Ms6127Bwithfigures

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High-resolution microwave frequency dissemination on an 86-
km urban optical link 
O. Lopez
1
, A. Amy-Klein
1
, M. Lours
2
, Ch. Chardonnet
1
 and G. Santarelli
2
 
1
Laboratoire de Physique des Lasers, Université Paris 13, CNRS, 99 Av. J.B. Clément, 93430 
Villetaneuse, France 
2
LNE-SYRTE, Observatoire de Paris, CNRS, UPMC, 61 Av. de l'Observatoire, Paris, France
Abstract : we report the first demonstration of a long-distance ultra stable frequency dissemination 
in the microwave range. A 9.15 GHz signal is transferred through a 86-km urban optical link with a 
fractional frequency instability of 1.3
×
10
-15
at 1 s integration time and below 10
-18
at one day. The 
optical link phase noise compensation is performed with a round-trip method. To achieve such a 
result we implement light polarisation scrambling and dispersion compensation. This link 
outperforms all the previous radiofrequency links and compares well with recently demonstrated 
full optical links. 
corresponding author Giorgio Santarelli, email : [e-mail] 
PACS : 42.62.Eh, 06.30.Ft, 42.81.Uv 

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1. Introduction 
Ultra-stable frequency transfer between remotely located laboratories is required in time and 
frequency metrology, fundamental physics, particle accelerators and astronomy. Distant clock 
comparisons are currently performed using satellites, by Two-Way Satellite Time and Frequency 
Transfer, or through the Global Positioning System. Both methods are limited to a 10
-15
fractional 
frequency instability for one day of averaging time [1]. This is insufficient to transfer the properties 
of modern cold atom microwave frequency standards which have demonstrated frequency 
instability of a few 10
-16
at one day [2, 3]. Beyond metrology, high-resolution clock comparison is 
essential for advanced tests in fundamental physics, such as tests of the stability of fundamental 
constants [4-6]. 
To overcome current free space link limitations, the transmission of standard frequencies 
over optical fibers has been investigated for several years [7-10]. This technique takes advantage of 
the low attenuation, high reliability and continuous availability of fibers.
Radio frequency (RF) transmission using amplitude modulation of an optical carrier at 
1 GHz have demonstrated a frequency instability as low as 5
×
10
-15
at 1 s and 2
×
10
-18
at one day 
over 86 km [11]. Direct optical frequency transfer [12-15] can provide even better stability and be 
extended to greater distances. For both methods a phase noise correction is needed to compensate 
for the fluctuation of the propagation delay due to mechanical perturbation and temperature 
variation along the fibre. For this purpose the so-called round-trip method is used. 
Radio frequency transmission over an optical link has already been demonstrated at 
100 MHz, 1 GHz and a few tens and hundreds of GHz [8, 9, 11, 16, 17]. In this paper, we report on 
the transmission of a microwave frequency reference signal at 9.15 GHz over a 86-km urban fiber 
link connecting our two laboratories, LPL and LNE-SYRTE. This work pursues the development of 
stable frequency distribution of a reference signal already demonstrated between our two 
laboratories in the RF domain at 100 MHz [8] and 1 GHz [11]. The fiber optical length fluctuations 
induce phase fluctuations proportional to the modulation frequency, thus moving to higher 
frequency potentially leads to an increase of the signal-to-noise ratio of the detected fiber phase 
fluctuations. Moreover a microwave frequency of about 10 GHz is well suited to applications 
concerning particle accelerators [18] and astronomy and is close to the 9.192 GHz caesium 
transition frequency used for the definition of the SI second. In the following, we describe the new 
set-up. Then we present the resulting performance, discuss the limitations and conclude. 

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