Properties of Uniform Bragg Gratings 151 It is also useful to note the approximate position of the side-lobe peaks at 1 aL ¼ i M þ p; M ¼ 1; 2; 3; . . . ; ð4:6:19Þ 2 which leads to l 2 pn eff v ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 2 u 2 u 1 u M þ p u 6 2 7 u k ac k à þ 6 7 : ac t 4 5 L dl sl ¼ Æ ð4:6:20Þ 4.6.1 Phase and Group Delay of Uniform Period Gratings Figure 4.12 shows the phase response of the two gratings in Fig. 4.11, as a function of detuning, in units of radians per meter of grating. In the region out- side of the band stop of the grating, the phase of the light changes according to the unperturbed material refractive index. Into the band stop, the phase velocity slows down with increasing strength of the grating. In Fig. 4.13 is the group delay of the same grating as in Fig. 4.12 in units of psec/m. This group delay per meter is