**a. Transmitter**

Transmitter transforms the electrical signal to an optical signal and it modulates the laser beam to transfer carrying data to the receiver through the atmosphere channel. The transmitter consists of four parts as shown in Fig. (2): laser modulator, driver, optical source and transmit telescope.

## **Laser modulator**

Laser modulation means the data were carried by a laser beam. The modulation technique can be implemented in following two common methods: internal modulation and external modulation [2].

**Internal modulation:** is a process which occurs inside the laser resonator and it depends on the change caused by the additive components and change the intensity of the laser beam according to the information signal.

**External modulation:** is the process which occurs outside the laser resonator and it depends on both the polarization phenomena and the refractive dualism phenomenon.
