Download Author: Vladimír Smotlacha, Milan Sova

Title:
Time and Frequency Transfer
in All-optical Network
Authors:
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]
Keywords:
time & frequency transfer, all-optical network
Extended abstract:
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.
Approach
The method of time and frequency transfer depends on the involved standards:

Currently existing atomic clocks (Cesium 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 5MHz) 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 wavelength is in 1500 nm band and corresponds to DWDM grid.
The ultimate stability of optical clocks introduces strong requirements on optical patch quality, e.g. the lowest possible
noise, which can be met only by dedicated fiber link. However, all-optical DWDM network can provide infrastructure
sufficient for common atomic standards.
Network requirements
Time and frequency transfer requires negligible jitter and noise. Any network used for such application must meet
several common conditions:

all-optical network with optical amplifiers (WDM channel or "dark" fiber)

no regenerators with OEO (optical-electrical-optical) conversion

ROADM or similar devices in nodes allowing to "bypass" standard network switches/routers
Experiments
We developed adapters dedicated for time transfer between two sources of 1PPS signal – typical application is
comparison of time scales represented by two distant atomic clocks. The adapter is based on FPGA chip and utilizes a
SFP transceiver.
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 1 – Map of used links
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 fiber
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 were measured. We utilized 744 km long bidirectional optical
loop – the route between cities Prague – Brno – Olomouc – Hradec Kralove – Prague.
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 fiber link is 504 km
excluding the fiber 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.
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 laboratory 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 fibers.
Figure 2 – Comparison with GPS/based method
References
[1] Smotlacha, V., Kuna, A., Mache,W., „Time Transfer Using Fiber Links”,
EFTF 2010, Noordwijk, April 2010.
Vitae:
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 monitoring, network time
services and communication protocols. He is a 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.