Abstract :
Laser communications
offer a viable alternative to RF communications for intersatellite links and other
applications where high-performance links are necessary. High data rate, small antenna
size, narrow beam divergence, and a narrow field of view are characteristics of
laser communication that offer a number of potential advantages for system design.
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. The narrow
beam divergence of affords interference-free and secure operation.
Free space laser
communications systems are wireless connections through the atmosphere. They work
similar to fibre 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.
SYSTEM CHARACTERISTICS
AND DESCRIPTION
The key system
characteristics which when quantified, together gives a detailed description of
a laser communications system. These are identified and quantified
for a particular application.
The critical parameters are grouped into five major categories: link, transmitter,
channel, receiver, and detector parameters.
LINK PARAMETERS
The link parameters
include the type of laser, wavelength, type of link, and the required signal criterion.
Today the lasers typically used in free space laser communications are the semiconductor
laser diodes, solid state lasers, or fibre amplifier lasers. Laser sources are described
as operating in either in single or multiple longitudinal modes. In
the single longitudinal
mode operation the laser
emits radiation at a single frequency,
while in the
multiple longitudinal mode, multiple frequencies are emitted. Semiconductor lasers
have been in development for three decades and have only recently (within the past
7 years) demonstrated the levels of performance needed for the reliable operation
as direct sources .typically operating in the 800-900 nm range(gallium arsenide/gallium
aluminium arsenide)their inherently high efficiency(50%)and small size
made this technology
attractive.
The key
issues have been the life times, asymmetric beam shapes, output power. Solid state
lasers have offered higher power levels and the ability to operate in high peak
power modes for the acquisition. When laser diodes are used to optically pump the
lasing media graceful degradation and higher overall reliability is achieved. A
variety of materials have been proposed for laser transmitters: neodyminium doped
yttrium aluminium garnet (Nd: YAG) is the most widely
used. Operating at 1064
nm, these lasers
require an external modulator leading to a slight increase
in the complexity
and reliability. With the rapid development
of terrestrial fibre communications, a wide array of components is available for
the potential applications in space. These
include detectors, lasers, multiplexers, amplifiers, optical pre amplifiers etc. Operating at 1550nm erbium
doped fibre amplifiers have been developed for commercial optical fibre communications
that offer levels of performance
consistent with many free space communications applications. There are
three basic link types: acquisition, tracking and communications. The major differences between
the link types are reflected in the required signal criterion for each. For acquisition the criterion is acquisition
time, false alarm rate, probability of detection. For the tracking link the key
considerations are the amount of error induced in the signal circuitry. This angle
error is referred to as the noise effective angle. For the communications link,
the required data and the bit error rates are of prime importance.
TRANSMITTER
PARAMETERS
The transmitter parameter consists
of certain key laser
characteristics, losses incurred in the transmitter optical path, transmit antennae
gain, and transmit pointing losses. The key laser characteristics include peak and average optical
power, pulse rate and pulse width. In a pulsed configuration the peak laser power
and duty cycle are specified, whereas in continuous wave application, the average
power is specified. Transmit optical path loss is made up of optical transmission
losses and the loss due to the wave front quality of the transmitting optics. The
wave front error loss is analogous to the surface roughness
loss associated with the
RF antennas. The optic transmit antenna gain is analogous to the
antenna gain in the RF systems and describes the on axis gain relative to an isotropic
radiator with the distribution of the transmitted laser radiation defining the transmit
antenna gain. The laser sources suitable for the free space communications tend
to exhibit a Gaussian intensity distribution in the main lobe. The reduction in
the far field signal strength due to the transmitter pointing is the transmitter
pointing losses. The pointing error is composed of bias (slowly varying) and
random (rapidly varying) components.
CHANNEL
PARAMETERS
The channel
parameters for an optical intersatellite link(ISL) consist of range and associated
loss ,background spectral radiance and spectral irradiance. The range loss is directly
proportional to the square of wavelength and inversely proportional to the square
of the separation between the platform in metres.
RECEIVER PARAMETERS
The receiver
parameters are the receiver antenna gain, the receive optical path loss, the optical
filter bandwidth and the receiver field of view. The receiver antenna gain is proportional
to the square of effective receiver diameter in metres
and inversely proportional to the square of
the wavelength. The receiver optical path
loss is simply the optical transmission loss for systems employing the direct detection
techniques. However for the lasers employing the coherent optical detection there
is an additional loss due to the wave front error. The preservation of the wave
front quality is essential for the optimal mixing of the received signal and the
local oscillator fields on the detector surface. The optical filter bandwidth specifies
the spectral width of the narrow band pass
filter employed in optical inter satellite
links. Optical filters reduce the amount of unwanted background entering the system.
The optical width of the filter must be compatible with the spectral width of
the laser source.
The minimum width will
be determined by the acceptable transmission level of
the filter. The final
optical parameter is the angular field of view (FOV), in radians which limits the
background power of an extended source incident on the detector. To maximize the
rejection, the FOV should be as small as possible. For small angles the power incident
on the detector is proportional to FOV square. The minimum FOV is limited by optical
design constraints and the
receiver pointing capability.
DETECTOR
PARAMETERS
The detector
parameters are the type of detector, gain of detector, quantum efficiency, heterodyne
mixing efficiency, noise due to the detector, noise due to the following pre amplifier
and angular sensitivity. For optical ISL systems based on semiconductor laser diodes
or Nd:YAG lasers the detector of choice is a p type intrinsic n type (PIN) or an
avalanche photodiode(APD) APIN photo diode can be operated in the photovoltaic or
photoconductive mode and has no internal gain mechanism. An APD is always operated
in the photo conductive mode and has
an internal
gain mechanism, by virtue
of avalanche multiplication. The quantum
efficiency of the detector is the efficiency with which the detector converts the
incident photons to electrons. The mean output current for both the PIN and APD
is proportional to the quantum efficiency. By definition the quantum efficiency
is always less tha unity.
Another detector parameter is the noise due to the detector alone. Typically in a detector there is a DC current even in the absence of signal or background. This DC dark current produces a shot noise current just as the signal and the noise currents do. In an APD there are two contributors to this DC dark current-an multiplied and an un multiplied current. The output of the detector is the input to the preamplifier that converts the detector signal current into a voltage and amplifies it to a workable level for further processing. Being the first element past the detector, the noise due to the preamplifier can have a significant effect on the systems sensitivity. The selection of the pre amplifier design and the internal transistor design and the device material depends on a number of factors.
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Laser communications
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