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.
