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

2. Outline Why time & frequency through optical fiber? (Ultra)stable fiber-optical frequency transfer Accurate fiber-optical time transfer Integration into high-capacity fiber-optical telecom infrastructure and application to VLBI

3. (Ultra)stable fiber-optical frequency transfer Partners/collaborators in the Netherlands: Tjeerd PinkertVU Amsterdam Chantal van TourVU Amsterdam Wim UbachsVU Amsterdam Kjeld EikemaVU Amsterdam Roeland NuijtsSURFnet Rob SmetsSURFnet Oliver BöllKVI Groningen Lorentz WillmannKVI Groningen Klaus JungmannKVI Groningen JK

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

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

6. Transport through telecom fiber Fiber attenuation: 20 dB/100 km, need amplifiers! Issue: bi-directional optical amplifiers needed, but telecom amplifiers are uni-directional (to avoid lasing) Two approaches: 1.Dark fiber (no other signals, us bi-di amp) 2.Dark channel (bi-di bypass amplifier) (Paris groups, O. Lopez et al., Appl. Phys. B 110, 3 (2012)) Location A Location B EDFA optical isolators Scattered Bidir amp

7. Part of the solution: out-of-band channels Use out-of-band wavelength channels – C-band: 1530 nm – 1565 nm erbium-doped fiber amplifier (EDFA) gain spectrum – Use semiconductor optical amplifiers (SOAs) for signal amplification <1530 nm – Ease of wavelength multiplexing with standard components … but does it work for optical frequency transfer? Lab test on 5 km spooled fiber (Amsterdam) EDFASOA Max. gain [dB]25-3020-25 Max. bi-di gain [dB]<25 Noise Figure [dB]6-88-10 Nonlinearity (keep P in low)

8. Results 5 km link + SOA 5 km link SOA adds a small amount of noise, but link stability still far below the stability of optical clocks (and masers)! Work in progress: compare performance SOAs with EDFAs YES H-maser

9. From lab to field: SURFnet optical fiber link Link part of SURFnet DWDM network Length 317 km, round trip 635 km Single -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-optical time transfer Partners/collaborators in the Netherlands: Nikos SotiropoulosTU Eindhoven Chigo OkonkwoTU Eindhoven Huug de WaardtTU Eindhoven Tjeerd PinkertVU Amsterdam Roeland NuijtsSURFnet Utrecht Rob SmetsSURFnet Utrecht Martin FransenVSL Delft Erik DierikxVSL Delft Henk PeekNIKHEF Amsterdam JK

11. Time transfer – the state of the art MethodDistanceAccuracyRef. GNSS>1000 km3 – 50 ns TWSTFT>1000 km1 ns T2L2>1000 km200 ps expectedFridelance et al., Exp. Astr. (1997) White Rabbit (fiber) (1 Gpbs Ethernet, PTP) 10 km0.1 - 1 ns www.ohwr.org Optical fiber (20 Mbps PRBS) 540 km100 - 250 psLopez et al., Appl. Opt. (2012) Optical fiber (20 Mbps PRBS) 73 km74 psRost et al., Metrologia (2012) Dedicated optical fiber (10 MHz + 1pps) 69 km (480 km) 8 ps (35 ps) Sliwczynski et al., Metrologia (2013)

12. Approach LaserLaB VU – TU Eindhoven Collaboration funded by SURFnet, setup at TU Eindhoven Find delays via XCOR of 10 Gb/s bit streams through 75 km fiber link Advantages: Transmit 10 Gb/s data, no telecom capacity sacrificed Time + data transfer Compatible with existing telecom methods & equipment 25 km 50 km Two round-trip delays measured: ( ) and ( ) Quasi-bidirectional amplifier (Amemiya et al., IEEE IFCSE 2005)

13. PRBS signals and correlation 75 km150 km 50 GS/s12.5 GS/s

14. Results Time difference= log BER Received power [dBm] 75 km 50 km 25 km 0 km Estimated accuracy: 4 ps (agrees with observations) Estimated accuracy: 4 ps (agrees with observations) Measurement number OWD t AB (t) [ps] 75 km link Bit-error rate (BER) below 10 -9 : Error free communication at 10 Gb/s Bit-error rate (BER) below 10 -9 : Error free communication at 10 Gb/s

15. Results log BER Received power [dBm] 75 km 50 km 25 km 0 km Measurement number OWD t AB (t) [ps] 75 km link N. Sotiropoulos et al. (submitted)

16. Time transfer – the state of the art MethodDistanceAccuracyRef. GNSS>1000 km3 – 50 ns TWSTFT>1000 km1 ns T2L2>1000 km200 ps expectedFridelance et al., Exp. Astr. (1997) White Rabbit (fiber) (1 Gpbs Ethernet, PTP) 10 km0.1 - 1 ns www.ohwr.org Optical fiber (20 Mbps PRBS) 540 km100 - 250 psLopez et al., Appl. Opt. (2012) Optical fiber (20 Mbps PRBS) 73 km74 psRost et al., Metrologia (2012) Dedicated optical fiber (10 MHz + 1pps) 69 km (480 km) 8 ps (20 ps) Sliwczynski et al., Metrologia (2013) Cross correlation of 10 Gbps optical data 75 km4 psSotiropoulos et al. (submitted)

17. Speed bonus Delay determination/synchronization requires a single shot of 10 Gb/s data lasting less than 1 ms – For comparison: state-of-the-art methods require 10-100 s of averaging to achieve 4 ps stability

18. Integration into high-capacity fiber-optical telecom infrastructure and application to VLBI Use out-of-band wavelengths integrate time and frequency transfer in hardware for high-capacity optical telecom Will require involvment of manufacturers of optical telecom network equipment and NRENs… … AND a convincing test case! eVLBI using fiber-optical synchronization? Fiber in Data out T&F out

19. Application to eVLBI? 10 Gb/s channel for antenna signal transport Synchronize LOs at telescope sites through fiber to 4 ps = (1/5) of a 50 GHz cycle – Useful for initial calibration? Phase-lock 10 Gb/s to stable Master clock and distribute through stabilized fiber links – Phase lock LO to recovered clock at remote sites Use low-noise TCXO/OCXO for short-term stability Use recovered clock for long-term stability – Do away with expensive H-masers? Master clock Special thanks to Paul Boven and Arpad Szomoru of JIVE for insightful discussions about eVLBI Disclaimer: not necessarily limited to Europe!

20. Work in progress… Demonstrate time transfer VSL-VU-SARA-NIKHEF Ultrastable frequency transfer VU – JIVE Dwingeloo – KVI Test new techniques that do not affect/sacrifice telecom capacity and performance Demonstrate an optical GPS-timing backup system Develop terrestrial optical- wireless positioning with cm accuracy (with TU Delft - SuperGPS 4 ps 2.4 mm accuracy (4D positioning) Aperture synthesis through mobile handsets?

21. Thanks!