Long-distance fiber-optical time transfer in the Netherlands Jeroen Koelemeij VU Amsterdam

Simultaneous fiber-optical delivery of picosecond time and 10 Gb/s data over 75 km distance Jeroen Koelemeij LaserLaB VU University, Amsterdam, The Netherlands Nikolaos SotiropoulosCOBRA Institute, Eindhoven University of Technology, Chigo OkonkwoThe Netherlands Huug de Waardt Roeland NuijtsSURFnet (Dutch Research & Education Network), Utrecht, The Netherlands

Timing is everything Time transfer & Atomic clocks PNT(GNSS)PNT(GNSS) InternetInternet Mobile telecom e-Financial transactions Power grids Oil & mineral exploration Power grids Oil & mineral exploration ScienceAstronomyAerospaceScienceAstronomyAerospace

Time transfer – the state of the art MethodDistanceTime uncertaintyRef. GNSS>1000 km3 – 50 ns TWSTFT>1000 km1 ns T2L2>1000 km0.2 ns expectedFridelance et al., Exp. Astr. (1997) White Rabbit (fiber) (1 Gpbs Ethernet, PTP) 10 km ns Optical fiber (20 Mbps PRBS) 540 km0.1 – 0.25 nsLopez et al., Appl. Opt. (2012) Dark optical fiber (20 Mbps PRBS) 73 km74 psRost et al., Metrologia (2012) Dark optical fiber (10 MHz + 1pps) 69 km (480 km) 8 ps (20 ps) Sliwczynski et al., Metrologia (2013) Satellite methods Optical fiber methods Satellite and optical fiber methods

Time transfer through optical fiber Need to correct for (unknown) propagation delay t AB Can measure only round-trip delay: t RT = t AB + t BA Generally, t BA  t AB (t BA = t AB +  ) Estimate: t AB = (t RT –  )/2 AB Optical fiber Optical signals Important requirement: Calibration of  Bidirectional light path through optical fiber to achieve <1  s

THE disadvantage (so far) of OFTT Hard to get access to fiber telecom infrastructure! Need ‘dark’ wavelength channel or unused ‘dark’ fiber, but… Telecom fiber infrastructure: mostly commercial business, fiber owners want to earn back their investment (and more!) Governmental institutions possess fiber infrastructure, but often interested only if it supports their mission/saves money Our approach to circumvent this: develop methods which are compatible with high-capacity optical telecom Not entirely new (IEEE 1588, SyncE can make use of optical connections) BUT key to sub-ns accuracy lies in the optical layer: bidirectional optical light path is essential!

Our approach Test bed: 75 km, 10 Gb/s telecom link (spooled fiber) at TU/e Find delays via XCOR of 10 Gb/s bit streams (captured with oscilloscope) Advantages: Time + 10 Gb/s data transfer functionality - no telecom capacity sacrificed! Compatible with existing telecom methods & equipment 25 km 50 km Quasi-bidirectional amplifier (Amemiya et al., IEEE IFCSE 2005)

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

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

Sources of delay asymmetry Nonreciprocal paths in instruments, fiber patches, amps:  I – Easily accumulates to >> 1 ns – Calibrate: remove fiber spools and measure – Calibrate dependence on ambient conditions and system parameters Choosing different wavelengths for downstream and upstream communication adds to compatibility with existing networks, BUT leads to asymmetry due to dispersion – Chromatic dispersion (1.6 ns/nm per 100 km) – Polarization mode dispersion (<10 ps for 100 km link) Calibrate chromatic dispersion by use of third wavelength 1 2  AB

Few ns size, sub-ps uncertainty Few ps size, sub-ps uncertainty Chromatic dispersion calibration 1.Measure round-trip delays t AC 12 and t AC 13 2.For each combination ( 1, J ), calibrate all other delay asymmetries (  =  I +  PMD ) 3.Measure wavelengths j (0.3 pm uncertainty) 4.Estimate one-way delay (  AB ) using formula: (Estimate n‴ separately; see Sotiropoulos et al. Optics Express 21, (2013)

Polarization mode dispersion Measure two different round-trip delays with opposite polarization states  PMD found by differencing the delays PMD leads to few-ps delay asymmetry in 75 km legacy fiber, much less in newer optical fiber types 25 km 50 km

Every millimeter counts… Effect of air-gap attenuator Measure signal propagation delays with 200 fs resolution! 

Every dB counts… SOA input power SOA bias current Received power 

Results Time difference=  e.g. OWDestimate = (4.2) ps OWDdirect = (0.8) ps Estimated delay uncertainty: 4 ps (agrees with observations) Estimated delay uncertainty: 4 ps (agrees with observations) Measurement number OWD  t AB (t) [ps] 75 km link  log BER Received power [dBm] 75 km 50 km 25 km 0 km Bit-error rate (BER) below : Error free communication at 10 Gb/s Bit-error rate (BER) below : Error free communication at 10 Gb/s

Results Time difference=  e.g. OWDestimate = (4.2) ps OWDdirect = (0.8) ps Estimated delay uncertainty: 4 ps (agrees with observations) Estimated delay uncertainty: 4 ps (agrees with observations) Measurement number OWD  t AB (t) [ps] 75 km link  log BER Received power [dBm] 75 km 50 km 25 km 0 km Bit-error rate (BER) below : Error free communication at 10 Gb/s Bit-error rate (BER) below : Error free communication at 10 Gb/s

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., Optics Express 21, (2013)

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

Dissemination of UTC(VSL) via WR from Delft to Amsterdam Tjeerd PinkertLaserLaB VU University, Amsterdam, Jeroen Koelemeij The Netherlands Erik DierikxVSL Delft Martin FranssenThe Netherlands Nikolaos SotiropoulosCOBRA Institute, Eindhoven University of Technology, Huug de WaardtThe Netherlands Rob SmetsSURFnet (Dutch Research & Education Network), Utrecht, The Netherlands Henk PeekNIKHEF Amsterdam, The Netherlands

Work in progress… Demonstrate optical time transfer from VSL (Delft) to NIKHEF (Amsterdam) Fiber link provided by SURFnet Amplified fiber link to distribute UTC(VSL) via WR

WR link VSL – NIKHEF (2 × 80 km) SOA WDM WR node 1 Atomic clock WDM Rx Tx SOA WDM WR node 2 WDM Rx Tx link VSL-TU DelftSURFnet link NIKHEF WR node 3 Rx Tx WR node 4 Rx Tx time & freq comparison Atomic clock time & freq comparison VSL Delft       nm   nm WDM Leiden Delft Amsterdam UTC(VSL) measurement Bidirectional (dark) fiber link

Towards large-scale depoyment in live networks v1 bi-di OLA (developed i.c.w. TU Eindhoven / SURFnet / VTEC Lasers and Sensors) Includes remote control & monitoring through Ethernet > 20 dB gain (fiber-to-fiber) Delay asymmetry calibrated at ps level Under development: Commercial v2 bi-di OLA  Remote control & monitoring, compatible with telecom infrastructure & standards  Including filters for bi-directional bypass in live DWDM networks (patent pending)  Create bi-di optical path for WR TFT w/o insertion loss  Delay asymmetry calibrated at ps level  Create bi-di optical path for ultrastable frequency dissemination  Commercially available (expected launch Q4 2014)

Outlook: SuperGPS for science & society Planned (VU Amsterdam, TU Delft & stakeholders) 4 ps  2.4 mm uncertainty (4D positioning)

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