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Free Space Laser Communications

Definition
Lasers have been considered for space communications since their realization in 1960. However, it was soon recognized that, although the laser had potential for the transfer of data at extremely high rates, specific advancements were needed in component performance and systems engineering, particularly for space-qualified hardware. Advances in system architecture, data formatting, and component technology over the past three decades have made laser communications in space not only a viable but also a attractive approach to intersatellite link applications. The high data rate and large information throughput available with laser communications are many times greater than in radio frequency (RF) systems.

The small antenna size requires only a small increase in the weight and volume of host vehicle. In addition, this feature substantially reduces blockage of fields of view of the most desirable areas on satellites. The smaller antennas, with diameters typically less than 30cm, create less momentum disturbance to any sensitive satellite sensors. Fewer onboard consumables are required over the long lifetime because there is less disturbance to the satellite compared with larger and heavier RF systems. The narrow beam divergence of affords interference-free and secure operation.

Features Of Laser Communications System
A block diagram of typical terminal is illustrated in Fig 1. Information, typically in the form of digital data, is input to data electronics that modulates the transmitting laser source. Direct or indirect modulation techniques may be employed depending on the type of laser employed. The source output passes through an optical system into the channel. The optical system typically includes transfer, beam shaping, and telescope optics. The receiver beam comes in through the optical system and is passed along to detectors and signal processing electronics. There are also terminal control electronics that must control the gimbals and other steering mechanisms, and servos, to keep the acquisition and tracking system operating in the designed modes of operation.

Operation
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. 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.

Acquisition And Tracking
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. High peak power pulses more easily overcome the receiver set threshold and keep the false alarm rate low. A low duty cycle transmitter gives high peak power, yet requires less average power, and is thus a suitable source for acquisition. As the uncertainty area becomes less, it becomes more feasible to use a continues source of acquisition, especially if the acquisition time does not have to be short.


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