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

Radio Interferometers X E1E1 E2E2 Cross-correlation of signals From the antennas D Interferometer: signals from each telescope brought together coherently for correlation Resolution= λ/D Groningen Use of several telescopes Enables aperture synthesis

X HomestationOutstation UHF Tx LO Synth UHF Rx Microwave Tx Correlator Rx 463 MHz 7.9 GHz TV Link A Radio Linked Interferometer 1960’s Rowson 1973 No phase lock loops at outstation! Groningen 20143

1960’s Ever Increasing Baselines: Cat and Fiddle, Mucklestone by Loggerheads, Pocklington….. 25 ft Dish (Donaldson) at Pocklington Yorkshire 1966 H. P. Palmer A. C. B. Lovell Groningen 20144

MkII-MkIII Interferometer MkIII 85 ft x 125 ft at Wardle 24 km from JB Also Defford 85 ft 127 km away Groningen 20145

Go and Return Phase -2πfτ Frequency f ~ PLL Delay τ Phase -4πfτ Options 1.Add to give 2f : same phase at both ends 2.Subtract to measure τ : correct in correlator x2 Phase -4πfτ Groningen 20146

Phase Stable Interferometer 1970’s Warwick, Davis and Spencer 1976 MNRAS 177, 335 Signals from 1 and 2 added, 30 MHz used to synthesise LO Groningen 20147

Phase Stability: ~ 1% of uncorrected Groningen 20148

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 MHz Microwave link used only for radio astronomy base-band signal Groningen 20149

Schematic: Groningen

L-band link (c !) Uses a pulsed system at 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 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

Groningen

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

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

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

Phase measurements Suresh Kumar and Matt Strong deg=1 ns Groningen

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

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

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)

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

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

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

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

Groningen 2014 Paul Boven Nov 2014

Tsingua Phase Transfer System NB patented Groningen

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

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

UWA Frequency Transfer Concept UWA Lab Tour ν OPT +1ν MW +2ν AOM-A +2ν Re.AOM ν OPT +1ν AOM-B +2ν Re.AO M +1ν AOM-A reflected 2ν AOM- A +2ν Re.AOM Optical Frequency at PD-B Intensity ν OPT ν OPT +2ν AOM-B +2ν Re.AOM ν OPT +1ν MW +1ν AOM-A +2ν Re.AOM +1ν AOM-B reflected reference 2ν AOM- B +2ν Re.AOM Optical Frequency at Re.PD ν OPT +1ν AOM-B +1ν Re.AOM ν OPT +1ν MW +1ν AOM- A +1ν AOM- C ν MW + (ν AOM-A − ν AOM- B ) Intensity Antenna Site Link SKA Central Site Laser Freq. Shift ν MW Re.PD ν MW + (ν AOM-A − ν AOM- B ) Mirror ν Re.AOM Re.AOM AOM-B ν AOM-B AOM-A ν AOM-A PD-A Mirror 2ν AOM-A +2ν Re.AO M Servo A Mirror 2ν AOM-B +2ν Re.AO M PD-B Servo B Optical Frequency at PD-A Intensity ν OPT +1ν MW reference

Questions? Groningen