© 2014 IBM Corporation IBM Research - Zurich VCSEL based Radio-over-Fiber Links for Radio Astronomy Jonas Weiss, IBM Zurich Research Lab, Switzerland

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Overview Background and Motivation Radio Astronomy Specifications vs. other RFoF Applications VCSEL based RoF Link Design Results and Discussion Outlook 2

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss 3 The Task: Directly Connect Remote Antennas to Signal Processing Data Processing Up to 50 km Current design: -250’000 antennas, 2 signals each -Outer stations on spiral arms, up to 50 km radius -Except for most outer stations: Direct analog optical link from each antenna to Data Processing. Why optical links? -Only «alternative» is electrical data/signal transmission, which has orders of magnitude higher losses → many repeaters (amplifiers) along the cable! Why not sample at the antennas and transmit many signals over a single fiber? -System is extremely noise- and interference sensitive! That’s why it’s in the desert! Any digital logic within the antenna area would pose a serious RFI risk. -System is extremely phase-sensitive. Phase accurate sampling-clock distribution and synchronization on that scale would be extremely difficult and costly.

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Overview Background and Motivation Radio Astronomy Specifications vs. other RFoF Applications VCSEL based RoF Link Design Results and Discussion Outlook 4

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Radio over Fiber Links for Different Applications 5 FTTx (Fiber to anywhere Transceivers) - 50 – 900 MHz, ~850 MHz BW → broadband (> 3 octaves) - < km few to 10’s km fiber cable Optical Transmitter Optical Receiver DriverLaserPDAmplifier Generic Radio/RF over Fiber/Glass Link SKA LFAA Telescope MHz, = 300 MHz BW → broadband (> 2 octaves) -10+ km -Veeery low cost (x 500’000) Source: emcore Comparable! Antenna Remoting e.g. for (microcellular) GSM - 2.5/5 GHz, < 168 MHz BW → < % relative bandwidth - < km? Compatible!

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Overview What is the Low Frequency Aperture Array (LFAA) Radio Astronomy Specifications vs. other RFoF Applications VCSEL based RoF Link Design Results and Discussion Outlook 6

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss VCSELs for RF over Fiber Links 7 Why use VCSELs? → Potentially significantly lower cost and not DFBs? (device testing and packaging) → High-Speed (> 25 Gbps) → Low-enough noise (RIN < -140 dB) → Good linearity (IIP3 > ~10 dBm) Challenges:→ almost exclusively short-wavelength (850 nm) multimode devices → Higher fiber-losses than single mode (2.3 vs. 0.3 dB/km) → Multimode fibers are more expensive ( x) Gain of the RoF Link: 21 dB Noise Figure of the RoF link:21.9 dB (at 350 MHz) Link IIP3:-20.7 dBm (incl. 10 dB RFI reserve) Link SFDR:79.7 dB∙Hz 2/3 (incl. 10 dB RFI reserve) 23.1 dB Link Requirements for LFAA:

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss 8 few to 10’s km fiber cable DriverLaserPDAmplifier VCSEL based RF over Fiber Link Design OM4 MM Fiber > 7 km Goal:- Keep it simple, small & low cost - meet/exceed performance requirements - off-the-shelf components and technologies - add fancy control and monitoring at a later time NF = 22 dBGain = 21 dB IIP3 = -21 dBmSFDR > 80 dB∙Hz 2/3 ~90 mW > 600 MHz < 15 € ~90 mW > 600 MHz < 10 €

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Overview What is the Low Frequency Aperture Array (LFAA) Radio Astronomy Specifications vs. other RFoF Applications VCSEL based RoF Link Design Results and Discussion Outlook 9

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Radio over Fiber Link Measurements and Results Measurements vs. Model ~2.5 km ! SFDR=2/3(OIP3 – Output_Noise_Density ) System Requirements ~ 7km ! SFDR [dBm∙Hz 2/3 ] Conclusions: -Can reach 7 km, possibly also 10 km -Attenuation and noise limited -Beyond ~2-3 km, cost of multi-mode fibers offsets VCSEL price advantage! -Speed and distance can be traded-off -Can also operate at few GHz Multimode Fiber (from 2x up to 10 x cost of single-mode fiber) 10

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Link Potential and Outlook 11 Spurious Free Dynamic Range SFDR > 92 dBHz 2/3 for 2+ km links «linear» trade-off between SFDR and link length Potential use for remote antenna-feeds or FTTx applications Low power, low cost, low/no RFI emission, high EMI immunity → suitable for various high-bandwidth and high- frequency remote sensor applications

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Acknowledgements IBM Zurich Research Lab: Martin Schmatz Ralph Heller Bert Jan Offrein, Folkert Horst Photonics Group Astron (Netherlands Institute for Radio Astronomy) Peter Maat 12

© 2014 IBM Corporation IBM Research - Zurich Jonas Weiss Thank You! 13