laser communication

Laser

Communication

Presented by, P.Anitha(08MCS204)

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

04/08/23 Technical Seminar-II 2

How does it Work


Signal Transmitter Laser

Receiver Signal

Laser Transmitter and Receiver


A one-way Laser communication system.


Laser Transmitter

The transmitter involves: Signal processing electronics(analog/digital) Laser modulator Laser( Visible, near visible wavelengths)


Modulation

AM Easy with gas lasers,hard with diodes

PWM PFM

Potentially the highest bandwidth(>100kHz)


Receiver

The receiver involves: Telescope(‘antenna’) Signal processor Detector

PIN diodes Avalanche Photo Diodes(APD) Single or multiple detectors


Gain Systems

Transmitter Maximum output power Minimum divergence

Receiver Maximum lens area Clarity Tight focus on

detector


Laser Diode

Laser Diodes include Photodiodes for feedback to insure consistent output


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, 2000 etc.


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.


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


Laser Communication System


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


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


Link parameters

Semiconductor laser diodes Reliable operations as direct sources Operating in 800-900 nm range High efficiency of about 50% Small size Output power


Link parameters

Solid state lasers Higher power levels, high peak power mode Operating at 1064 nm Increase in complexity and reliability


Link parameters

Types of link: Acquiston

Acquiston time,false alarm rate,probability of detection Tracking

Amount of error induced in the signal circuitry Communications

Bit error rates


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


Channel Parameters

Consists of Range, associated loss background spectral radiance spectral irradiance


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


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


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


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


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


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)


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.


References

www.mindstein.net Laser communication.pdf www.bestneo.com Lasercommunicationsystem.pdf server4.oersted.dtu.dk/courses/31825/Project11.pdf www.freepatentsonline.com/4717828.html www.qsl.net/k7kw/DEMOS/LaserCommunications.ppt www.mseconference.org/.../

mse03_2P_Uherek_Microoptoelectronicscurricula.pdf opticalcomm.jpl.nasa.gov/PAPERS/ATP/gospi03b.pdf


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