Abstract

Undesirable losses in highly reflective fiber Bragg gratings due to coupling to bound cladding modes or lossy radiation modes are well documented¹-⁵. Several approaches have been proposed for reducing this coupling, including high LP₀₁ mode confinement fibers (high numerical aperture single-mode fibers⁶ and two-mode fibers⁷), fibers with equally photosensitive core and cladding regions⁸, and, recently, fibers with wide depressed cladding regions⁵. However, the solution detailed by Komukai, et al.⁶ is undesirable due to mode-field mismatch losses when splicing to conventional single-mode telecommunication fibers operated at 1550 nm (e.g, Corning SMF-28) and the solution of Okude, et al.⁷ requires advanced fabrication techniques. Additionally, while the solutions detailed by Dong, et al.⁵ and Delevaque, et al.⁸ reduce cladding mode losses for gratings with no variation of the photoinduced index transverse to the fiber axis, it is well known that strong asymmetries in the transverse photoinduced index profile can occur in Type I⁹ and Type II¹⁰ gratings due to the sidewriting process. In fact, elaborate writing schemes have been proposed to reduce this asymmetry¹¹-¹². Additionally, in practice, small tilts of the grating fringe planes can be introduced during fabrication due to mask-fiber misalignment. It is demonstrated in this paper that the solutions detailed by Dong, et al.⁵ and Delevaque, et al.⁸ are not optimum for azimuthally asymmetric transverse photoinduced index distributions, and a new narrow depressed cladding fiber design that has superior loss suppression characteristics for moderate grating asymmetries is presented.

© 1997 Optical Society of America