precise time transfer on the ipe vugkt line a detailed

Infrastructure Precise time transmission Sagnac effect

Precise time transfer on the IPE–VUGKT line– a detailed analysis

Josef Vojtech, Martin Šlapák, Pavel Škoda, Vladimír Smotlacha,Radek Velc, Petr Münster, Jan Kundrát, Ondrej Havliš, Lada

Altmannová and Michal Altmann

Department of Optical networks, CESNET z.s.p.o., Prague 6, Czech Republic

TSP 2016

Martin Šlapák Precise time transfer on the IPE–VUGKT line


Why we need a precise time?

I Broad range of fields eg.: legal time, sensing, metrology, navigation,astronomy, seismology, fundamental physic. . .

I Standards: IEEE1588 – Precise time synch for realtime automationapplications

I Optical transfer outperforms radio frequencyI high carrier frequency (hundreds of THz)I shorter propagation paths (vs. GPS)I accuracy in order of ps or betterI no needs for antennas or clear sky view

http://eandt.theiet.org/magazine/2015/05/60-years-of-atomic-time.cfm



Infrastructure

I CESNET provides e-Infrastructure for R&E in Czech Republic.

I The 1st fibres were lit-up in 1999 with several EO regenerators.

I Now the optical amplifiers are widely used on backbone.

I Currently backbone contains ~6400 km of dark fiber (1510 km withcommercial transmission systems and 4890 km CzechLigh).

Photonics servicesPhotonic services are end–to–end connection between two or more sitesin network defined by photonic–path and allocated bandwidth. Optimalycarried with no or minimal impact to signal.

I Deployment of multiple applications into a single path is no problem.

I PSes can share the fibers→ efficient use of optical paths,sometimes the only one solution (we cannot afford separated fiber).



Map of CESNET2 network topology



CzechLight R© devices

I Developed and still improved byCESNET.

I Great monitoring capabilities.

I ROADM – DWDMReconfigurable Add–DropMultiplexers

I BiDi CLA – bidirectional fulloptical amplifiers

I WSS – wavelength selectiveswitches

I TDC – tunable dispersionconmensator

I Used standard chasis.

http://www.root.cz/clanky/czechlight-cesky-opticky-zesilovac-rizeny-linuxem/



Precise time transmission

I The first application of T/F infrastructure was made on link IPE–BEV(Wiena) – 550 km.

I An one way method has high uncertainty→ transfer signal from A toB and back.

I By two way method we eliminate uncertainty of propagation time butwe need to ensure that propagation time is the same for both ofdirections or need to know the difference. (It is better to use the samefiber .)

I Propagation time depends mainly on refractive index and length offiber and also on temperature, wavelength and polarization.

I Pilot link: IPE–CESNET–VUGKT total 95 kmI 12 months of observation (2015/03–2016/02)I Delay was changed of about 70 ns→ annual transmission time

fluctuation 736 ps/km.I Sunny days brings greater day difference ~0.4 ns.



Pilot implementation – link overview

Two spans: IPE–CESNET (17 km) and CESNET–VUGKT (78 km)Martin Šlapák Precise time transfer on the IPE–VUGKT line


Precise time transmission – link map



Corrections

I Time corrections for one way time transfer over optical fiber arederived in and lead to formula:

∆t± =1

c

∫ L

0ndl ± 1

c2

∫ L

0v · sldl +

1

c3

∫ L

0n(w +

v2

2)dl (1)

I 1st term represents influence of refractive index ~5 ms (1000 km)

I 2nd term describes th Sagnac effect ~±5 ns

I 3rd term is Shappiro’s correction ~3 ps

I The realworld two way transmission needs different channel’swavelengths – time propagation difference: ∆T = (d · L + D)∆λ

I where: d = chromatic dispersion coefficient, L = fiber length,D = chrom. disp. of passive components

I for IPE–VUGKT line: L =95 km, d =17.5 ps·nm−1 · km−1,∆λ = 0.8 nm, D = 200 ps·nm−1→ ∆T = 1.49 ns



Sagnac effect

I The c−2 term in eq. 1 is the Sagnac correction and correct sign from± depends on direction of tranfer (eastward or westward).

|∆t| =2ΩS

c2(2)

I where Ω is angular velocity of the Earth numerlicaly 7.292E-5,I the variable is S corresponds to area defined projection fiber path.



Automated computation

I Input: GPX file with fiber path (approx.) and some meta information(a real length of fiber).



Conclusion

I The IPE–VUGKT line shows directional difference:I of 1490 ps based on wavelenght diff,I of 79.7±0.004 ps based on Sagnac effect.

I The main uncertainty 29.8 ps comes from laser wavelength drift,

I A Polarization Mode Dispersion brings uncertainty of 0.85 ps



Contact

I https://photonics.cesnet.cz

Josef [email protected]

This slides


https://photonics.cesnet.cz

precise time transfer on the ipe vugkt line a detailed

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