“Optics rule: everything seen from a distance look larger”
Extract from Nouvelles pensées échevelées,
Stanislaw Jerzy Lec (1909–1966)
We discuss the “physical layer” of wireless optical terminals in this chapter. The digital communication part and the communication protocol part will be discussed in Chapter 10 (OSI layers 1 and 2 and the TCP/IP layer 1).
The other layers are application oriented and will not be considered in this book.
The transmitter and receiver components of this section are components of very small dimensions (volumes below 1 mm3) processed by deposition of thin films on semiconductor materials.
A crystal of semiconductor material is a poor conductor. The electrons which are the charge carriers are not free to move within the material when they occupy energy states in the low-energy valence band. They can only contribute to the current flow when they get enough energy in excess of the bandgap and are free in the conduction band as shown in Figure 8.3.
This excess in energy needed to gain kinetic energy and be admitted into ...