Reduction of Bragg grating-induced coupling to cladding modes

We discuss fiber designs that have been suggested for the reduction of Brag g-grating induced coupling to cladding modes. The discussion is based on a theoretical approach that includes the effect of asymmetry in the W-induced index grating made by UV-side writing. Experimental results from gratings in a depressed-cladding fiber are compared with simulations. The model give s good agreement with the measured transmission spectrum and accounts for t he pronounced coupling to asymmetrical cladding modes, even when the gratin g is written with smallest possible blaze. The asymmetry causing this is ac counted for by the unavoidable attenuation of the UV light. It is found for the considered fiber designs that a high numerical-aperture fiber increase s the spectral separation between the Bragg resonance and the onset of clad ding-mode losses. A depressed-cladding fiber reduces the coupling strength to the lower order cladding modes, and the W-sensitive cladding design redu ces the cladding-mode coupling loss. The analysis suggests that the UV-sens itive cladding design is the most effective in reducing the cladding-mode-c oupling losses.