LASER SATELLITE COMMUNICATION

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Jodie Parsons
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1 LASER SATELLITE COMMUNICATION

2 INTRODUCTION a)transmission at frequencies in b)advantage Greater bandwidth Smaller beam divergence angles Smaller antennas c)modes of communication Aerial Fiber optical communication Optical computer

3 ARIEL Ariel :data and images are transferred using low power beams Impossible to jam by known means Weather dependent Clear day several miles Rain,fog g,,mist limited to shorter distance

4 Fiber optical communication& optical computers Guided media 4 Giga bits of information/sec over a span of 120Km Optical computers I. Light is used instead of electrical circuit II. Light can be encoded dwith much more information III. Zero resistance to flow,more information than the equivalent sized electric circuit IV. Optical signal can be used in parallel

5 Use Communication between the satellite themselves Can not be used between earth station and geostationary satellite being atmospheric dependent

6 LASER SATELLITE COMMUNICATION GS3 LASER LASER RF ES13 GS1 RF GS2 ES23 ES12 users RF ES22 ES21 ES11 GSS =GEAOSTATIONARY SATELLITE ESS= EARTH OBSERVATION SATELLITE

7 LINK ANALYSIS Atmospheric Effects: Attenuation due to energy absorption Beam spreading due to scattering of light waves Beam bending due to refocusing of optical beams Beam break up due to loss of coherence

8 ATMOSHERIC Dependent on wavelength Dependent on elevation angle

9 Complete link design Up link and downlink RF is used to satellite Two satellite cross link (optical link) RF up link wave form s ( t ) = u ( t ) + nu ( t ) u( t) = uplink u. carrier n u ( t) = uplink. Noise. and. Interferen ce

10 ( t) isaveragep r The the R R = r (1 + reciever photo βs( t)) owerand satellite detector βi sin the detects signal the tensity [ β s ( t ) ] = Rβ [ u ( t ) + n ( t) ] s s = ns = = r r u ) photo satellite α = 2 s α detector [( Rβ 2 n t [( Rβ t responsiti downlinkpo r ) r 2 ) 2 cu nu ] l + wer D of mod ulation β 1 optical intensity vity )] L reciever modulated by photo signal α is signal and noise suppression ns =total downlink retransmitted noise power L is the downlink losses cu is the uplinkpower ofu(t) pd is photo detector noise nu additional noise by the down link as detecting it

11 ( C N ) / = ( C / N ) T / = ( C / N ) ( C / N ) α 2 s = u 1 op r + ns cu nu L C t s D 1. / + α nd s N 2 nd op ( ) ( ) 1 ( ) 1 C / N C / N + C / N + ( C / N ) T 1 [ ] 1 = u op r

12 Satellite beam and acquisition, tracking and pointing Beam is narrow ointing problem ointing within the pointing error±θc radians Optical beacon( unmodulated light source) Transmitter satellite receives the beacon from the receiving ii satellite Transmits its modulated laser beam back to the receiving satellite Angle of drifting of the receiving satellite(point ahead angle)

13 transmitter Receiving satellite

14 Satellite beam and acquisition, tracking and pointing Vt is the tangential velocity of the receiving satellite α= Vt/150 micro radians oint ahead angle exceed the one half of the laser modulated dbeam width then the use of point ahead angle is made

15 Transmitter satellite Beacon laser Receiver satellite laser Modulator Modulator TELESCOE detect or Beam steer TELESCOE Beam steer detect or Tracking system Tracking system receiver receiver

16 OTICAL SATELLITE LINK TRANSMITTER LASER SOURCE MODULATOR ANTENNAS

17 LASER LASER SOURCE: a. GAS LASER, b. SOLID STATE LASER, c. SEMICONDUCTOR LASER

18 Semiconductor laser AlGaAs and InGaAs are also being used AlGaAs is reliable between 0.78 and 0.86 μm InGaAs emits between 1.2 and 1,65 μm Lasers are low powered devices Used in arrays to increase output

19 Small size Weight High ihefficiency i Reliability Easily modulated LASER Advantage

20 DISADVANTAGE Beam combining problem Integrated optical technology has developed coherent combining technology Increasing the power Decreasing the beam divergence

21 Laser commonly used in satellite communication LASER TYE WAVELENG AVERAGE EFFICIENCY CHARACTERISTICS TH OWER OUTUT Nd YAG 1.06 μ 0.5 1W 0.5 1% Requires elaborate modulation equipment, diode d or solar pumping 10,000 life hours Crystal 0.532μ 100MW 05 1% 0.5 GaAs μ 40MW 5 10% Life hours 5000,reliable, small, rugged, compact, directly and easily modulated Easily combined into arrays Nano second pulsing

22 Laser commonly used in satellite communication LASER TYE WAVELENG AVERAGE EFFICIENCY CHARACTERISTICS TH OWER OUTUT CO2(gas laser) 1.06 μ 1 2W 10 15% Life hours 20,000 used in IR range, dt detectors t are poor, Uses a discharge tube, modulation is difficult HeNe 063μ 0.63μ 10MW 1% Life hours 50,000.requires000 (Helium Neon) external modulation, has gas tube,is power limited and is inefficient

23 MODULATORS Direct intensity modulation Driving current is varied in accordance with the type of modulation

24 Various optical laser modulation method Modulation Analog pulse digital type Information Signal Time Continuous Time Continuous Or sampled Time sampling Carrier arameter Continuous Continuous Quantized or coded Or Quantized Example Intensity modulation ulse intensity modulation ulse code modulation, intensityi modulation

25 ANTENNA Conventional Telescopes Size and geometry as per the wavelength and geometry Narrow light beams Lensing system for transmission and focusing

26 Optical Antenna Transmission Φb=λ/dt Laser t φb Ar Optical Receiver Z

27 b r t r z A = 2 2 φ b t b g = 2 4 φ π φ ( ) ( ) p b d d z L λ π ( ) r t t r z d d = λ o r r hf n = fo is optical frequency Nr photo electrons per second

28 Optical satellite link reciever telescope: focus the optical signal on to the photo detector Optical filter: eliminate back ground radiation that is not of same wavelength as the optical signal Δ λ λ B o 0 f 0

29 Optical detection Direct detection System Heterodyne system

30 Direct Detection System Respond to the signal intensity h f f ll f ld b hoto focusing interference collimating field objective Detector lens filter lens stop lens IRIS

31 rinciple of heterodyne detection IF AMLIF IER demodula tor Local oscillator

32 Heterodyne receiver Optical receiver field view: Field arriving angles over which lenses will focus theimpinging field onto thephoto detector surface Detector area and focal llength Ωfv= Ad/f 2 c=ad/ar=(ad/λ 2 )(λ 2 /Ar) (λ 2 /Ar) diffraction limited field of view

33 Heterodyne receiver i n diode and avalanche photo diode Detection efficiency, gain, responsivity and bandwidth Wave length dependent, material used for photo emission i Detected count rate of optical receiver Ns=(η/hfo)r

34 hoto detector Gain is increased by cascading photo emmissive surface noise increases Excess noise F= 1+σ 2 d/(g) 2 G mean gain σ 2 d gain variance Responsivity : current produced for a given output R=eη Reη G /hf o

35 hoto detector noise model r b Short noise Dark current i1(t) Thermal current Detector output current Back ground radiation

36 hoto detector Ns(w)=G 2 FeR Ndc(w)=eI dc Nt(w)= 4KT o eq /R L R L is impedance load T o eq noise equivalent ttemperaturet Intensity modulation so s(t) information wave form modulated don the laser field r (t)= r[1+βs(t)]

37 hoto detector After detection photo detector current will be i(t)=r[r(t)+b]+i sn (t)+i dc (t)+i i (t) s=(rrβ) 2 signal power n=n 0 (2Bm) total noise power SNR=s/n =(Rrβ) 2 /[ G 2 FeR(r+b)+ ei o dc + 2KT eq /R L ]2Bm

Fiber Optic Communications

Fiber Optic Communications ( Chapter 2: Optics Review ) presented by Prof. Kwang-Chun Ho 1 Section 2.4: Numerical Aperture Consider an optical receiver: where the diameter of photodetector surface area