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Towards an international optical clock

comparison between NPL and SYRTE

using an optical fibre network

1NPL, UK - 2LNE-SYRTE, France - 3LPL, France - 4INRIM, Italy - 5DANTE, UK - 6PTB, Germany

Giuseppe Marra1, Jochen Kronjaeger1, Paul-Eric Pottie2, Anne Amy-Klein3,

Olivier Lopez3, Davide Calonico4, Guy Roberts5 and Harald Schnatz6

NPL Management Ltd - Public

Comparing optical clocks

Caesium

standard

Co-located

Optical

clock

10-15@1d

10-16 acc

10-16@1s

10-17 acc

Two-way satellite

transfer

10-15@1d

Optical fibre

transfer

10-15@1s

10-18@100s

Co-located optical clock: Bloom et al, Nature 506, 72 (2014)

Caesium standard: K Szymaniec, IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT 60, 2475 (2011)

Two-way satellite:P Gill, Phil. Trans. R. Soc. A 369, 4109 (2011)

Optical fibre transfer: Droste et al, PRL 111, 110801 (2013)

NPL

SYRTE

Map based on “English Channel location map.svg”

by NordNordWest/Wikipedia, license CC-BY-SA-3.0-DE

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Optical fibre transfer Transmit optical carrier in telecom C-band (1550nm)

Stabilise optical length of the fibre

Need bi-directional amplifiers (EDFA or Brillouin)

Need bidirectional amplifiers (EDFA or Brillouin)

Temperature and strain

Doppler shift

End marker Frequency

correction

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Bi-directional Amplifiers

Pump

980nm

Signal 1550nm Signal 1550nm

bad splice connector

Rayleigh

scattering

Gain G

Allow signal to propagate both ways

Required by noise cancellation scheme

Refl R1 Refl R2

Laser threshold

E.g. Rayleigh scattering -35dB – occasionally much higher

In practice G = 15…20dB max

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The London-Paris link

~812 km

~210 dB loss

9 or 10 commercial bi-directional EDFAs

Pair of dark fibres

Fibre provided by European research

and education network GÉANT

2 high-gain Brillouin amplifiers (PTB)

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Scheme for clock comparison

Independent

fibres

Compare fibres for real-

time servo monitoring

Transmit ultra-stable light

locked to NPL optical clock

Compare received light

to SYRTE optical clock

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Status of the project

Link established from

NPL to Folkestone

(and back)

Commercial bidirectional EDFA

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Preliminary fibre noise

NPL to Central London

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The future of clock comparison

Challenges of ever longer links

A European network of fibre links?

Residual loss Amplifier gain can’t fully compensate loss

as this would lead to spontaneous

oscillation – e.g. 3dB loss per stretch

Loop bandwidth Detection delay for London-Paris link is

about 8ms – bandwidth of 20Hz

Cost Typical fibre rental cost

~£100k/yr

Court

esy o

f iS

tockphoto

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Thank you for your attention.

The research leading to these results has received funding from the European

Community’s Seventh Framework Programme (FP7 2007-2013) under Grant Agreement

No. 605243 (Gn3plus). It has also received funding from Action Spécifique GRAM.