Free space laser communications systems are wireless connections through the atmosphere. They work similar to fiber optic cable systems except the beam is transmitted through open space. The carrier used for the transmission of this signal is generated by either a high power LED or a laser diode.
Description of Free Space Laser Communications
The laser systems operate in the near infrared region of the spectrum. The laser light across the link is at a wavelength of between 780 - 920 nm. Two parallel beams are used, one for transmission and one for reception.There are three basic steps to laser communication: acquisition, tracking, and communications. Of the three, acquisition is generally the most difficult; angular tracking is usually the easiest.
Communications depends on bandwidth or data rate, but is generally easier than acquisition unless very high data rates are required. Acquisition is the most difficult because laser beams are typically much smaller than the area of uncertainty. Satellites do not know exactly where they are or where the other platform is located, and since everything moves with some degree of uncertainty, they cannot take very long to search or the reference is lost.
Instability of the platforms also causes uncertainty in time. In the ideal acquisition method, the beam width of the source is greater than the angle of uncertainty in the location of receiver. The receiver field of includes the location uncertainty of the transmitter. Unfortunately, this ideal method requires a significant amount of laser power.It is possible to operate a number of laser types at high peak power and low duty cycle to make acquisition easier.
This is because a lower pulse rate is needed for acquisition than for tracking and communications.