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

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!

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.

Properties of links: Typical loss of fibre = 0.25 dB per km at 1550 nm. Dispersion (NZDSF) = 4 ps/km/nm. Power output of 10 Gbps laser diode+modulators is 1 mW Typical receiver sensitivity for error rate is 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 ’s and needs EDFAs for ~20 km links.

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

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 limit at ~80km –Can be overcome using NZ-DSF and dispersion compensation methods to reach spans of hundreds of kms Polarisation Mode Dispersion limit at ~400km –Cannot easily be compensated. Regeneration required at this limit.

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

Some current prices (10 Gbps link per antenna)

Some Possible SKA Configurations O Why ? Circle Y (cf. VLA) Spiral r~  2

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

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

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

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

Llano de Chajnantor Simon Radford’s photo

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.