Simultaneous fiber-optical delivery of picosecond time and 10 Gb/s data over 75 km distance

Transcription

1 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 Sotiropoulos Chigo Okonkwo Huug de Waardt COBRA Institute, Eindhoven University of Technology, The Netherlands Roeland Nuijts SURFnet (Dutch Research & Education Network), Utrecht, The Netherlands

2 Timing is everything PNT (GNSS) Science Astronomy Aerospace Mobile telecom Internet Time transfer & Atomic clocks e-financial transactions Power grids Oil & mineral exploration

3 Optical fiber methods Satellite methods Time transfer the state of the art Satellite and optical fiber methods Method Distance Time uncertainty Ref. GNSS >1000 km 3 50 ns TWSTFT >1000 km 1 ns T2L2 >1000 km 0.2 ns expected Fridelance et al., Exp. Astr. (1997) White Rabbit (fiber) (1 Gpbs Ethernet, PTP) 10 km ns Optical fiber (20 Mbps PRBS) Dark optical fiber (20 Mbps PRBS) 540 km ns Lopez et al., Appl. Opt. (2012) 73 km 74 ps Rost et al., Metrologia (2012) Dark optical fiber (10 MHz + 1pps) 69 km (480 km) 8 ps (20 ps) TDEV= s Sliwczynski et al., Metrologia (2013)

4 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 + D) Estimate: t AB = (t RT D)/2 Optical fiber A B Optical signals Important requirement: Calibration of D Bidirectional light path through optical fiber to achieve <1 ms

5 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!

6 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) Quasi-bidirectional amplifier (Amemiya et al., IEEE IFCSE 2005) 25 km 50 km Advantages: Time + 10 Gb/s data transfer functionality - no telecom capacity sacrificed! Compatible with existing telecom methods & equipment

7 PRBS signals and cross correlation 50 GS/s 12.5 GS/s 75 km 150 km

8 PRBS signals and cross correlation 50 GS/s 12.5 GS/s 75 km 150 km

9 Sources of delay asymmetry Nonreciprocal paths in instruments, fiber patches, amps: D 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 A l 2 l 3 l 1 B

10 Chromatic dispersion calibration 1. Measure round-trip delays t AC 12 and t AC For each combination (l 1, l J ), calibrate all other delay asymmetries (D = D I + D PMD ) 3. Measure wavelengths l j (0.3 pm uncertainty) 4. Estimate one-way delay (q AB ) using formula: Few ns size, sub-ps uncertainty 1 q D D l D l D l [ t t t t AB 2 2 AC AC 1 AC 2 AC 3 2 l2 l3 Ll l l l l l l n ( l ) / c Few ps size, sub-ps uncertainty (Estimate n separately; see Sotiropoulos et al., to appear in Optics Express) ]

11 Polarization mode dispersion 25 km 50 km Measure two different round-trip delays with opposite polarization states D PMD found by differencing the delays

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

13 Every db counts SOA input power SOA bias current Received power D

14 log BER OWD t AB (t) [ps] Results Time difference= <OWDestimate> <OWDdirect> 75 km link Measurement number Link length [m] (1) (1) (1) Source a q [ps] q [ps] q [ps] D I DPO time base stability Fit uncertainty VOAs PMD correction Wavelength measurement XCOR b interpolation Estimate n SPM and XPM <0.1 <0.1 <0.1 Fast fiber length fluctuations <0.1 <0.1 <0.1 Link length uncertainty <10 4 <10 4 <10 4 Total e.g. OWDestimate = (4.2) ps OWDdirect = (0.8) ps Estimated delay uncertainty: 4 ps (agrees with observations) Bit-error rate (BER) below 10-9 : Error free communication at 10 Gb/s 25 km 0 km 50 km 75 km Received power [dbm]

15 OWD t AB (t) [ps] log BER Received power [dbm] Results Delivery of 10 Gb/s optical data with 4 ps uncertainty over 75 km distance 75 km link Measurement number 75 km N. Sotiropoulos et al. (to appear in Optics Express) 25 km 0 km 50 km

16 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

17 Work in progress Demonstrate time transfer VSL-VU-SARA-NIKHEF [White Rabbit link to UTC(VSL)] Wish list: Integrate timing in DSP/coherent receivers? Develop terrestrial opticalwireless positioning with cm accuracy (with TU Delft) 4 ps 2.4 mm uncertainty (4D positioning)

18 Outlook - ETPS Enhanced Terrestrial Positioning System 4 ps 2.4 mm uncertainty (4D positioning)

19 Outlook - ETPS GNSS PNT backup system?

20 Credits TU Eindhoven (COBRA) Nikos Sotiropoulos Chigo Okonkwo Huug de Waardt SURFnet Roeland Nuijts (now at Ciena) Rob Smets Ronald van der Pol Funding VU Amsterdam (LaserLaB) Tjeerd Pinkert Kjeld Eikema

21 Thank you! Contact: