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TIME AND FREQUENCY TRANSFER
IN ALL-OPTICAL NETWORK
Vladimír Smotlacha
CESNET
Zikova 4, 160 00 Prague 6, Czech Republic
[email protected]
Alexander Kuna
Institute of Photonics and Electronics, AS CR, v.v.i.
Chaberská 57, 182 51 Prague 8, Czech Republic
[email protected]
Paper type
Research paper
Abstract
This paper describes usage of all-optical network for time metrology application – time and frequency transfer between
two geographically distant sites. Although several approaches exist, there is no production implementation yet. Our
method is based on newly developed adapters utilizing channels in a DWDM (Dense Wavelength-Division
Multiplexing) network. We present results of tests performed in real production all-optical network including the time
transfer between atomic clocks in Prague and Vienna over more than 500 km long optical path.
Keywords
time transfer, frequency transfer, all-optical network
1. Introduction
Accurate time and frequency transfer between two geographically distant sites is one of common tasks in time and
frequency metrology. It is dominated by dedicated two-way satellite links and GPS based systems. However, there
exists a request for an alternative technique and optical network utilization is studied. Currently deployed all-optical
DWDM networks offer suitable and relatively inexpensive infrastructure.
This text introduces new set of optical network applications and also specifies requirements of community using such
applications, metrologists and other scientists and researchers involved in time and frequency transfer.
2. Approach
The method of time and frequency transfer depends on the involved standards:

Currently existing atomic clocks (Caesium and Rubidium beam, H-maser, Cs or Rb fountain, ...) internally
work in microwave frequency range but have usually two common outputs: 1PPS (1 pulse per second) signal,
where rising edge of each pulse defines the beginning of the second and 10 MHz (eventually 5 MHz) is
nominal frequency.

New (resp. future) generation of standards is represented by optical clocks, e.g. devices that contain optical
clockwork. Output of the optical clock lies in optical frequency range – the wavelength is in 1500 nm band
and corresponds to the DWDM grid.
The ultimate stability of optical clocks introduces strong requirements on optical patch quality, e.g. the lowest possible
noise, which can be complied with dedicated fibre link. However, a channel in all-optical DWDM network provides
infrastructure sufficient for time and frequency transfer of common atomic standards. It is subject of research if
DWDM channel can satisfy also the optical clock requirements of stability.
In order to eliminate slow changes of communication channel delay (introduced by the thermal dilatation of fibre), the
signal is transmitted in both directions – this standard method is called “two-way time transfer” and is commonly used
in time metrology.
3. Network requirements
Time and frequency transfer requires negligible jitter and noise. Any network used for such application must also meet
several common conditions:

all-optical network (WDM channel or "dark" fibre) delivering optical signal end-to-end

no regenerators with OEO (optical-electrical-optical) conversion – optical amplifiers (e.g., EDFA) must be
used instead

ability to transfer signal with alternative modulation - it is utilized neither Ethernet nor SDH/SONET framing

ROADM or similar devices in nodes allowing to "bypass" standard network switches/routers
We performed our experiments in the Cesnet2 network, where provided us by DWDM channels on the backbone and in
cross-border link to Vienna. Figure 1 shows skeleton of the network including optical amplifiers.
Figure 1. Cesnet2 network skeleton
4. Adapters
We have designed and developed adapters (Figure 2) dedicated for time transfer between two sources of 1PPS signal –
typical application is comparison of time scales represented by two distant atomic clocks.
Main parts of the adapter are:



development board based on FPGA chip (Virtex-5)
daughter board with interfaces
SFP transceiver
Figure 2. Adapter prototype
5. Experiments
We performed several experiments in order to verify the time transfer method, assess developed adapters and evaluate
accuracy of our method. Experiments focused on testing the method at long optical loop, demonstrating long distance
time transfer between Prague and Vienna and evaluating the time transfer accuracy. In all cases we utilized Cesnet2
production network.
Figure 3. Map of used links
5.1 Measurements on Optical Loop
The goal of this experiment was to measure the delay of a long optical path in order to predict the influence of the fibre
thermal dilatation on changes of the link asymmetry. 1PPS from a local clock was transmitted in both directions and
using two time interval counters, the delays δAB and δBA (Figure 4) were measured. We utilized 744 km long
bidirectional optical loop – the route between cities Prague – Brno – Olomouc – Hradec Kralove – Prague. Concerning
the time transfer it is important to know the difference Δ of particular delays δAB and δBA. Ideally, Δ should be
constant. Its variation (about 1 nanosecond in our experiment – see Figure 5) represents accuracy of the time transfer.
Figure 6 displays stability of the time transfer – we see that time deviation value is about 100 ps for 1 s and the lowest
value is 8.1 ps for averaging time of 500 s.
Figure 4. One-way delays
Figure 5. Assymetry of optical loop
Figure 6. Time stability of optical loop
5.2 Time Transfer between Prague and Vienna
Cesnet operates also a DWDM fiber link from Brno to Vienna, where it ends in the premises of ACOnet (Austrian
national research and education network) located in Vienna university campus. The length of this fibre link is 504 km
excluding the fibre compensating chromatic dispersion.
This experiment aimed at time transfer between Prague and Vienna. In Prague, we used GPS-disciplined Rb clock as in
previous experiment. In Vienna, the situation was complicated by not yet operational fiber link between Vienna
University and BEV (Austrian time and frequency laboratory). Therefore, BEV transported their Rb clock to ACOnet,
where it was operated as free-running clock. Figure 7 shows time offset between both clocks. As clock in Prague was
disciplined by GPS, we can conclude that free running Rb clock in Vienna had a relative frequency offset of about
8.1*10-12.
Figure 7. Time transfer between Rb clocks in Prague and Vienna
5.3 Comparison with GPS based time transfer
Our intention was to verify optical time transfer and compare method performance with standard time transfer device.
We used two GPS dual-frequency receivers GTR-50. One site of measurement was located in the Laboratory of the
National Time and Frequency Standard in Prague (in Institute of Photonics and Electronics), the second site of
measurement was temporarily set up in the campus of the University of West Bohemia in Pilsen - we installed there
free-running Rubidium clock, the GTR-50 device with GPS antenna and the optical transfer adaptor. The geographical
distance of both sites is 94 km, while the optical cable length is 153 km.
We measured the difference between optical transfer and Common View GPS (CV GPS) time transfer for 10 days. All
results were in the range ±2 ns. Declared accuracy of GTR-50 based measurement is 2 ns, therefore the observed
difference of both methods is in range of GTR-50 inaccuracy. Despite the fact of GTR-50’s accuracy, we can observe
daily periodicity caused probably by temperature dilatation of dispersion compensation fibres.
Figure 8. Comparison with GPS/based method
Time stabilities of two-way optical transfer and CV GPS in terms of Time deviation are shown in Figure 9. For
comparison, time stability of our Rb clock measured directly against Cs clock is also included.
T ime D eviation, S ec onds
1.E -09
1.E -10
G P S C ode Meas urements
1.E -11
G P S C arrier P has e
Meas urements
Two-Way Optic al Trans fer
Direc t Meas urement
1.E -12
1
10
100
1 000
10 000
Averag ing Interval, S ec onds
Figure 9. Optical time transfer stability
6. Conclusions
We verified functionality of the method and adapters on time transfer in a 740-km fibre in real production network and
we proved that the system is compatible with DWDM technology and does not interfere with other data channels.
We also compared the accuracy of our optical time transfer with Common View GPS time transfer and confirmed that
despite daily fluctuation the inaccuracy is in range ±2 nanoseconds.
We proved that our method implemented in Cesnet2 network can be used for comparison of atomic time scales of
Czech and Austrian national time standards.
References
[1] V. Smotlacha, A. Kuna, W. Mache: “Time Transfer Using Fiber Links”, EFTF 2010, Noordwijk, April 2010.
Biographies
Vladimir Smotlacha received his MSc degrees in Computer Science from Czech Technical University and from
Charles University. He received his PhD degree in Information Science and Computer Engineering from the Czech
Technical University. He is with CESNET since 1996, currently in Research and Development Department. His
research interests include network time services, network monitoring and communication protocols. He is member of
the ACM, SIGCOMM and IEEE.
Alexander Kuna was born in 1978. He graduated from the Faculty of Electrical Engineering (FEE), Czech Technical
University, Prague, in 2004. He has been with the Institute of Photonics and Electronics AS CR, v.v.i. (IPE) since
2005, where he is working on precision frequency stability measurement. Currently he is head of the Time and
Frequency Department of IPE.