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Optical Communication with Laser Beams for HAPs

This paper discusses the use of optical communication with laser beams for High Altitude Platforms (HAPs) in various scenarios, including HAP to HAP, HAP to GEO, and LEO to GEO links. It explores the advantages, challenges, and building blocks of optical links, as well as the influence of the atmosphere on the communication. The paper also presents optical transceiver designs and discusses the importance of Pointing, Acquisition, and Tracking (PAT) for successful optical communication in space missions.

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Optical Communication with Laser Beams for HAPs

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  1. Cost 297HAPCOS Meeting, Friedrichshafen, GermanyOct. 8 – 10, 2008 Communicationsto and from HAPs –with laser beams?Walter Leebwalter.leeb@tuwien.ac.atVienna University of TechnologyInstitute of Communications and Radio-Frequency EngineeringGusshausstrasse 25/389, 1040 Vienna

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

  3. 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 f2)  10 rad, G  130 dB  small antenna diameter expecting: low terminal mass low power consumption W. Leeb

  4. 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 W. Leeb

  5. Scenarios distance L = 45000 to 83000 km data rate R = 3 Gbit/s distance L > 1000000 km data rate R = 2 Mbit/s GEO ... geostationary orbit LEO ... low earth orbit ISS ... International Space Station W. Leeb

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

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

  8. 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 W. Leeb

  9. 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 W. Leeb

  10. 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 W. Leeb

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

  12. Optical transceiver for space missions W. Leeb

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

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

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

  16. Input-output multiplexing (2) shared antenna aperture simple duplexing scheme  increased telescope diameter W. Leeb

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

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

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

  20. Influence of atmosphere • absorption by molecules attenuation •  scattering (molecules, waterdroplets, fog, snow) attenuation  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 pronounced influence within first 15 km above the Earth's surface, but relatively small influence above 15 km W. Leeb

  21. Beam spread far field: diffraction limited beam divergence in vacuum beam divergence including influence of turbulence r0 ... Fried-Parameter  ... wavelength W. Leeb

  22. Fried parameter Fried parameter r0 characterises the degree of turbulence, integrated over beam path  for a transmit antenna diameter DT equal to the Fried parameter r0, the turbulence causes an increase of the divergence by a factor of , i.e. a gain reduction by 3 dB  large r0 means little influence of turbulence  examples (medium turbulence,  = 1.5 m): - HAP(at 17 km)-to-satellite link r0 = 10 m - ground-to-satellite link r0 = 15 cm - downlink (satellite to HAP): in general negligible influence of turbulence - uplink: typically < 0.1 dB additional loss due to turbulence-induced beam spread W. Leeb

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

  24. 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 2characterises the temporal behaviour of intensity (I) fluctuations (normalized variance of I(t)) typically 2 < 0.025 for HAP-to-satellite link  temporal mean W. Leeb

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

  26. Sensitivity of receivers Optical on-off keying: BEP = 10-9 requires an average of 10 photons per bit (absolute physical limit) rule of thumb for detecting one bit of information: required is an energy ofeither 10 hf or 10 kT,whatever is larger optical regime requires 100 times larger input power! h ... Planck`s constant k ... Boltzmann`s constant T ... system temperature W. Leeb

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

  28. Transmission bandwidth - examples HAP (20 km)  GEOsatellite (36000 km) distance L = 50000 km (zenith angle 45°) achievable bandwidth B for optical and RF links = ? W. Leeb

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

  30. Bandwidth L = 50 000 km, SNR = 16 dB RF: f = 17 GHz, tRtR = 0.35, noise figure 3 dB, e.g. DT = 2.8 m DR = 2.0 m PT = 10 W PT = 10 W  = 1 W  W. Leeb

  31. Bandwidth L = 50 000 km, SNR = 16 dB RF: f = 17 GHz, tRtR = 0.35, noise figure 3 dB, Optical:  = 0.85 µm, tRtR = 0.25, MAPD,opt, in.el = 12 pA/Hz, Nback = 2·10-25 W/Hz, Bopt= 1nm e.g. DT = 2.8 m DR = 2.0 m PT = 10 W PT = 10 W PT = 0.1 W   = 1 W  W. Leeb

  32. Bandwidth L = 50 000 km, SNR = 16 dB RF: f = 17 GHz, tRtR = 0.35, noise figure 3 dB, Optical:  = 0.85 µm, tRtR = 0.25, MAPD,opt, in,el = 12 pA/Hz, Nback = 2·10-25 W/Hz, Bopt= 1nm Optical:  = 1.55 µm, tRtR = 0.25, in,el = 12 pA/Hz, Nback = 4·10-25 W/Hz, Bopt= 0.5 nm e.g. DT = 14 cm DR = 23 cm e.g. DT = 2.8 m DR = 2.0 m PT = 1 W = 0.3 W  PT = 10 W PT = 10 W PT = 0.1 W    = 1 W  W. Leeb

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

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

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

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

  37. Summary very small disturbance by atmosphere for • HAP  GEO link (zenith angle < 45°) • HAP  HAP link (hHAP = 20 km) 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 W. Leeb

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