Vladimír Smotlacha, CESNET Alexander Kuna, IPE Time and Frequency Transfer in All-optical Network TNC 2011 Prague 17 May 2011

Time and frequency standards pendulum –frequency standard until 1930 s quartz crystal atomic clock (Cesium, Rubidium) –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 distribution time and frequency can be distributed by electromagnetic field –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 –e.g. DCF77 (radio controlled clock)

Time transfer time transfer = comparison of two clocks satellite navigation systems (GPS, Galileo, GLONASS,...) –time broadcast – tens of nanoseconds noise –timestamping of local clock – “common-view” (CV), “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 Fiber Goal Design alternative method for accurate time transfer Accuracy in ~1000 km range comparable or better than CV GPS Use DWDM all-optical networks Features Two-way transfer Optical signal modulation for 1-pps encoding Uses SFP transceivers

Adapter structure

Adapter prototype

Experiments We made 3 experiments: Optical loop measurement Time transfer Cesnet – BEV (Prague – Vienna)‏ Comparison with CV 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 University of West Bohemia, Pilsen

Cesnet DWDM network

Optical loop experiment Both endpoints in one laboratory, common clock Bidirectional optical loop length 744 km DWDM production network 12 optical amplifiers Segment Praha - Hradec Kralove on top of electricity distribution poles – high length dilatation

Optical network - geography

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 (University of West Bohemia) Site B: Cs clock in IPE – UTC(TP) 150 km of fibre, WDM, production network GTR50 installed in both sites 95 km geographical distance

Methods comparison - I Optical and CV GPS comparison Measured clock offset Frequency drift eliminated

Methods comparison - II Optical and GPS measurements performed simultaneously Direct measurement performed separately

Methods comparison - III 10-day difference between optical transfer and CV GPS (CGGTTS data)

Conclusions Adapter prototypes successfully tested and method verified Utilization of all-optical network No interference with other DWDM traffic observed Fiber length dilatation canceled in two-way transfer (residual asymmetry less than 1 ns at 700+ km fiber) TDEV 8.7 ps / 500 s at 700+ km fiber Good agreement with CV GPS

Future work Build all-optical path between BEV and IPE (Austrian and Czech time and frequency laboratories) Convert experimental method to service – timescale comparison Design adapters suitable for time distribution – no data processing at server side Utilize optical service in GN3 network

Thank you