It can be seen, from Fig. 2, the absorption edge locates at the bandgap. The absorption

It can be seen, from Fig. 2, the absorption edge locates at the bandgap. The absorption coefficient is small in the transparent region away from the absorption edge, such as 1.55 !liD. A low-loss material is possible in this case. On the other hand, the change of refractive index is large near the absorption edge. So one can get a small absorption and large index change at certain optical frequency (wavelength). It is possible to get an optimisation of the absorption edge by designing the semiconductors bandgap so that the required operating wavelength (1.55 11m) falls in the transparency region.