| 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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