Characterization of UV-induced birefringence in photosensitive Ge-doped silica optical fibers

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Abstract
The anisotropic nature of the UV-induced index change achieved in photosensitive optical fibers is characterized by a direct measurement of the induced birefringence in three different fibers. The birefringence, which grows with UV exposure proportionally to the total induced index change, is measured to be as small as 0.2% and as large as 8% of the total index change, depending on the fiber type and the exposure geometry. Such large values of birefringence are especially notable, since they can lead to substantial resonant effects such as polarization-dependent reflectivity in a fiber Bragg grating. The birefringence is predominantly determined by the polarization of the UV irradiation. We demonstrate an order-of-magnitude reduction of birefringence in an actual fiber phase grating by rotating the UV polarization to lie in the plane of the irradiating beams and the fiber axis.