Using single wavelength light to improve the
synchronization accuracy of the White Rabbit system
Xu Yuan (袁 绪) and Bo Wang (王 波)*
State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision
Instrument, Tsinghua University, Beijing 100084, China
*Corresponding author: bo.wang@tsinghua.edu.cn
Received May 12, 2017; accepted July 3, 2017; posted online July 20, 2017
We demonstrate a new synchronization method for the White Rabbit system. Signals are transmitted in a single
mode fiber in both directions with the same light wavelength. Without the complex calibration process of the
fiber asymmetry parameter, the new method reduces the effect of chromatic dispersion and improves the syn-
chronization accuracy. The experiment achieves timing synchronization accuracy below 200 ps over 50 km fiber
constructed by different companies’ fiber spools. The proposed method would make White Rabbit technology
immune to the chromatic dispersion of fiber links and can be applied to long distance synchronization.
OCIS codes: 120.3930, 120.3940, 060.4265, 060.4510.
doi: 10.3788/COL201715.101202.
High precision time synchronization technology has
been widely required in phased array antenna and related
astronomical observations
[1,2]
, navigation
[3,4]
telecommuni-
cation
[5]
, and timing control systems
[6]
. Satellite-based
global positioning and navigation systems (GNSS), such
as the Global Positioning System (GPS)
[7]
, Beidou
[8]
,
and the GLObalnaya NAvigatsionnaya Sputnikovaya
Sistema (GLONASS)
[9]
, can provide nanosecond (ns) level
synchronization precision. However, in some applications,
satellite signals are unavailable. For example, the
radio astronomical telescope Square Kilometer Array
(SKA)
[10,11]
requires precise time synchronization under
the radio silent condition. Furthermore, for the under-
ground accelerator and indoor localization system, they
also cannot receive the satellite signals. Fiber-based
White Rabbit (WR)
[12]
time synchronization technology
is an excellent alternative synchronization solution. It
can provide sub-ns time synchronization within 10 km.
Since its high precision and compliance with Ethernet
standards, WR now has a wide range of applications in
cosmic particle detectors and timing control systems
[13–17]
.
But in practical applications, the one-way transmission
delay is hard to estimate precisely because different wave-
lengths of light are used in the uplink (slave to master) and
downlink (master to slave). Furthermore, the long distance
urban fiber link is connected section by section. Different
fiber sections may be supplied by different companies.
All of these factors limit the synchronization distance,
which means the synchronization accuracy will be de-
creased at longer distances.
In this Letter, we demonstrate a synchronization
method for the WR system to eliminate the effect of
chromatic dispersion. Different to the conventional WR
method, two laser lights with the same wavelength are
used to transmit and receive signals in a single fiber. A pair
of optical transceiver modules is used to modulate the elec-
tric signals on the laser light. The proposed method brings
significant improvement on the long distance synchroniza-
tion accuracy. On a 50 km fiber link, which is constructed
by different companies’ fiber spools, the synchronization
error maintains below 200 ps. As a comparison, the timing
synchronization accuracy using the conventional WR
method is around 1. 5 ns. It makes the proposed synchro-
nization method available for the already deployed fiber
link without complicated calibration.
WR is a solution to provide sub-ns synchronization ac-
curacy and reliable data transfer for multi-users over an
Ethernet-based topology network. Initially, the WR system
is used as a timing system for the experimental physics
facilities
[18]
. A typical WR link consists of the WR master
and the WR slave
[19]
, as is illustrated in Fig. 1(a). Based
on the Synchronous Ethernet technique
[20]
, the slave’s
Fig. 1. Link model of WR system. (a) Schematic diagram of WR
system, (b) delay model of WR system. Δ
TXM
, Δ
RXM
are the
transmission and reception delays of the master device. Δ
TXS
,
Δ
RXS
are the transmission and reception delays of the slave
device. ε
M
, ε
S
are the bitslide values while aligning the received
data stream. δ
MS
, δ
SM
are the one-way propagation latencies in
fiber. WDM, wavelength division multiplexer; CDR, clock data
recovery circuits.
COL 15(10), 101202(2017) CHINESE OPTICS LETTERS October 10, 2017
1671-7694/2017/101202(5) 101202-1 © 2017 Chinese Optics Letters