Immersionless single-mode preform index profiling.
Refractive-index profiles of optical fibers and their preforms can be measured by the focusing method which, in its original form, required that the preform (or fiber) be immersed in index-matching fluid to ensure that incident, collimated light reaches its core undistorted. Since immersion of the preform may be undesirable, we propose to keep the preform dry and obtain the required collimated light by means of a cylindrical matching lens. For simplicity, we are using homogeneous cylindrical glass rods as matching lenses and have achieved excellent agreement between index profiles obtained by the new technique and by the conventional focusing method employing index-matching fluid. The new technique works well with single-mode preforms, but it may not be applicable to index profiling of multimode preforms. This limitation arises for two reasons: (1) The requirement that the light beam be collimated inside the preform limits the region that is accessible to a light beam coming from the outside. (2) Aberrations of the matching lens (correctable to some extent) restrict the region inside the preform over which light collimation can be achieved by optical means. In practice, only half of the central preform cross section is accessible to the focusing method if a matching lens replaces the index-matching fluid. Where applicable, the new technique eliminates the possibility of damaging the surfaces of the preforms during the cleaning operation. Damaged preform surfaces could result in fibers with reduced tensile strength. In addition, the new technique can provide information on special preforms with high refractive index or long operating wavelengths for which matching fluids may not be available.
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