Analysis of Narrow Depressed-Cladding Fibers for Minimization of Cladding and Radiation Mode Losses in Fiber Bragg Gratings

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.