1. Cost 297 HAPCOS Meeting, Friedrichshafen, Germany Oct. 8 – 10, 2008 Communications to and from HAPs – with laser beams? Walter Leeb walter.leeb@tuwien.ac.at Vienna University of Technology Institute of Communications and Radio-Frequency Engineering Gusshausstrasse 25/389, 1040 Vienna

2. Oct. 8, 2008 2 W. Leeb Overview Introduction Building blocks PAT Influence of channel (= atmosphere) Bandwidth offered by optical and microwave links Summary

3. Oct. 8, 2008 3 W. Leeb Motivation for optical links transmission bandwidth  f (small) percentage of carrier frequency f f = 200 to 350 THz   f  300 GHz beam divergence  proportional to 1/f (antenna gain G proportional to f 2 )    10  rad, G  130 dB  small antenna diameter expecting: low terminal mass low power consumption

4. Oct. 8, 2008 4 W. Leeb Basic differences to microwave links  so far no frequency regulations  no electromagnetic interference  difficult eavesdropping  quantum nature dominates (hf >> kT)  dimension of devices (D >> )  antenna pointing, terminal acquisition, mutual tracking (PAT) (  two-way optical link)  influence of atmosphere  background radiation (Sun, Moon, etc.)  h... Planck's constant k... Boltzmann's constant T... system temperature

5. Oct. 8, 2008 5 W. Leeb Scenarios GEO... geostationary orbit LEO... low earth orbit ISS... International Space Station distance L = 45 000 to 83 000 km data rate R = 3 Gbit/s distance L > 1 000 000 km data rate R = 2 Mbit/s

6. Oct. 8, 2008 6 W. Leeb HAP – HAP – GEO Scenario GEO... geostationary orbit HAP... high altitude platform HAP  HAP L = 5... 100 km HAP  GEO L = 50 000 km R = 1 Gbit/s

7. Oct. 8, 2008 7 W. Leeb LEO-GEO link 2001 European Space Agency ARTEMIS (GEO)  SPOT-4 (LEO) mean distance: 40 000 km = 0.85 µm R = 50 Mbit/s [2 Mbit/s] 2005 ARTEMIS  OICETS (LEO, Japan) SILEX... Semiconductor Laser Intersatellite Link Experiment ARTEMIS SPOT 4

8. Oct. 8, 2008 8 W. Leeb Balloon-to-ground link 2005 German Aerospace Centre (EU project CAPANINA) STROPEX balloon (at 22 km) to ground, distance = 64 km = 1.5 µm (InGaAs diode laser) R = 622 Mbit/s and 1.25 Gbit/s

9. Oct. 8, 2008 9 W. Leeb Airplane to GEO satellite 2006 European Space Agency, France "LOLA" airplane (10 km height) to ARTEMIS (GEO) = 0.85 µm, diode laser successful pointing and tracking, video transmission

10. Oct. 8, 2008 10 W. Leeb LEO-LEO link 2008 intersatellite laser communication: TerraSAR-X (LEO, Germany)  NFIRE (LEO, USA), 5 000 km = 1.06 µm (Nd:YAG laser) coherent receiver (homodyne) BPSK (binary phase shift keying) R = 5.5 Gbit/s

11. Oct. 8, 2008 11 W. Leeb Overview Introduction Building blocks PAT Influence of channel (= atmosphere) Bandwidth offered by optical and microwave links Summary

12. Oct. 8, 2008 12 W. Leeb Optical transceiver for space missions

13. Oct. 8, 2008 13 W. Leeb TX, RX for = 0.85 µm direct modulation APD... avalanche photodiode

14. Oct. 8, 2008 14 W. Leeb TX, RX for = 1.5 µm EDFA... Erbium doped fiber amplifier

15. Oct. 8, 2008 15 W. Leeb Input-output multiplexing (1) duplexing: spectrally, or via polarization, or both to keep crosstalk TX  RX low: high isolation within duplexer (e.g. P T = 1 W, P R = 10 nW)  95 dB duplex operation between two moving terminals required, at least for acquisition and tracking  single antenna for RX and TX

16. Oct. 8, 2008 16 W. Leeb Input-output multiplexing (2) simple duplexing scheme    increased telescope diameter  shared antenna aperture

17. Oct. 8, 2008 17 W. Leeb Overview Introduction Building blocks PAT Influence of channel (= atmosphere) Bandwidth offered by optical and microwave links Summary

18. Oct. 8, 2008 18 W. Leeb PAT e.g.: = 1.55 µm, D T = 20 cm  2  T = 10 µrad  satellite position uncertainty and vibrations ( > 2  T ) require:  initial pointing of transmit and receive antenna  mutual search and acquisition of terminal position  closed loop tracking of antenna direction (accuracy: 1 µrad!) beam divergence 2  T (antenna directivity) PAT possibly:  extra acquisition laser  separate tracking beam and tracking sensor (CCD)

19. Oct. 8, 2008 19 W. Leeb Overview Introduction Building blocks PAT Influence of channel (= atmosphere) Bandwidth offered by optical and microwave links Summary

20. Oct. 8, 2008 20 W. Leeb Influence of atmosphere  absorption by molecules  attenuation  scattering (molecules, waterdroplets, fog, snow)  attenuation pronounced influence within first 15 km above the Earth's surface, but relatively small influence above 15 km  turbulence (random variation of index of refraction)  increased beam divergence ("beam spread" & "breathing" of beam)  attenuation, fading  random beam deflection ("beam wander")  fading  phase front distortion  fading, scintillation

21. Oct. 8, 2008 21 W. Leeb Beam spread r 0... Fried-Parameter... wavelength diffraction limited beam divergence in vacuum beam divergence including influence of turbulence far field:

22. Oct. 8, 2008 22 W. Leeb Fried parameter Fried parameter r 0 characterises the degree of turbulence, integrated over beam path  large r 0 means little influence of turbulence  examples (medium turbulence, = 1.5  m): - HAP(at 17 km)-to-satellite link r 0 = 10 m - ground-to-satellite link r 0 = 15 cm  for a transmit antenna diameter D T equal to the Fried parameter r 0, the turbulence causes an increase of the divergence by a factor of, i.e. a gain reduction by 3 dB  - downlink (satellite to HAP): in general negligible influence of turbulence - uplink: typically < 0.1 dB additional loss due to turbulence-induced beam spread

23. Oct. 8, 2008 23 W. Leeb Beam wander caused by large-scale turbulence near the transmitter, leading to deflection of entire beam

24. Oct. 8, 2008 24 W. Leeb Scintillation caused by small-scale turbulence, leads to interference between parts of the beam,  disturbance of intensity profile ("speckle")  distortion of beam phasefront, mode de-composition (  reduced coupling into single-mode receiver) scintillation index  2 characterises the temporal behaviour of intensity (I) fluctuations (normalized variance of I(t)) typically  2 < 0.025 for HAP-to-satellite link  temporal mean

25. Oct. 8, 2008 25 W. Leeb Overview Introduction Building blocks PAT Influence of channel (= atmosphere) Bandwidth offered by optical and microwave links Summary

26. Oct. 8, 2008 26 W. Leeb Sensitivity of receivers rule of thumb for detecting one bit of information: required is an energy of either 10 hf or 10 kT, whatever is larger 10 hf10 kT optical = 1 µm, T = 300 K 2  10 -18 Ws4  10 -20 Ws microwave f = 10 GHz, T = 300 K 7  10 -23 Ws4  10 -20 Ws h... Planck`s constant k... Boltzmann`s constant T... system temperature optical regime requires 100 times larger input power! Optical on-off keying: BEP = 10 -9 requires an average of 10 photons per bit (absolute physical limit)

27. Oct. 8, 2008 27 W. Leeb Background radiation  sources: Sun, Moon, planets (including Earth), scattering atmosphere  received background power P B = N back  B o  m Optical links: noise increase due to background N back... power density (in one spatial mode) e.g. at = 1.5  m - N back,Sun = 4  10 -20 W/Hz - N back,Earth = 4  10 -25 W/Hz - N back,atm@20 km = 10 -27 W/Hz B o... bandwidth of optical filter [Hz] m... number of modes accepted by receiver

28. Oct. 8, 2008 28 W. Leeb Transmission bandwidth - examples HAP (20 km)  GEO satellite (36 000 km) distance L = 50 000 km (zenith angle 45°) TX: GaAs laser diode RX: avalanche photodiode TX: InGaAs laser diode RX: EDFA reamplifier RF in K-band wavelength0.85 µm1.55 µm1.76 cm carrier frequency353 THz194 THz17 GHz achievable bandwidth B for optical and RF links = ?

29. Oct. 8, 2008 29 W. Leeb Link geometry variable parameters: antenna diameters, transmit power... wavelength  T,  R... terminal troughput SNR... signal-to-noise ratio B... bandwidth

30. Oct. 8, 2008 30 W. Leeb Bandwidth P T = 10 W L = 50 000 km, SNR = 16 dB RF: f = 17 GHz,  R  R = 0.35, noise figure 3 dB, P T = 10 W e.g. D T = 2.8 m D R = 2.0 m   = 1 W

31. Oct. 8, 2008 31 W. Leeb Bandwidth P T = 10 W Optical: = 0.85 µm,  R  R = 0.25, M APD,opt, i n.el = 12 pA/  Hz, N back = 2·10 -25 W/Hz, B opt = 1nm RF: f = 17 GHz,  R  R = 0.35, noise figure 3 dB, P T = 10 W e.g. D T = 2.8 m D R = 2.0 m   = 1 W  P T = 0.1 W L = 50 000 km, SNR = 16 dB

32. Oct. 8, 2008 32 W. Leeb Bandwidth P T = 10 W Optical: = 0.85 µm,  R  R = 0.25, M APD,opt, i n,el = 12 pA/  Hz, N back = 2·10 -25 W/Hz, B opt = 1nm RF: f = 17 GHz,  R  R = 0.35, noise figure 3 dB, Optical: = 1.55 µm,  R  R = 0.25, i n,el = 12 pA/  Hz, N back = 4·10 -25 W/Hz, B opt = 0.5 nm P T = 10 W e.g. D T = 2.8 m D R = 2.0 m   e.g. D T = 14 cm D R = 23 cm = 1 W    = 0.3 W P T = 0.1 W P T = 1 W L = 50 000 km, SNR = 16 dB

33. Oct. 8, 2008 33 W. Leeb Antenna gain and beam spread loss HAP(at 20 km)-to-GEO uplink, = 1.5 µm antenna gain antenna gain minus beam spread loss, h HAP = 20 km antenna gain minus beam spread loss, h HAP = 1 km

34. Oct. 8, 2008 34 W. Leeb Sun as background SNR degradation due to sun as background [dB] APD receiver (large field-of-view) 15 10 5 0 16 dB EDFA receiver (single transverse mode) 0.7 dB N back = 4  10 -20 W/Hz

35. Oct. 8, 2008 35 W. Leeb Beam spread loss (  bs ) for HAP-to-HAP links = 1.55 µm, D T = D R = 13,5 cm  bs = 0.3 dB... up, medium turbulence  bs = 0.7 dB... down, medium turbulence  bs = 0.3 dB... weak turbulence  bs = 0.7 dB... strong turbulence  bs  with D T , because ratio D T /diameter of turbulent eddies ... but much less than antenna gain!

36. Oct. 8, 2008 36 W. Leeb Entangled photons for cryptography AliceBob aim: global distribution of cryptographic keys using a source of entangled photons onboard the International Space Station (ISS) or on a HAP?

37. Oct. 8, 2008 37 W. Leeb Summary large bandwidth obtainable with  low antenna diameter  small prime power (?)  compact terminal (?) challenges  mutual acquisition, tracking of terminals strategies towards implementation  adapt demonstrated systems and technologies  systems should have potential for further development very small disturbance by atmosphere for  HAP  GEO link (zenith angle < 45°)  HAP  HAP link (h HAP = 20 km)