TFBGs correspond to a refractive index modulation angled by a
few degrees relative to the perpendicular to the propagation axis
(inset of Fig. 1). In addition to the self-backward coupling of the
core mode at the Bragg wavelength, TFBGs redirect some light to
the cladding whose diameter is so large that several tens of
cladding modes can propagate, each with its own effective refractive
index neff,clad. The phase matching condition for the ithcladding mode resonance is given by λclad,i¼(neff,coreþneff,clad,i)Λ