1. Presented by, P.Anitha(08MCS204)
Laser Communication
Presented by,
P.Anitha(08MCS204)

2. Introduction
Laser communications systems are wireless connections through the atmosphere.
Use Laser Beams to transmit information between two locations
No fibres 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
4/25/2017
Technical Seminar-II

3. How does it Work Signal Transmitter Laser Receiver Signal 4/25/2017
Technical Seminar-II

4. Laser Transmitter and Receiver
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Technical Seminar-II

5. A one-way Laser communication system.
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Technical Seminar-II

6. Laser Transmitter The transmitter involves:
Signal processing electronics(analog/digital)
Laser modulator
Laser( Visible, near visible wavelengths)
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Technical Seminar-II

7. Modulation AM PWM PFM Easy with gas lasers,hard with diodes
Potentially the highest bandwidth(>100kHz)
4/25/2017
Technical Seminar-II

8. Receiver The receiver involves: Telescope(‘antenna’) Signal processor
Detector
PIN diodes
Avalanche Photo Diodes(APD)
Single or multiple detectors
4/25/2017
Technical Seminar-II

9. Gain Systems Transmitter Receiver Maximum output power
Minimum divergence
Receiver
Maximum lens area
Clarity
Tight focus on detector
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Technical Seminar-II

10. Laser Diode
Laser Diodes include Photodiodes for feedback to insure consistent output
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Technical Seminar-II

11. 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
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Technical Seminar-II

12. 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, etc.
4/25/2017
Technical Seminar-II

13. Security Aspects
Free space laser communications systems have narrow optical beam paths, which are not accessible unless viewing directly into the transmitter path.
Any potential eavesdropping will result in an inter-ruption of the data transmission.
The existence of laser beams cannot be detected with spectrum ana-lyzers.
4/25/2017
Technical Seminar-II

14. Safety Aspects
The free space laser communications systems do not require certification for handling or operation.
Although the emitted laser beam is invisible to the unaided eye, it can cause eye damage if viewed directly at close range for extended periods of time
4/25/2017
Technical Seminar-II

15. Laser Communication System
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Technical Seminar-II

16. Laser Communication System
Input: Digital data
Direct or indirect modulation
Source output passes through the optical system into the channel
Optical system: transfer, beam shaping, telescope optics
Receiver beam comes through optical system and passed to detectors and signal processing electronics
4/25/2017
Technical Seminar-II

17. System Characteristics
Link parameters:
Type of laser, wavelength, type of link
Semiconductor laser diodes, solid state lasers, fiber amplifier lasers.
Lasers operate in single or multiple longitudinal modes.
Single longitudinal mode
laser emits radiation at a single frequency
Multiple longitudinal mode
multiple frequencies are emitted
4/25/2017
Technical Seminar-II

18. Link parameters Semiconductor laser diodes
Reliable operations as direct sources
Operating in nm range
High efficiency of about 50%
Small size
Output power
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Technical Seminar-II

19. Link parameters Solid state lasers
Higher power levels, high peak power mode
Operating at 1064 nm
Increase in complexity and reliability
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Technical Seminar-II

20. Link parameters Types of link: Acquiston Tracking Communications
Acquiston time,false alarm rate,probability of detection
Tracking
Amount of error induced in the signal circuitry
Communications
Bit error rates
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Technical Seminar-II

21. Transmitter Parameters
Laser characteristics, losses incurred in the transmit optical path, transmit antennae gain, transmit pointing losses.
Laser characteristics
peak and average optical power
pulse rate
pulse width
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Technical Seminar-II

22. Channel Parameters Consists of Range, associated loss
background spectral radiance
spectral irradiance
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Technical Seminar-II

23. Receiver Parameters The receiver parameters are the
Receiver antenna gain
proportional to the square of effective receiver diameter in meters and inversely proportional to the square of the wavelength.
Receive optical path loss
optical transmission loss for systems employing the direct detection techniques.
Optical filter bandwidth
the spectral width of the narrow band pass filter employed in optical inter satellite links
Receiver field of view
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Technical Seminar-II

24. Advantages of Laser Communication Technology
Higher data rates
Compared to RF technology LC provides much higher data rates
Higher data rates are essential as more and more data is moved between diff. locations
Key Driver for investments in Laser Communication Technology
High security regarding interception
A focused laser beam is hard to intercept without notice
Path to Quantum Cryptography
4/25/2017
Technical Seminar-II

25. Advantages of Laser Communication Technology
Less frequency restrictions
RF spectrum is crowed and heavily used
Smaller aperture dimensions and thus reduced size and mass
Less weight and power per bit
Autonomous alignment agility resulting in less platform manoeuvres
Less fuel or more flexibility
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Technical Seminar-II

26. Applications for Laser Communication
Data Relay Services for UAVs
UAV transmits its data to a GEO Stationary Satellite
Data Relay Services for Satellites
LEO Satellite transmit their data to a GEO Satellite
Inter-Satellite Links
Data Exchange between GEO/LEO Satellites
Deep Space Data Transmissions
Scientific data is transmitted down to Earth, e.g. Mars -> EarthGEO
4/25/2017
Technical Seminar-II

27. Situation
Situation
Demand for more information requires more and higher resolution sensors/cameras on UAVs
Data transmission becomes the limiting factor to acquire and distribute information from UAV to Operation Center at diff. location
RF solution reach data rate limits
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Technical Seminar-II

28. Problem, Need
Problem
Limited information is available to Operation Centers
Information not available when needed as transmission time is a bottle neck
Real-time decision making not possible or only limited possible
Need
Solution for higher data rate transmissions from UAV to Operation Center of long distances (span continents)
4/25/2017
Technical Seminar-II

29. Conclusions
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.
4/25/2017
Technical Seminar-II

30. References www.mindstein.net Laser communication.pdf
Lasercommunicationsystem.pdf
server4.oersted.dtu.dk/courses/31825/Project11.pdf
opticalcomm.jpl.nasa.gov/PAPERS/ATP/gospi03b.pdf
4/25/2017
Technical Seminar-II