Proceedings PaperMeasurement of refractive index and stress birefringence in optical fibers and preforms
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Sevralmethods for testthg optical fibers and preforms recently developed by the author [1,2,3,4,5)will be rewieved in this presentation. The first presented method enables measurement of refracitve index profile, residual sUess birefngence, and stress components in axially symmelñcal fiber optic preforms. Dynamic spatial-filtering technique, which is used typically to determine deflection angle $(x) of optical rays passing through the tested preform, was modified to achive bifimctionality of measuring system. Further to a diaphragm rotating in a back focal planeofanobjective imaging the tested preform ( typical configuration of the dynamic spatialltering technique), a modulator of ellipticity is used in a beam iliuminatizg the tested preform which enables measurement ofoptical retardation R(x) introduced by the preform between axially and tiansversally polarized components ofiliuminaling beam. The system may be easily switched (with mechanical switches) from an operating mode enablig measurement of the ray deflection function (x) to an operating mode enablig measurement of the retardation function R(x). Knowing 4(x) and R(x) refractive index profile, residual stress birefringence, and stress components is calculated with the help ofinverse Abel transformation. Examples ofmeasurement results iflustarting the system perfomience are shown. Veiy similar concept is applied to determine refractive index profile, residual siress birefringence, and siress components in optical fibers. In this case, however, wavefront derivative dW(x)/dx (proportional to a ray deflection angle (x)) is directely measured in a scanning-type differentialing interferometcr with a Woliastone prism as a beam splitting element Vciy high Sensitivity requied in this measuring system is achived by applying a sinusoidal modulation of elliplicity of an illuminating beam and a hornodyne detection ofthe first harmonic component ofan output beam. The system can be SWitChed to measuring the retardation R(x) intrOduced by the tested fiber simply by removing the Wollaston prism. As in the pievious case all fiber characterisctics, i.e., refractive index profile, residual stress bireuiingence, and stress components, are obtained by computer pcocessing ofmeasured functions dW(x)/dx and R(x). A method for measuring modla birefringence and polarization mode dispersion in highly birefringent (IIR) fibers is also presented It employs a white.hght interference phenomenon arising between polaiization rriodcs as a results ofinode coupling irduced by pointlike force applied in approximately one halfthe length ofthe tested permits the use of a Wollaston prism without a delay line as an analyzing interferometer. Results of measurements of modal birefringence and polarization mode disperssion at ?826nm are reported for four commercially available fibers: the York Bow-Tie, the Fujikura Panda, the Andrew E-type, and the Andrew D4ype.