Chirped and Step-Chirped Gratings 1550.0 0 ­5 Reflectivity, dB ­10 ­15 ­20 ­25 ­30 Wavelength, nm C D 15 5 ­5 1550.2 B 1550.4 A 1550.6 1550.8 1551.0 45 (delay), ps 35 25 317 Figure 7.14 Comparison between the delay and reflection spectra of asymmetrically (B, D), and symmetrically (A, C) apodized gratings. the rear end of the grating, so that the ripple reduces still further. However, the curvature also increases. Figure 7.14 shows the reflection and group delay dif- ference spectra of a symmetric and an asymmetric tanh apodized grating. The bandwidth of the asymmetric apodized grating, B, is now wider than that of the symmetric apodised grating, A, and so is the group delay difference, D. The peak reflectivity of B is 98%, whereas that of A is 90%. There is also a