Effective-index increase, form birefringence and transition losses in UV-side-illuminated photosensitive fibers.

We present a simple variational method for the analysis of UV-side-illuminated single-mode fibers. We use a shiftable, two-dimensional elliptical Gaussian function as a field trial for the fundamental-mode. By this method, the actual UV-induced peak refractive-index increase can be quantitatively related to the measured effective-index increase. The asymmetry of the UV-induced refractive-index profile due to the absorption of the writing UV light causes both form birefringence and transition losses between UV-illuminated and non-illuminated fiber sections. These characteristics are easily calculated from the results of the variational method. We show that UV-illumination of the fiber from two opposite sides reduces both the form birefringence and the transition losses.