Infrastructure for Precise Time Transmission

1

Infrastructure for Precise Time Transmission

Josef Vojtěch

Vladimír Smotlacha, Pavel Škoda

http://www.cesnet.cz http://czechlight.cesnet.cz

Optické Komunikace 2013

October 24-25, 2013

http://www.cesnet.cz/

http://czechlight.cesnet.cz/

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Work was supported by Czech institutional funding of

research by project

Large Infrastructure CESNET LM2010005

(www.cesnet.cz)

CESNET also participate in NEAT-TF (JRP-S11)

project: Accurate time / frequency comparison

and dissemination through optical telecommunication

networks

OK 2013 October 24-25, 2013




3 OK 2013 October 24-25, 2013


Outline

• Motivation

• Infrastructure

• UFE – BEV

• CESNET – VUGKT

• Amplified bidirectional single fibre transmission

• Conclusion & Further steps

• Q&A

4 OK 2013 October 24-25, 2013


Motivation

• Why better clocks?

Schnatz H., Comparison of clocks using optical fiber links: recent results and future projects

5 OK 2013 October 24-25, 2013


Motivation cont.

• Why better clocks?

Schnatz H., Comparison of clocks using optical fiber links: recent results and future projects

6 OK 2013 October 24-25, 2013


Motivation cont.

• Higher number interconnected clocks (Cesium

primary standards and Hydrogen masers)

improve accuracy and stability of the time scale

• “Interconnection” means time transfer

• Traditional satellite based methods (GPS,

TWSTF) are reaching their limits

7 OK 2013 October 24-25, 2013


Goals

• Transfer time from existing Caesium primary

standard to Czech national TF laboratory in

Institute of Photonics and Electronics (UFE)

• Compare national approximation of UTC with

neighbouring countries

• Distribute accurate time and stable frequency to

users

8 OK 2013 October 24-25, 2013


Transport Media

• Pair of channels in a production DWDM optical network

– the same wavelength in both directions

• Pair of DWDM channels in single fiber bidirectional

transmission system

– different wavelengths

• Pair of DWDM channels (both uni- and bi-directional) in

experimental links

• Dark fiber – last mile / urban area links

9 OK 2013 October 24-25, 2013


Infrastructure

10 OK 2013 October 24-25, 2013


Infrastructure

UFE – National time and frequency laboratory, Prague – UTC(TP)

• 3x Caesium clocks

• dark fibre, 16 km VUGTK – Geodetic observatory, Pecný

• Caesium clock

• single bidirectional dark fibre with installed DWDM, 78 km

CESNET

• Caesium clock

• Central point of network BEV – Bundesamt für Eich - und Vermessungswesen, Vienna

• Austrian National time and frequency laboratory - UTC(BEV)

• 2x Caesium clocks

• Hydrogen maser

• Lit pair of unidirectional channels, DWDM, 550 km UPT – Institute of Scientific Instruments, Brno

• Hydrogen maser

• Lit pair of unidirectional channels, DWDM, 340 km

11 OK 2013 October 24-25, 2013


Adapters

• Use of commercially available HW whenever possible

• FPGA Virtex-5

• Integrated time interval counter

• SFP (SFP+) transceiver

12 OK 2013 October 24-25, 2013


UFE-BEV

• Comparison of time scales UTC(TP) and UTC(BEV) operational since

Aug 2011, Cesium beam 5071A/001 atomic clocks

• Path - one way 550km ~ 137 dB, contains of 220km cross border fibre

• Mixture of fibre types (G.652/655)

• Mixture of transmission systems Cisco/OpenDWDM Czechlight

• Mixture of CD compensation types (DCF, FBG)

13 OK 2013 October 24-25, 2013


Parallel transmissions of time transfer and

coherent 100G

• Field verification 2011 with vendor 1

• Over 300km of G.655 fibre

• Full operation Feb 2013 with vendor 2

• Same line as above + spectral displacement from coherent used

• No observable influence

14 OK 2013 October 24-25, 2013


CESNET - VUGKT

• Single fibre line 78km, since Jun 2013

• Unamplified wavelengths for time transfer

• Propagation delay difference can be calculated and

compensated: 1.16 ns

• Parallel amplified 10GE channels

15 OK 2013 October 24-25, 2013


Fibre vs. GPS transfer

• Transfer time

CESNET – VUGTK UFE – BEV

• Better results than GPS (PPP)

• About 3 times lower phase white noise

16 OK 2013 October 24-25, 2013


Amplified single fibre bidirectional transmission

• Telecom EDFA intentionally designed as unidirectional

• Reflections and back-scattering from line create feedback

• In case of gain is high enough -> unwanted lasing

17 OK 2013 October 24-25, 2013



• Bidirectional amplified transmission

• EDFA – 540km

• Raman – single hop 200km

• EDFA + Brilouin – 2x920km reported for frequency

• References:

A. Amy-Klein, et al”, ”Time transfer through optical fibers”, project NEAT-TF workshop,

2012

C. Clivati, et al, ”Distributed Raman Amplification for long-haul optical frequency

dissemination”, Proceedings of EFTF, Prague, Czech Republic, 2013

S. Droste, et al, “Optical Frequency Transfer over a single span 1840 km Fiber Link,” in

Proceedings of EFTF, Prague, Czech Republic, 2013

18 OK 2013 October 24-25, 2013



• Tested EDFAs from 3 different vendors in lab

• 100km/20dB spools of SSMF

• EDFAs from telecom vendors – prone to lasing

Tx

Rx

Tx

Rx

19 OK 2013 October 24-25, 2013



• EDFA from dedicated vendor, prone to lasing, but….

Error free Operation over 5x100km

20 OK 2013 October 24-25, 2013



• Looking for cost effective solution – SOA

• Output powers kept low to avoid saturation

• SOAs from three different vendors tested

Error free Operation over 4x100km

• Lasing again, however…

21 OK 2013 October 24-25, 2013


Conclusions & Further Steps

• Better time stability results than GPS (PPP)

• Bidirectional amplified transmission verified in lab

• Connection UPT – CESNET/UFE is planned to be

established soon

• Bidirectional amplified transmission in

experimental facility

• Line delay stabilisation

22 OK 2013 October 24-25, 2013


Acknowledgement

Lada Altmannová, Jan Gruntorád,

Miloslav Hůla, Jakub Kostelecký,

Alexander Kuna, Werner Mache, Martin

Míchal, Jan Nejman, Anton Niessner,

Václav Novák, Jan Radil, Karel

Slavíček, Stanislav Šíma

23 OK 2013 October 24-25, 2013


Thank you for kind attention!

Questions?

josef.vojtech(zavináč)cesnet.cz

mailto:[email protected]




24 OK 2013 October 24-25, 2013


Limits of sattelite based methods

Source: S.Droste et al, “Optical Frequency Transfer over a single span

1840 km Fiber Link”

25 OK 2013 October 24-25, 2013


UFE – BEV propagation time changes

Left: Seasonal October 7 2011 - March 14 2012

approximately 350ns, 1.3 ∙ 10−4 of avg. delay 2788 µs

Right: Daily changes 4-7ns

26 OK 2013 October 24-25, 2013


Loop tests

• Loop tests over standard DWDM systems in 2010

• Optical loop 744km/462mil, two unidirectional channels

• 12 EDFAs, G.652, G.555, one span aerial fibre on power distribution

poles, high dilatation

• Fluctuation ~130 ns (temperature changes about 12 deg. C)

• Residual asymmetry < 2 ns (resp. TDEV 8.7 ps / 500 s)

27 OK 2013 October 24-25, 2013


CESNET

• National Research and Educational Network in Czech Republic

• Non-profit organization

• Connects > 40 partners - universities, hospitals and research institutions

• Optical network DWDM based ~ 5000km lit fibers

• 250 researchers and staff

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Photonic Service

TNC 2013 Maastricht, June 2-6, 2013

Fibre Capacity Increase for New Services

End-to-end connection between two or more places in network

– Described by allocated bandwidth and photonic-path

– Maximal transparency - minimal impact of network on transmitted content

– Path all-optical, no OEO except special cases

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Photonic Service



• Photonic service can transport real time data

• Dedicated bandwidth with no or only special OEO

• Transparency to transmitted signals

• Only transport latency shortest photonic path

• Constant latency (i.e. negligible jitter), non or only

specially tailored electrical processing

• Stable service availability (due allocated bandwidth)

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Available Transport



• Signals can be untypical (slow, non-modulated)

• Dedicated fibre – OK, but price of rental

Annualized avg. cost of fibre rental: cf = 0.5 €/m/year [1]

• Long distances - Dedicated lambda – OK

Annualized costs of 10Gbps transmission

system: ct = 0.12 €/m/year [1]

• [1] S. Sima et al.: Deliverable D3.2v3 of Porta Optica project: Economic

analysis, dark fibre usage cost model and model of operations http://www.porta-

optica.org/publications/POS-D3.2_Economical_analysis.pdf

•

http://www.porta-optica.org/publications/POS-D3.2_Economical_analysis.pdf





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Long-haul Transport



• Alien wavelength – lot of parallel lambdas posibly with 100G

• Indicated: (slow) OOK (amplitude modulated) signals have

negative impact on coherent DP-QPSK through non-linear

interactions, but precise numbers difficult to find

• Vendors typically don’t give any warranties on system

performance with parallel transmission with PSs

• Guard-bands generally improve the situation but consume

system bandwidth (200GHz)

•

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Influence of Slow OOK Signals on 100G



• Lab tested 100G DP-QPSK systems vendors 2 and 3

• Interaction with slow signals – 1 Mb, 100 Mb and 1Gb

– 100Mb/s signals similar to PS accurate time transfer

(comparison of atomic slocks)

– 1Mb/s tested as the worst alternative, on 50GHz spacing

– preFEC was very slightly different but probably due to

changes in power per channel (lab EDFAs)

• Over 450km of fibre (G.652 and G.655) non principal

harmful effects, when 100G ‚surrounded‘ by slow signals

• May be different on 2000km

•

-60

-50

-40

-30

-20

-10

1549,5 1550 1550,5 1551 1551,5

dB

nm

100M

1G

10G

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Transmission Channels Shortage



• 2.5G systems able of operation over 25GHz grid ~ 160ch in

35nm of C band

• Typical 10G systems with 50GHz offered 88 (92) channels,

undersea systems over 33GHz grid ~ 120 channels

• Present 100G DP-QPSK fits into 50GHz

• Probably no chance to fit 400G or 1T streams into

50GHz

•

Infrastructure for Precise Time Transmission

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