Investigation of Differential Induced Attenuation of Orthogonal Axes in Polarization Maintaining Fibers
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INVESTIGATION OF DIFFERENTIAL INDUCED ATTENUATION OF ORTHOGONAL AXES IN POLARIZATION MAINTAINING FIBERS M.E. GINGERICH,* S.J. HICKEY,** C.C. HARRINGTON,** M.J. MARRONE,* E.J. FRIEBELE,* L.D. LOONEY,*** and J.R. ONSTOTT**** * Naval Research Laboratory, Washington DC 20375 ** SFA, Inc., Landover, MD 20785 *** Los Alamos National Laboratory, Los Alamos, NM 87545 **** 3M Center, St. Paul, MN 55144 ABSTRACT Recent studies have indicated that the birefringent-inducing stress of polarization-maintaining (PM) fibers decreases the long term, permanent loss induced by ionizing radiation' and that light polarized along the two orthogonal axes of PM fibers may be attenuated differently by exposure to irradiation sources. 2 This paper reports the results of specific studies of this differential attenuation induced in a series of PM fibers by both steady state and transient irradiations. It has been found that the response to ionizing radiation depends on the materials properties of the fiber, i.e. the core and clad dopants and/or degree of stress, and that the magnitude of the differential attenuation is small relative to the total or one-axis incremental loss. INTRODUCTION Many applications of optical fibers require the ability to maintain the input polarization state, e.g. in coherent communications, interferometric sensors, and fiber optic 3 gyroscopes. Since some of these may also require that the fibers operate in a radiation environment, it is necessary to characterize the degradation of their polarization-maintaining ability upon exposure. Several studies have documented the radiation response of PM fibers,1 2' 4 ' 5 but most' 4' 5 have studied the total induced attenuation without differentiating the orthogonal axes' response. One study4 evaluated the effect of 60Co radiation on beat length but found no effect up to 6 x 10s rads. The PM fibers in these previous studies exhibited varying degrees of recovery of the radiation-induced attenuation following exposure. In particular, when the recovery data of a set of unstressed and symmetrically-stressed fibers were fit to nth order kinetic behavior (see ref. 1 and eq. 1 of ref 6), the highly stressed PM fibers had the lowest levels of permanent induced loss, Af. This result suggested the possibility that the differential stress on the two orthogonal axes of a PM fiber might cause differential permanent incremental loss. Recent data of Taylor et al. 2 appear to support this hypothesis: in this study a 1.3 pm PM fiber irradiated with a 200 Mev proton source showed differential induced loss for the two orthogonal axes of about 160 dB/km after a total exposure of 18 krad. These results suggest that the incremental loss of both axes of these fibers needs to be individually characterized to determine if there is an optimum axis to be used in radiation environments and if changes in fiber manufacturing process or dopants can improve their radiation hardness. Mat. Res. Soc. Symp. Proc. Vol. 172. ©1990 Materials Research Society
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