Large-scale multiplexing of a FBG array with
randomly varied characteristic parameters for
distributed sensing
XIN GUI,
1,2
ZHENGYING LI,
1,2,
*XUELEI FU,
2
CHANGJIA WANG,
2
HONGHAI WANG,
1
FAN WANG,
2
AND XIAOYI BAO
3
1
National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan, Hubei 430070, China
2
School of Information Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, China
3
Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
*Corresponding author: zhyli@whut.edu.cn
Received 25 July 2018; revised 21 September 2018; accepted 26 September 2018; posted 28 September 2018 (Doc. ID 340442);
published 18 October 2018
Large-scale multiplexing of a fiber Bragg grating (FBG)
array with randomly varied characteristic parameters
(RVCPs) is proposed. The center wavelength of each
FBG and the grating spacing between adjacent FBGs are
randomly varied within a controlled range. Due to
the low reflectivity (< −45 dB) and RVCPs of the FBGs,
the sensing array presents low spectral-shadowing crosstalk
and multiple-reflection crosstalk, implying high FBG mul-
tiplexing capacity. The numerical and experimental results
show that the RVCP-FBG array significantly impr oves the
signal-to-noise ratio and demodulation accuracy of the dis-
tributed sensing system. Large-scale multiplexing of 10,000
FBGs along a 10 m long fiber is achieved.
© 2018 Optical
Society of America
https://doi.org/10.1364/OL.43.005259
Multiplexed fiber Bragg grating (FBG)-based distributed
optical fiber sensing (DOFS) is widely used for structural health
monitoring, due to its excellent multiplexing capacity and low
cost [1,2]. Thousands of identical weak FBGs can be rapidly
fabricated along a single fiber using the novel in-line FBG
fabrication technique during fiber drawing [3]. The gratings
fabricated by an in-line method have low reflectivity and nearly
identical reflection wavelengths. Optical time domain reflec-
tometry (OTDR) [4], optical frequency domain reflectometry
(OFDR) [5], and frequency-shifted interferometry [6] are
commonly used to demonstrate DOFS with the in-line fabri-
cated FBG arrays. The reflectivity of each FBG affects the
multiplexing capacity of a DOFS system. High reflectivity
results in severe spectral-shadowing crosstalk and multiple-
reflection crosstalk and, eventually, limits the multiplexing
capacity. To achieve large-scale multiplexing, using FBGs with
weak reflectivities between −35 and −40 dB has been proven to
suppress the crosstalk [7]. Nevertheless, a decrease of the
reflectivity will reduce the signal level and, hence, expensive
low-noise detection is needed. Thus, the practical application
of DOFS based on a large-scale multiplexed FBG array cannot
be achieved by merely using weak reflection gratings. Recently,
the use of several phase masks to produce an FBG array with a
few different central wavelengths has been demonstrated to im-
prove the multiplexing capacity [6]. In this case, the experimen-
tally demonstrated maximum multiplexing capacity is 2000
FBG sensors in a single array [4].
In this Letter, we describe a distributed sensing system with
a large-scale multiplexed FBG array. FBG arrays with ran-
domly varied characteristic parameters (RVCPs) are proposed
in order to r educe the spectral-shadowing and multiple-
reflection crosstalk effects. The fabrication system is a modi-
fied version based on an in-line FBG writing system, which
originally fabricates FBG arrays with identical center wave-
lengths and fixed grating spacing. Such an improved in-line
system guarantees reliable fabrication of large-capacity RVCP-
FBG a rrays with the center wavelengths and grating spacing
randomized within a controlled range. A comprehensive
numerical a nalysis and experimental investigations are con-
ducted to assess the system performance, which show that
the use of an RVCP-FBG array significantly improves the
signal-to-noise ratio (SNR) and demodulation accuracy of
the distributed sensing system. Multiplexing of 10,000 FBGs
along a 10 m fiber is achieved, presenting a large-scale multi-
plexed FBG array to future distributed sensing solutions.
Since the OFDR distributed sensing system uses continuous
light, the resulting multiple-reflection effect is more detrimen-
tal than that in the pulsed light-based distributed sensing
system. Therefore, the analysis conducted in this Letter is also
applicable to an OTDR demodulation system.
SNR is a key parameter in a distributed sensing system. An
improved SNR leads to improved accuracy, sensing range, and
spatial resolution [7 ]. Multiple reflections are some of the
main contributors to the noise in distributed sensing system.
In an FBG array with large grating spacings, the length of a
single FBG is often overlooked when we consider the impact
on multiple reflections. However, in the cases concerning
ultra-high spatial resolution, the grating length and grating
spacing jointly determine the spatial resolution. Therefore,
the length of the FBGs is taken into consideration in the
following analysis.
Letter
Vol. 43, No. 21 / 1 November 2018 / Optics Letters 5259
0146-9592/18/215259-04 Journal © 2018 Optical Society of America