Vladimír Smotlacha, CESNET Accurate Time Transfer over Optical Network 6 th CEF Networks Workshop Prague 13 September 2010

Time and frequency standards pendulum –frequency standard until 1930 s quartz crystal Cesium (Rubidium) clock –elements of IA periodical table group (single electron in level s) –hyperfine levels transition emits microwave frequency photon Hydrogen maser near future: quantum logical clock –transition (in optical frequency) of isolated ion kept in electromagnetic field trap (Al, Hg, Sr,...)

Time transfer 1 ns represents 30 cm light path in vacuum, ~20 cm in cable or fiber cable, optical link –short distance, negligible environmental influence (e.g. thermal dilatation) radio broadcast systems satellite navigation systems (GPS,...) –time broadcast – tens of nanoseconds noise –timestamping of local clock – common-view, all-in-view, similar signal propagation in geographically close localities, accuracy below 1 ns (e.g. GTR50) two-way satellite transfer (TWSTFT) –assumes equal propagation delay in both directions two-way optical link time transfer

Time transfer over fibre Goal Design device and method for accurate time transfer – alternative to satellite based methods Comparable or better accuracy and stability on range ~1000 km Use existing DWDM all-optical networks

Adapters Features Two-way transfer Optical signal modulation for 1-pps encoding Uses SFP transceivers Based of FPGA Virtex-5 Requires two time interval counters

Time transfer principle x A = Tr A – T A x B = Tr B – T B ε A = Ts A – T A ε B = Ts B – T B δ AB = (x A – ε B – Θ AB )δ BA = (x B – ε A + Θ AB ) assume δ AB = δ BA Θ AB = ((x A – x B ) + ( ε A – ε B )) / 2

Adapter prototype

Cesnet DWDM network

Experiments We made 3 experiments: Optical loop measurement Time transfer Cesnet – BEV (Prague – Vienna) Comparison with GPS time transfer Participants: IPE (Institute of Photonics and Electronics), Czech national time and frequency laboratory, Prague BEV (Bundesamt für Eich - und Vermessungswesen), Austrian national time and frequency laboratory, Vienna ACOnet, Austrian NREN, Vienna West Bohemian University, Plzen

Optical loop experiment Both endpoints in one laboratory, common clock Bidirectional optical loop length 744 km DWDM production network 4 segments, 3 fiber providers 12 optical amplifiers Various optical elements (Cisco, CzechLight) Segment Praha – Hradec Kralove on top of electricity distribution poles

Optical loop - geography Praha - Brno 284 km Brno - Olomouc 113 km Hradec Králové - Olomouc 197 km Praha - Hradec Králové 150 km km

Optical loop - results One-way delay in both directions fluctuation ~130 ns (temperature changes about 12 °C) aerial fiber on top electricity distribution poles residual asymmetry < 2 ns (resp. TDEV 8.7 ps / 500 s)

Optical loop – results II

Prague – Vienna experiment Time transfer between Cesnet and BEV Site A: Rb clock in Cesnet, Prague, GPS disciplined Site B: Rb clock, BEV (resp. Vienna university), free running 506 km, DWDM in production network, (Prague – Brno – Vienna)

Prague – Vienna results Step in one-way delay ( :13 UTC) –direction to Prague +72 ns (cca +14 m) –direction to Vienna +16 ns(cca +3 m) Free running Rb clock: relative frequency offset 8.08 *10 -12

Optical x GPS time transfer Comparison with GPS time transfer Site A: free running Rb clock in Plzen (West Bohemian University) Site B: Cs clock in IPE – UTC(TP) 150 km of fibre, WDM, production network GTR50 installed in both sites 95 km geographical distance

Optical x GPS transfer - results 10 days measurement comparison with GTR50 – exact calibration system based on GPS difference in range ± 2 ns short time TDEV (time stability) 120 ps

Optical x GPS transfer – results II Optical transfer is more stable for averaging time up to 100 s then common-view method Equal stability for longer averaging time interval (influenced by Rb clock rather then the measurement method)

Conclusions Adapters prototypes successfully tested and method verified No influence to other DWDM traffic Fiber length dilatation cancels in two-way transfer (residual asymmetry less than 1 ns at 744 km fiber in 6 day test) TDEV 8.7 ps / 500 s at 744 km fiber Better TDEV then common-view method (averaging interval up to 100 s, distance about 100 km)

Future work Build all-optical path between BEV and IPE Convert experimental method to service – timescale comparison TIC integration into FPGA structure Design adapters suitable for time distribution – no data processing at server side

Thank you