<title>Ultralow-birefringence measurement in optical fibers by use of a fiber optical loop mirror</title>

Abstract

ABSTRACTWe describe a new method for measuring ultralow-birefringence in fibers. It is based on the fiber twistin a fiber optical loop mirror. The method allows birefringence measurement within the range from106 to 108,SUMMARY The most ofthe experimental techniques proposed to measure fiber bireflingence use a section of a fiber with a length of several beat lengths [1,2J. These techniques are convenient for high-birefringent fibers, but it isdifficult to use them for low-birefringent fibers. Recently the proposal to use a twist method for low- birefringent fibers has been published [3]. High accuracy of this technique was demonstrated but it requires aquite difficult procedure ofthe adjustment ofpolarizers and principal axes ofthe fiber.It is well known that a Fiber Optical Loop Mirror (FOLM) does not require polarized light [5] at an input. Thisfeature has been used for polarization independent sensors and WDMs [4,5]. Polarization independence maysimplify drastically the adjustment when the FOLM is used for polarization measurements. in this report weshow for the first time that the FOLM consisted from a coupler with output ports connected by the fiber to bemeasured provides a very simple and accurate measurement of the birefringence of low-birefringent fibers.Fig.!. Experimental setup.Fig.! presents the experimental configuration. A laser beam from a cw Nd:YAG laser is splitted by a