Phase and Temporal Response of Bragg Gratings 429 uniform Gaussian intensity apodized chirped apodized Figure 9.26 Four commonly encountered grating refractive index modulation profiles. The assumption of Eq. (9.3.19) restricts the application of this technique to gratings with slowly varying chirp by a variation of the grating period and to average refractive index (dl/L < 1 À3 ), with no saturation in Dn. The chirp is limited by the numerical aperture of the focused beam. Figure 9.26 shows the refractive index modulation profiles of four gratings commonly encountered in Bragg grating technology. 9.3.2 Measurement of Internal Stress