Modal field deformation and transition losses in UV side-written optical fibers

The asymmetry of the UV-induced refractive-index profile in photosensitive optical fibers causes a deformation of the modal fields, resulting in transition losses between UV-exposed and unexposed fiber sections up to 0.1 dB for UV-induced index changes of the order of ≤10-2. A numerical analysis of the transition losses is developed. It is verified experimentally that the loss is caused by coupling of power of the fundamental mode to cladding modes at such a transition.