [9]. Using a simple Faraday effect experiment and a corresponding theoretical model of the Faraday rotation in a birefringent fiber we have shown that this can be used to infer the intrinsic birefringence of the fiber. In particular, the upper limit of the total birefringence that is permitted to retain sensor sensitivity can be determined. This method is simple, fast and requires short lengths of fiber, and is potentially applicable to a wide variety of fibers. In this paper, we use the same model to extend the analysis and investigate the thermal properties of the Faraday effect in low- birefringence fibers. The results demonstrate the detrimental effect of the fiber coating to the sensor temperature dependence. We focus on the means to address these issues, as we anticipate that they can potentially be of the greatest impact. II. FARADAY EFFECT SETUP The Faraday effect is a phenomenon which arises from the interaction between a magnetic field and the light propagated through a medium. It causes the rotation of the plane of Low birefringence measurement and temperature dependence in metre-long optical fibers Martha Segura, Natasha Vukovic, Nicholas White, Tim May-Smith, Wei H. Loh, Francesco Poletti and Michalis N. Zervas S