Phase Stable Interferometers at Jodrell Bank

1. Phase Stable Interferometers at Jodrell Bank Ralph Spencer Jodrell Bank Centre for Astrophysics University of Manchester 3rd VLBI Technical Meeting Groningen Nov 2014 Groningen 2014

2. Radio Interferometers Interferometer: signals from each telescope brought together coherently for correlation E1 E2 D Use of several telescopes Enables aperture synthesis X Resolution= λ/D Cross-correlation of signals From the antennas <E1xE2> Groningen 2014

3. Homestation Outstation A Radio Linked Interferometer 1960’s LO Synth 7.9 GHz TV Link X Correlator Rx Microwave Tx UHF Rx UHF Tx 463 MHz No phase lock loops at outstation! Rowson 1973 Groningen 2014

4. 1960’s Ever Increasing Baselines: Cat and Fiddle, Mucklestone by Loggerheads, Pocklington….. H. P. Palmer 25 ft Dish (Donaldson) at Pocklington Yorkshire 1966 A. C. B. Lovell Groningen 2014

5. 1968 - • MkII-MkIII Interferometer • MkIII 85 ft x 125 ft at Wardle 24 km from JB • Also Defford 85 ft 127 km away Groningen 2014

6. Go and Return Frequency f Phase -2πfτ Delay τ PLL Phase -4πfτ ~ Options Add to give 2f : same phase at both ends Subtract to measure τ : correct in correlator x2 Phase -4πfτ Groningen 2014

7. Phase Stable Interferometer1970’s Signals from 1 and 2 added, 30 MHz used to synthesise LO Warwick, Davis and Spencer 1976 MNRAS 177, 335 Groningen 2014

8. Phase Stability: ~ 1% of uncorrected Groningen 2014

9. MERLIN • Multiple telescopes • Multi frequency licences untenable • Need repeaters since distances > 30 km • Separate signals using pulse width encoding • Uses L-band go-and-return link at 1486.3 MHz • Microwave link used only for radio astronomy base-band signal Groningen 2014

10. Schematic: Groningen 2014

11. L-band link (c. 1977 !) • Uses a pulsed system at 1486.3 MHz, measure go and return delay • Typical pulse switching cycle: • Pulse 1 transmitted from Jodrell, received by repeater A. • Pulse 2 transmitted from repeater A, received by repeater B. • Pulse 3 transmitted from repeater B, received by outstation. • Pulse 4 transmitted from outstation, received by repeater B. • Pulse 5 transmitted from repeater B, received by repeater A. • Pulse 6 transmitted from repeater A, received by Jodrell. • Logical timing synchronised via pulse width. Pulse cycle repeats at 88 Hz • Received 1486.3 Mhz signal phase locks oscillators at 75.3 MHz at repeaters and remote telescopes • Received signal at the remote telescope used to lock a quality 5 Mhz reference oscillator with 10-sec loop constant. Freq. synthesiser generates LO’s etc. Anderson, Davis, Bentley, Speed, Spencer…. Groningen 2014

12. Groningen 2014

13. L-band link performance Typically achieves 1-2 ps rms equivalent timing accuracy over 10 mins

14. Can we use optical fibre? • Transfer phase information using fibre optics in a Phase Transfer System • Inherent problem: fibres do not conserve phase – e.g PMD is non-reciprocal • Need to develop a system to monitor and correct for phase variations • Laboratory tests on fibre: York MSc tech proj, 2002, Strong PhD thesis 2005 Groningen 2014

15. Lab set up 1 GHz modulation, 1550nm laser, 20 km of fibre Groningen 2014

16. Phase measurements 360 deg=1 ns Suresh Kumar and Matt Strong 2004 Groningen 2014

17. Go and return phase minus 2x one way phase 2 deg= 5.5 ps • Limited by circulators: eliminate! • Use pulsed L-band link on fibre? Groningen 2014

18. Schematic diagram of the e-MERLIN fibre network Pickmere Darnhall Plumley Winsford JBO Crewe Burlton Nottingham Peterborough Knockin Wolverhampton Birmingham Sandy Hatton Cambridge Defford Global Crossing PoP Interconnect to Global Crossing network Antenna Dark fibre New fibre build 570 km of dark fibre 100 km of new build tails. Groningen 2014

19. e-MERLIN Phase Transfer • Requires ~ 2 ps rms stability over ~250 km to maintain coherence at 22 GHz • Existing MERLIN L-band link works very well, producing stable fringes for 25 years! • Licences and tower rental becoming a problem • So use L-band link to modulate optical link • RF over fibre system, go and return at 1.5 micron , WDM with data transfer (Anderson, Spencer, Garrington, McCool 2008)

20. Tests on Optical Phase transfer • ~1-2ps over 1s-10min demonstrated in SKADS ( R McCool et al. 2008, 2009) Back to Back 28.6 km @ 1550 nm 28.6 km @ 1310 nm 110 km no thermal control 110 km thermal control Groningen 2014

21. ALAMA – uses coherent laser Go-and-return measurements with reflected IR laser light. Active compensation with optical line-stretcher Shillue et al SPIE 2012 Groningen 2014

22. SKA Requirements • Generate sampler clocks to 1 ps equivalent timing accuracy • Compensate for link path variations where necessary • reference 1 pps ticks coherent (jitter < 0.1 ns) with central clock LO maser. • measure 1pps against local timing (GPS or WR) ~10ps • provide optical timecode as required (cf WIDAR correlator TC) • measure the round trip delay to sampler • provide tick-timing offsets to the correlator/data acquisition system for corrections in software Groningen 2014

23. General scheme • 3 solutions considered: • Modify e-MERLIN – correct in correlator • Tsingua - automatic correction • Uni. W Australia - automatic correction Groningen 2014

24. Groningen 2014

25. Tsingua Phase Transfer System NB patented Groningen 2014

26. Tsingua scheme: Delay τ f1 Φ=f1τ 2 GHz ~ f1-f2 f2 1 GHz Φ=0 Reflect PLL 1 GHz Φ=2f2τ f1=2f2 Groningen 2014

27. University of Western Australia Scheme • Laser light from master is frequency shifted by M-Z modulator • Reflected through AOM at antenna • Returned signal used to dive correction servo • MW signals have same phase at each end Groningen 2014

28. UWA Frequency Transfer Concept PD-A PD-B 2νAOM-A+2νRe.AOM Servo A νMW Freq. Shift AOM-A νMW+ (νAOM-A − νAOM-B) Link νAOM-A Mirror Re.PD Laser Re.AOM Mirror νAOM-B νRe.AOM Mirror AOM-B Antenna Site 2νAOM-B+2νRe.AOM Servo B SKA Central Site 2νAOM-A+2νRe.AOM 2νAOM-B+2νRe.AOM νMW+ (νAOM-A − νAOM-B) νOPT+1νMW +1νAOM-A+2νRe.AOM +1νAOM-B νOPT+1νMW +1νAOM-A+1νAOM-C νOPT+2νAOM-B+2νRe.AOM νOPT+1νAOM-B+1νRe.AOM νOPT+1νMW +2νAOM-A+2νRe.AOM νOPT +1νAOM-B +2νRe.AOM +1νAOM-A νOPT νOPT+1νMW Intensity Intensity Intensity reflected reflected reference reference Optical Frequency at PD-A Optical Frequency at PD-B Optical Frequency at Re.PD UWA Lab Tour - 2014-10-04

29. Questions? Groningen 2014