What are different types of Laser used for satellite communication? Explain acquisition link model for optical communication.

Q 1] What are different types of Laser used for satellite communication? [5 Marks]

Answer: Laser sources used for optcal transmission ar Gas laser, Solid state laser, Semiconductor laser.

Lasers are low power devices used in array to increase output, having various advantages than other optical sources such as small size, weight, high efficiency, reliability and easily modulated.

No.	Laser Type	Wavelength	Average output power	Efficiency	Characteristics
1)	Nd-YAG	1.06 µ	0.5 - 1W	0.5 - 1%	Requires elaborated modulation, diode or solar pumping 10,000 Life Hours
2)	Crystal	0.532 µ	100 mW	0.5 - 1%	-
3)	GaAs	0.8 -0.9 µ	40 mW	5 - 10%	Reliable, small, rugged, directly and easily modulated. 5000 Life Hours.
4)	CO2	1.06 µ	1.2 W	10 - 15%	Life hours 20000, used in IR range, detectors are poor, uses discharge tube.
5)	He-Ne	0.63 µ	10 mW	-	Life Hours 50000

Q 2] Explain acquisition link model for optical communication. [5 Marks]

Answer: Acquisition Link Model:

1. Link aquisition in optical laser communication requires searching an uncertain area to locate and establish link between satellite.
2. Acquisition is accomplished by using a pilot signal and detectable receiver. The receiving system must be in line of sight distance.
3. Satellite optical link work as free space optical link which transmits an optical signal through atmosphere or deep space.
4. Optical link is concentrated on one or more narrow beams. The narrow beam gaurantee high special selectivity so there is no interference with other link.
5. The main advantages of Acquisition Link Model are: Higher frequencies, higher bandwidth, smaller beam divergence, smaller antenna and high data rates.
6. The main disadvantages of Acquisition Link Model are: Free space optical link requires Line of Sight between transceiver, Bird and Scintillation(sparkle of light) cause beam interruption.