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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.
Lasers 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 page link applications.
Information transfer 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.
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Free Space Laser Communications
Introduction
¢ Free space laser communications
“ a.k.a. Free Space Optics, Optical
Presented By
Phillip Dykstra
WareOnEarth Communications Inc.
Wireless
¢ Basically a laser and a telescope
¢ Most are protocol independent
¢ Typical range < 4 km (.5, 1, 2, 4,
8km)
¢ Typical bandwidths: 10, 100, 1000
Mbps
“ Lucent 10 Gbps DWDM (TeraBeam)
Transmitters
¢ LED (single or multiple)
“ 1 mW typical
¢ Laser (single or multiple)
“ 10-20 mW typical, up to 100 mW
¢ Usually 785 or 850 nm, some 1310
nm
¢ Eye safety limit: 1.5 mW/cm^2 at
785 nm
¢ Multiple transmitters boost power,
may also reduce scintillation fades
Detectors
¢ PIN Diodes
-43 dBm typical
¢ Avalanche Photo Diodes (APD)
-53 dBm typical (also greater dynamic
range)
¢ Single or multiple detectors
¢ Larger aperture increases receive
power, reduces scintillation fades
¢ Design goal: BER < 10^-9 typical
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INTRODUCTION
:Light Amplification by Stimulated Emission of Radiation
A device produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher energy levels
When they return to lower energy levels by stimulated emission, they emit energy.
PROPERTIES
Monochromatic
Concentrate in a narrow range of wavelengths (one specific colour).
Coherent
All the emitted photons bear a constant phase relationship with each other in both time and phase
Directional
A very tight beam which is very strong and concentrated.
computer science crazy
Guest
please make a look on these page for more about LASER Communication
http://studentbank.in/report-laser-commu...tems--6013 i am sure you never miss a second for reading this report
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Laser Communications
Laser Classes
Class I – Sealed systems
Class II – Output <1mW
Class IIIa – Output 1mW - 5mW
Class IIIb – Output 5mW – 500mW
Harmful to eyes, diffuse viewing OK
Class IV – Output >500mW
Harmful to skin and eyes, diffuse viewing hazardous
Diode Lasers
Laser diodes emit an elliptical beam with astigmatism
Better units will include corrective lenses for astigmatism and to make the dot appear round
Neither of these problems are inherently bad for DX purposes but correcting them also improves divergence, a big win (more gain).
Human Spectral Response
Perceived Intensities
Laser Diode
Pointer Design
Pointer Innards
Modulation
AM
Easy with gas lasers, hard with diodes
PWM (Pulse Width Modulation)
Used by Ramsey in their kit
PFM (Pulsed FM)
Potentially the highest bandwidth (>100kHz)
Gain Systems
Transmitter
Maximum output power
Minimum divergence
Receiver
Maximum lens area
Clarity
Tight focus on
detector
Filters
Sun shade over detector
Shade in front of lens
Detector spectral response
Colored filters
Absorb ~50% of available light
Difficult to find exact frequency
Mounting Systems
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, vibration, wires)
1mR is 1.5' of divergence every 1000', 3' at 2000 ', etc.
Pointing
GPS and Compass
Scopes and Binoculars
Strobe lights, large handheld floods, headlights
HTs to yell when laser light is seen at remote location
Weak Signal Modes
Laser DX
Applications
Transmit voice for miles line-of-sight
Use weak signal modes for “cloud scatter”
Transmit video with cheap pens
Transmit high speed data without WEP
Blind flies for easy extermination
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