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• Laser communications systems are wireless connections through the
• Use Laser Beams to transmit information between two locations
• No fibers need, a wireless technology
• Communication over long distances, e.g. between planets
• Laser Communication Terminals (LCTs) transmit a laser beam and are
capable of receiving laser beams.
How does it Work ?
Signal Transmitter Laser
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Laser Transmitter and Receiver
Laser Transmitter Receiver
Optical fiber link
• The Transmitter involves a signal
processing circuit, and a laser.
• A laser diode is used to create the laser
• Laser Diodes include Photodiodes for
feedback to insure consistent output.
The receiver involves:
• Signal processor
Avalanche Photo Diodes(APD)
Single or multiple detectors
Easy with gas lasers, hard with diodes
Potentially the highest bandwidth(>100kHz)
Maximum output power
Maximum lens area
Tight focus on detector
• Sun shade over detector
• Shade in front of lens
• Detector spectral response
• Colored filters
Absorb ~50% of available light
Difficult to find exact frequency
• Mounts and stands need only be as accurate as beam divergence
• Good laser diodes will be 1-2mR (milliRadian)
• A 32 pitch screw at the end of a 2' mount will yield 1mR per revolution.
• Since quarter turns (even eighth turns) are possible, this is more than accurate enough
• Higher thread pitches allow shorter mounts which may be more stable (against wind,
• 1mR is 1.5 of divergence every 1000, 2000 etc.
• Thus system is set up to
send voice data.
• A person's voice gets put
into a conditioning
circuit so that the full
eight bit range of the
analogue to digital
converter is utilized.
• Once the digital signal is
obtained by the ADC, the
MCU passes the signal to
• The UART sets a transmit
pin high or low according
to the serial protocol.
• Some conditioning is
applied to this signal as
well in order to ensure
constant current to the
• On the receiver side, the
signal is read by a photo
transistor and basically the
signal goes through an
opposite sequence to
output a sound instead of
receiving one and using the
DAC will change the digital
signal to an analog one.
• Not always possible to lay fiber lines
Physically / Economically not practical
• Laser Communication being incorporated into fiber optic networks
when fiber is not practical.
Why not Fiber Optics?
• for Laser Communication (LC) is 100 times greater than for RF.
• in LC is directed at target, so much less transmission power required.
• Also the power loss is less.
• Size / Weight
• LC antenna is much smaller than RF.
• Due to low divergence of laser beam, LC is more secure than RF.
Why not RF?
ISP (Internet Service Provider) Industrial Use
• Defense and sensitive areas.
• At airports for communication across the
• Mass communication
• Free-space optical communication
• Space probe are being designed to use
optical rather than radio communication.
• Laser communication has also been
demonstrated on aircraft and high altitude
Lunar Atmosphere and Dust Environment Explorer
• Lunar Laser Communication Demonstration
(LLCD) equipment on LADEE set a space
communication bandwidth record in
• Early tests using a pulsed laser beam to
transmit data over the 385,000 kilometres
(239,000 mi) between the Moon and Earth.
• Passed data at a "record-breaking download
rate of 622 megabits per second (Mbps)“
• Demonstrated an error-free data upload
rate of 20 Mbps from an Earth ground
station to LADEE in Lunar orbit.
• Free space laser communications systems have narrow optical beam
paths, which are not accessible unless viewing directly into the
• Any potential eavesdropping will result in an interruption of the data
• The existence of laser beams cannot be detected with spectrum
• Ease of deployment
• Can be used to power devices
• License-free long-range operation (in contrast with radio communication)
• High bit rates
• Low bit error rates
• Immunity to electromagnetic interference
• Full duplex operation
• Protocol transparency
• Increased security when working with narrow beam(s)
• No Fresnel zone necessary
• For terrestrial applications, the principal limiting factors are:
• Beam dispersion
• Atmospheric absorption
• Fog (10..~100 dB/km attenuation)
• Interference from background light sources (including the Sun)
• Pointing stability in wind
• Pollution / smog
• With the dramatic increase in the data handling requirements for
satellite communication services, laser
• inter satellite links offer an attractive alternative to RF with virtually
unlimited potential and an unregulated spectrum.
• The system and component technology necessary for successful inter
satellite link exists today.
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