Laser Communication

Laser Communication Presented by, P.Anitha

Introduction 

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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

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How does it Work

Signal

Transmitter

Receiver

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Laser

Signal

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Laser Transmitter and Receiver

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A one-way Laser communication system.

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Laser Transmitter 

The transmitter involves:   

Signal processing electronics(analog/digital) Laser modulator Laser( Visible, near visible wavelengths)

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Modulation 

AM 

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Easy with gas lasers,hard with diodes

PWM PFM 

Potentially the highest bandwidth(>100kHz)

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Receiver 

The receiver involves:   

Telescope(‘antenna’) Signal processor Detector   

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

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Gain Systems 

Transmitter  

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Maximum output power Minimum divergence

Receiver   

Maximum lens area Clarity Tight focus on detector

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Laser Diode 

Laser Diodes include Photodiodes for feedback to insure consistent output

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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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Mounting Systems 

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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.

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Security Aspects 

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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.

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Safety Aspects 

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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

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Laser Communication System

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Laser Communication System   

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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

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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 

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laser emits radiation at a single frequency

Multiple longitudinal mode 

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multiple frequencies are emitted

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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

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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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Link parameters 

Types of link: 

Acquiston 

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Tracking 

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Acquiston time,false alarm rate,probability of detection Amount of error induced in the signal circuitry

Communications 

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Bit error rates

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Transmitter Parameters 

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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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Channel Parameters 

Consists of   

Range, associated loss background spectral radiance spectral irradiance

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Receiver Parameters 

The receiver parameters are the 

Receiver antenna gain 

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Receive optical path loss 

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optical transmission loss for systems employing the direct detection techniques.

Optical filter bandwidth 

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proportional to the square of effective receiver diameter in meters and inversely proportional to the square of the wavelength.

the spectral width of the narrow band pass filter employed in optical inter satellite links

Receiver field of view

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Advantages of Laser Communication Technology 

Higher data rates  

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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

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Advantages of Laser Communication Technology 

Less frequency restrictions 

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Smaller aperture dimensions and thus reduced size and mass 

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RF spectrum is crowed and heavily used Less weight and power per bit

Autonomous alignment agility resulting in less platform manoeuvres 

Less fuel or more flexibility

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Applications for Laser Communication 

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Data Relay Services for UAVs  UAV transmits its data to a GEO Stationary Satellite Data Relay Services for Satellites 

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Inter-Satellite Links 

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LEO Satellite transmit their data to a GEO Satellite Data Exchange between GEO/LEO Satellites

Deep Space Data Transmissions 

Scientific data is transmitted down to Earth, e.g. Mars -> EarthGEO

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Situation 

Situation 

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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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Problem, Need 

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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)

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Conclusions 

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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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References      

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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_Microop toelectronicscurricula.pdf opticalcomm.jpl.nasa.gov/PAPERS/ATP/gospi03b.pdf

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