Robust fiber-based frequency synchronization system
immune to strong temperature fluctuation
Xi Zhu (朱 玺)
1,2
, Bo Wang (王 波)
1,
*, Yichen Guo (郭倚辰)
1
, Yibo Yuan (袁一博)
2
,
Romeo Gamatham
3
, Bruce Wallace
3
, Keith Grainge
4
, and Lijun Wang (王力军)
1,2
1
State Key Laboratory of Precision Measurement Technology and Instruments, Department of
Precision Instrument, Tsinghua University, Beijing 100084, China
2
Department of Physics, Tsinghua University, Beijing 100084, China
3
SKA South Africa, Blend on Baker, Rosebank 2196, Johannesburg, South Africa
4
Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics & Astronomy,
The University of Manchester, Oxford Road, Manchester M13 9PL, UK
*Corresponding author: bo.wang@tsinghua.edu.cn
Received September 8, 2017; accepted November 16, 2017; posted online December 5, 2017
In order to make the fiber-based frequency synchronization system suitable for the use of large-scale scientific
and engineering projects in which the ambient temperature of the fiber links change dramatically, we design a
non-harmonic frequency dissemination system immune to strong temperature fluctuation. After the lab tests, in
which the ambient temperature of the fiber fluctuates 40°C/day and 20°C/h, respectively, the relative frequency
stabilities of this system reaches 4.0 × 10
−14
∕s and 3.0 × 10
−16
∕10
4
s. It is demonstrated that the proposed
non-harmonic scheme shows a strong robustness to complicated working environment with strong temperature
fluctuation.
OCIS codes: 060.0060, 120.0120.
doi: 10.3788/COL201816.010605.
As the world’s largest radio telescope under construction,
the Square Kilometre Array (SKA) consists of thousands
of parabolic dishes and aperture arrays, which work
together to form a one-square-kilometer collecting area.
SKA will surpass any current telescope and help us further
understand the universe. To ensure sufficient imaging fidel-
ity, all of the telescopes should keep time and be frequency
synchronized with stabilities superior to 2.3 × 10
−12
∕s,
3.8 × 10
−14
∕ min; and 1.9 × 10
−14
∕10 min
[1]
. In 2015,
we proposed and demonstrated a precision reference fre-
quency synchronization scheme via 1f–2f dissemination,
according to the requirements and characteristics of
SKA
[2]
. Here, we name it a harmonic system. It features
the phase-noise compensation performed at the client site
to reduce space requirement and unnecessary complexity at
central station. Considering our previous work along with
the existing fiber-based time and frequency transfer
schemes demonstrated by different groups
[3–17]
, they almost
utilized fiber spools or urban telecommunication fibers,
which basically run in buried cables. While for large-scale
scientific and engineering facilities, such as SKA, on ac-
count of attractive infrastructure cost and simple laying,
overhead fiber links will be adopted. Unfortunately, the
overhead fiber links are affected by atmospheric tempera-
ture variation and mechanical stress. Consequently, they
are much noisier than fiber spools and buried fiber
[18–25]
.
Taking the SKA South Africa site for an example, it is con-
structed in the desert region. From the meteorological
parameters captured by the local weather station, rapid
temperature change caused by a thunderstorm, gale,
sunrise, and sunset is recorded frequently in the past.
According to statistical analysis on the temperature data
in the period from January 1, 2005 to March 31, 2011,
the daily temperature fluctuation can reach 40°C
[26]
.There-
fore, a robust frequency dissemination system immune to
strong temperature fluctuation is required.
After the performance test in the lab, the harmonic sys-
tem was shipped to the SKA South Africa site to perform
an outfield test on several overhead fiber links from the 19th
to 27th September 2015. The frequency dissemination sta-
bility with phase-noise compensation via the 64 km over-
head fiber is shown in Fig.
1. For easy comparison, the
dissemination stability of 32 km overhead fiber without
compensation is also shown in Fig.
1.Duringthefieldtest,
we did not measure the dissemination stability on 64 km
overhead fiber without compensation, but it will be worse
than that on 32 km overhead fiber
[27]
. It can be seen that
there is a bump on the Allan deviation plot of dissemination
stability at the averaging time between 10 and 100 s, which
had not shown up in previous lab tests using fiber spools.
Through theoretical analysis and experimental verification,
as described later, we find that the fiber-induced phase fluc-
tuation due to strong temperature change cannot be com-
pletely compensated.
In this Letter, we analyze the reason why the previous
harmonic scheme is not perfectly immune to temperature
fluctuation and attribute this imperfection to limited iso-
lation and nonlinear performance of the radio frequency
(RF) components in the system. According to the analy-
sis, we design a non-harmonic system and test its perfor-
mance in an environment with temperature fluctuating
40°C per day, which is similar to the temperature
COL 16(1), 010605(2018) CHINESE OPTICS LETTERS January 10, 2018
1671-7694/2018/010605(5) 010605-1 © 2018 Chinese Optics Letters
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