Laser communications offer a viable alternative to RF communications for
inter satellite links and other applications where high-performance links are
a necessity. High data rate, small antenna size, narrow beam divergence, and a
narrow field of view are characteristics of laser communications that offer a
number of potential advantages for system design.
have been considered for space communications since their realization in 1960.
Specific advancements were needed in component performance and system 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 viable but also an attractive approach into inter
satellite link applications.
is driving the requirements to higher data rates, laser cross -link technology
explosions, global development activity, increased hardware, and design maturity.
Most important in space laser communications has been the development of a reliable,
high power, single mode laser diode as a directly modulable laser source. This
technology advance offers the space laser communication system designer the flexibility
to design very lightweight, high bandwidth, low-cost communication payloads for
satellites whose launch costs are a very strong function of launch weigh. This
feature substantially reduces blockage of fields of view of most desirable areas
on satellites. The smaller antennas with diameter typically less than 30 centimeters
create less momentum disturbance to any sensitive satellite sensors. Fewer on
board consumables are required over the long lifetime because there are fewer
disturbances to the satellite compared with heavier and larger RF systems. The
narrow beam divergence affords interference free and secure operation.
communication systems offer many advantages over radio frequency (RF) systems.
Most of the differences between laser communication and RF arise from the very
large difference in the wavelengths. RF wavelengths are thousands of times longer
than those at optical frequencies are. This high ratio of wavelengths leads to
some interesting differences in the two systems. First, the beam-width attainable
with the laser communication system is narrower than that of the RF system by
the same ratio at the same antenna diameters (the telescope of the laser communication
system is frequently referred as an antenna). For a given transmitter power level,
the laser beam is brighter at the receiver by the square of this ratio due to
the very narrow beam that exits the transmit telescope. Taking advantage of this
brighter beam or higher gain, permits the laser communication designer to come
up with a system that has a much smaller antenna than the RF system and further,
need transmit much less power than the RF system for the same receiver power.
However since it is much harder to point, acquisition of the other satellite terminal
is more difficult. Some advantages of laser communications over RF are smaller
antenna size, lower weight, lower power and minimal integration impact on the
satellite. Laser communication is capable of much higher data rates than RF.
laser beam width can be made as narrow as the diffraction limit of the optic allows.
This is given by beam width = 1.22 times the wavelength of light divided by the
radius of the output beam aperture. The antennae gain is proportional to the reciprocal
of the beam width squared. To achieve the potential diffraction limited beam width
a single mode high beam quality laser source is required; together with very high
quality optical components throughout the transmitting sub system.
antennae gain is restricted not only by the laser source but also by the any of
the optical elements. In order to communicate, adequate power must be received
by the detector, to distinguish the signal from the noise. Laser power, transmitter,
optical system losses, pointing system imperfections, transmitter and receiver
antennae gains, receiver losses, receiver tracking losses are factors in establishing
receiver power. The required optical power is determined by data rate, detector
sensitivity, modulation format ,noise and detection methods.
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