Methods for data, time and ultrastable frequency transfer through long-haul optical fiber links

1. Methods for data, time and ultrastable frequency transfer through long-haul optical fiber links Jeroen Koelemeij LaserLaB & Depart. PhysicsandAstronomy VU University Amsterdam, The Netherlands

2. Outline • Why time & frequencythroughoptical fiber? • (Ultra)stable fiber-opticalfrequency transfer • Accurate fiber-optical time transfer • Integration into high-capacity fiber-optical telecom infrastructureandapplicationto VLBI

3. (Ultra)stable fiber-opticalfrequency transfer Partners/collaborators in the Netherlands: Tjeerd Pinkert VU Amsterdam Chantal van Tour VU Amsterdam Wim Ubachs VU Amsterdam Kjeld Eikema VU Amsterdam Roeland NuijtsSURFnet Rob Smets SURFnet Oliver Böll KVI Groningen Lorentz Willmann KVI Groningen Klaus Jungmann KVI Groningen JK

4. Noise detection +compensation power power Laser frequency out Laser frequency in Optical path length stabilization Compensation of frequency fluctuations due to length fluctuations*: *L.-S. Ma, P. Jungner, J. Ye, J.L. Hall, Opt. Lett. 19, 1777(1994) 1.5 mm clock laser Optical fiber (~ 100 km) Partial reflector roundtrip contains 2× noise! PLL Clock laser + noise power Laser frequency out

5. Example: 920 km link PTB group (Braunschweig, Germany): K. Predehlet al., Science 336, 441 (2012) 1840 km link: S. Droste et al., Phys. Rev. Lett. 111, 110801 (2013) Free-running link Germany H-maser Stabilized link

6. Transport through telecom fiber • Fiber attenuation: 20 dB/100 km, needamplifiers! • Issue: bi-directionalopticalamplifiersneeded, but telecom amplifiers are uni-directional (toavoidlasing) • Twoapproaches: • Dark fiber (no othersignals, us bi-di amp) • Dark channel (bi-di ‘bypass’ amplifier) (Paris groups, O. Lopez et al., Appl. Phys. B110, 3 (2012)) Scattered EDFA optical isolators Location A  Both approaches work  Both approaches sacrifice telecom capacity  Approach 2: additionalinsertionloss  Telecom operators oftenreluctant Location B Bidir amp

7. Part of the solution: out-of-band channels • Useout-of-band wavelength channels • C-band: 1530 nm – 1565 nm erbium-doped fiber amplifier (EDFA) gain spectrum • Use semiconductor opticalamplifiers (SOAs) forsignalamplification <1530 nm • Ease of wavelength multiplexingwith standard components … but does itworkforopticalfrequency transfer? Lab test on 5 km spooled fiber (Amsterdam)

8. Results H-maser 5 km link + SOA 5 km link SOA adds a small amount of noise, but link stabilitystill far below the stability of opticalclocks (andmasers)! YES Work in progress: compareperformance SOAswithEDFAs

9. From lab to field: SURFnet optical fiber link • Link part of SURFnet DWDM network • Length 317 km, round trip 635 km • Single l-channel (1559.79 nm) • Fiber carrying live data traffic • Optical clocks under development at both ends of fiber link • Fiber connects to JIVE Dwingeloo • Future: bi-directional fiber link

10. Accurate fiber-opticaltime transfer Partners/collaborators in the Netherlands: Nikos Sotiropoulos TU Eindhoven ChigoOkonkwo TU Eindhoven Huug de Waardt TU Eindhoven Tjeerd Pinkert VU Amsterdam Roeland NuijtsSURFnet Utrecht Rob SmetsSURFnet Utrecht Martin Fransen VSL Delft Erik Dierikx VSL Delft Henk Peek NIKHEF Amsterdam JK

11. Time transfer – the state of the art

12. Approach LaserLaB VU – TUEindhoven • CollaborationfundedbySURFnet, setup at TU Eindhoven • Finddelaysvia XCOR of 10 Gb/s bit streamsthrough75 km fiber link Quasi-bidirectional amplifier (Amemiyaet al., IEEE IFCSE 2005) 50 km 25 km Tworound-trip delaysmeasured: t12 (l1, l2) andt13 (l1,l3) • Advantages: • Transmit 10 Gb/s data, no telecom capacitysacrificed • Time +data transfer • Compatible withexisting telecom methods & equipment

13. PRBS signalsandcorrelation 50 GS/s 12.5 GS/s 75 km 150 km

14. Results Time difference= <OWDestimate> - <OWDdirect> Estimatedaccuracy: 4 ps (agreeswithobservations) OWD-tAB(t)[ps] Bit-error rate (BER) below 10-9 : Error free communication at 10 Gb/s 75 km link Measurementnumber 75 km 25 km -log BER 50 km 0 km Received power [dBm]

15. Results Delivery of 10 Gb/s optical data with 4 ps accuracy over 75 km distance OWD-tAB(t)[ps] 75 km link Measurementnumber 75 km 25 km -log BER 50 km 0 km N. Sotiropouloset al. (submitted) Received power [dBm]

16. Time transfer – the state of the art State-of-the-art delay determination + Error-free optical data transfer at 10 Gbps

17. Speed bonus • Delay determination/synchronizationrequires a single shot of 10 Gb/s datalastinglessthan 1 ms • For comparison: state-of-the-art methodsrequire 10-100 s of averagingtoachieve 4 ps stability

18. Integration into high-capacity fiber-optical telecom infrastructureandapplicationto VLBI Use out-of-band wavelengths integrate time andfrequency transfer in hardware for high-capacityoptical telecom Will requireinvolvment of manufacturers of optical telecom network equipment andNRENs… … AND a convincing test case! eVLBIusing fiber-opticalsynchronization? Data out T&F out Fiber in

19. Application toeVLBI? Disclaimer: notnecessarilylimitedto Europe! • 10 Gb/s channelforantennasignal transport • SynchronizeLO’s at telescope sites through fiber to 4 ps = (1/5) of a 50 GHz cycle • Usefulforinitialcalibration? • Phase-lock 10 Gb/s tostable ‘Master clock’ anddistributethroughstabilized fiber links • Phaselock LO torecoveredclock at remote sites • Use low-noise TCXO/OCXO for short-term stability • Userecoveredclockfor long-term stability • Do awaywithexpensive H-masers? Master clock Special thanksto Paul Boven andArpadSzomoru of JIVE forinsightfuldiscussionsabouteVLBI

20. Work in progress… • Demonstratetime transfer VSL-VU-SARA-NIKHEF • Ultrastable frequency transfer VU – JIVE Dwingeloo – KVI • Test new techniquesthat do notaffect/sacrifice telecom capacityand performance • DemonstrateanopticalGPS-timing backup system • Developterrestrialoptical-wireless positioning with cm accuracy (with TU Delft - SuperGPS 4 ps  2.4 mm accuracy (4D positioning) Aperturesynthesisthrough mobile handsets?

21. Thanks!