Performance enhancement of Brillouin optical correlation
domain analysis based on frequency chirp magnification
Bin Wang (王 彬), Xinyu Fan (樊昕昱)*, Jiangbing Du (杜江兵), and Zuyuan He (何祖源)
State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao
Tong University, Shanghai 200240, China
*Corresponding author: fan.xinyu@sjtu.edu.cn
Received July 17, 2017; accepted September 22, 2017; posted online November 2, 2017
We propose a method to enhance the performance of Brillouin optical correlation domain analysis (BOCDA)
with a broad chirp span of optical sources based on the frequency chirp magnification technique. In BOCDA
systems, the number of effective sensing points is proportional to the chirp span of the light source, which is
normally limited by the characteristics of the laser diode. We demonstrate a chirp span of 126 GHz with the
proposed method, to double the effective sensing points of BOCDA. By combining with differential measurement
schemes, a spatial resolution of <10 cm over a 1 km range is achieved.
OCIS codes: 060.2370, 290.5900, 190.4380.
doi: 10.3788/COL201715.120601.
Distributed fiber-optic sensing technology is now attract-
ing considerable attention in many fields since it provides
continuous information along the whole fiber used for
sensing applications. To realize the distributed measure-
ment for locating the “event,” there are three main
methods using different locating principles in different
domains: time, frequency, and correlation
[1–3]
. Among
these methods, the time-domain locating technique is
widely used due to its cost effectiveness and long measure-
ment distance
[4–8]
. Several sensing systems using the time-
domain locating technique, such as optical time-domain
reflectometry (OTDR) and Brillouin optical time-domain
analysis (BOTDA), achieved great successes in many ap-
plications
[9–11]
, such as border security monitoring, oil pipe-
line monitoring, etc. However, the spatial resolution of the
time-domain method is limited to meter level because of
the poor signal-to-noise ratio (SNR) when a narrow opti-
cal pulse is launched into the fiber under test (FUT),
which refrains from meeting the requirements of high-
resolution measurements and restricts its applications.
On the contrary, the frequency-domain method can
achieve high spatial resolution
[12,13]
, but it has a limited
measurement range since it is susceptible to the phase
noise of the optical source and the accumulated phase
noise from the FUT added by the environmental pertur-
bations along the fiber.
The correlation-domain method, which provides ultra-
high (millimeter-level) spatial resolution and a relatively
long measurement range
[14–17]
, is another possible substi-
tute for the time-domain method and frequency-domain
method. It is based upon a reflectometric technique called
the synthesis of optical coherence function (SOCF)
[18,19]
.In
this method, the optical frequency of the laser diode (LD)
is modulated, usually step by step or sinusoidal, by
controlling the current injected into the LD. The spatial
resolution of correlation-domain sensing systems is
inversely proportional to the chirp span of the light source,
and a broader bandwidth helps to obtain a higher
spatial resolution. For the typical sinusoidal modulation,
a broad modulation bandwidth can be obtained by in-
creasing the current injected into the LD. However, the
increase of injection current added risks of damaging
the LD, and the modulation bandwidth is usually less than
60 GHz
[20,21]
.
In this Letter, we propose a method to improve the spa-
tial resolution of Brillouin optical correlation domain
analysis (BOCDA) based on frequency chirp magnifica-
tion (FCM) by using the four-wave-mixing (FWM) proc-
ess in highly nonlinear fiber (HNLF)
[22,23]
. In the FWM
process, when the pump has a frequency chirp, the gener-
ated idlers also have a frequency chirp but the span is mag-
nified
[24]
. The broadened frequency span helps to obtain a
high performance (high spatial resolution or more effective
sensing points) in correlation-domain sensing systems. In
BOCDA systems, the maximum chirp span of the LD is
∼63 GHz (90 kHz modulation frequency), and the span
of the generated Idler-1 is broadened to ∼126 GHz, which
is the largest chirp span ever reportedly achieved in
BOCDA systems. We have doubled the number of effec-
tive sensing points from 3300 to 6600, and achieved a spa-
tial resolution of 17 cm over a 1 km measurement range.
We believe that higher performance can be obtained by
generating higher-order idlers or utilizing a cascaded
FWM process. Moreover, the capability of combining dif-
ferent performance-enhancing techniques in BOCDA is
also verified. By combining the FCM technique with
the differential measurement scheme
[25–27]
, a spatial resolu-
tion of <10 cm over a 1 km measuremen t range is ob-
tained. We believe that the proposed technique may be
introduced into the state-of-the-art BOCDA systems to
achieve more than one million effective sensing
[28,29]
.
In conventional BOCDA systems, the frequency of
the light source is sinusoidal modulated to generate peri-
odical coherence peaks (CPs). The measurement range
COL 15(12), 120601(2017) CHINESE OPTICS LETTERS December 10, 2017
1671-7694/2017/120601(4) 120601-1 © 2017 Chinese Optics Letters
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