SKA and Optical Fibre Links

1. SKA and Optical Fibre Links R.E. Spencer JBO Dec 2001 • Fibre links • Fibre optics and link design • Array configurations • Cost implications

2. Fibre Links • Microwave links and satellite links limited to <~200 Mbps. Fibres have Terabit capability, using multiple wavelength channels (WDM –wavelength division multiplexing). • Current fibre optic technology has 10 Gbps components per channel available in 2001, 40 Gbps in 2 years time (but expensive and dispersion effects are more severe). • A conservative approach would use 10 Gbps per wavelength launched. ~100 wavelengths possible per fibre. • Commercial internet provision costs for virgin sites are exhorbitant!

3. Link Design • The link design is constrained by fibre transmission characteristics such as attenuation and dispersion, which are dependant on length. • Error rates depend on signal:noise and distortion due to dispersion. • Non-linear effects restrict total input power to < few mW. In WDM four wave mixing etc. gives cross-talk between channels.

4. Properties of links: • Typical loss of fibre = 0.25 dB per km at 1550 nm l. • Dispersion (NZDSF) = 4 ps/km/nm. • Power output of 10 Gbps laser diode+modulators is 1 mW • Typical receiver sensitivity for 10-10 error rate is 10-2 mW. • These limit maximum span of fibre to ~50 km before amplification required. • Amplifiers can be Erbium doped fibre amplifiers (EDFAs) or Raman effect amplifiers (just becoming available). • Multiple wavelengths on a link require optical multiplexers and de-multiplexers – with extra loss. • E.g ALMA has 12 l’s and needs EDFAs for ~20 km links.

5. Effect of losses and dispersion on error rates • Error rate given by the complementary error function: =Electrical SNR

6. Limits of 10Gbps transmission over SMF fibre • Attenuation limit at ~80km • Can be overcome using Erbium Doped Fibre Amplifiers (EDFAs) to reach spans of hundreds kms • Dispersion limitat ~80km • Can be overcome using NZ-DSF and dispersion compensation methods to reach spans of hundreds of kms • Polarisation Mode Dispersion limitat ~400km • Cannot easily be compensated. Regeneration required at this limit.

7. Effects of signal/noise ratio: -Eye diagram for 10 Gbps Data transmission (phase switched 5 GHz signal). -Using Multiplex pin diode Detector as for ALMA

8. Some current prices (10 Gbps link per antenna)

9. 400 200 0 -200 -400 -600 -400 -200 0 200 400 600 Some Possible SKA Configurations • O Why ? Circle Y (cf. VLA) Spiral r~q2

10. A Random Array • Antenna position chosen at random • 100 antennas • Max spacing ~900 km • Gives low, noise-like sidelobes for snapshot mapping

11. Random array: connections to centre and beam Compass diagram Array Beam of antenna positions

12. Connecting a random array Another random array, The travelling salesman solution 25000 km radial dig 7500 km dig

13. Costs of Links in k$: random Circle: Spiral: Y:

15. Llano de Chajnantor Simon Radford’s photo

16. Conclusion • Optical fibres only sensible solution for high data rate systems. • Costs depend heavily on the dig cost (average ~$100/m in UK) -- choosing the terrain can help e.g. plough in for sand. • Small change in configuration could lead to major changes in costs of fibre links – links should be considered when deciding the configuration.