Fig. 1. Rectangular grating and attached coordinate system

Fig. 2. Four types of symmetric mode profiles with grating parameters of d=1965 nm, c=g=932.5 nm, nc=1.4496, ng=1 and λ=1310 nm. (a) (b) First two modes under incidence of the first order Bragg angle; (c) (d) first two modes under incidence of the second order Bragg angle

Fig. 3. Graphs of four functions under grating parameters of d=1979 nm, c=1385 nm, g=594 nm, nc=1.4496, ng=1 and λ=1064 nm. (a) f1(neff2); (b) f2(neff2); (c) f3(neff2); (d) f4(neff2)

Fig. 4. Effective refractive index of the first three modes obtained by two methods versus duty ratio under grating parameters of d=1500 nm, nc =1.4496, ng=1 and λ=1064 nm