INSTITUTE OF PHYSICS PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY
Meas. Sci. Technol. 15 (2004) 1501–1505 PII: S0957-0233(04)74201-0
Distributed temperature sensing using a
chirped fibre Bragg grating
Pei Chin Won, Jinsong Leng, Yicheng Lai and J A R Williams
Photonics Research Group, Aston University, Aston Triangle, Birmingham B4 7ET, UK
E-mail: wonpc@aston.ac.uk
Received 30 December 2003, in final form 10 March 2004
Published 19 July 2004
Online at stacks.iop.org/MST/15/1501
doi:10.1088/0957-0233/15/8/012
Abstract
A fully distributed temperature sensor consisting of a chirped fibre Bragg
grating has been demonstrated. By fitting a numerical model of the grating
response including temperature change, position and width of localized
heating applied to the grating, we achieve measurements of these parameters
to within 2.2 K, 149 µm and 306 µm of applied values, respectively.
Assuming that deviation from linearity is accounted for in making
measurement, much higher precision is achievable and the standard
deviations for these measurements are 0.6 K, 28.5 µm and 56.0 µm,
respectively.
Keywords: distributed temperature sensor, chirped fibre Bragg grating, fibre
optic sensors
1. Background
Optical fibre distributed temperature monitoring is particularly
attractive in several situations, such as when electrical
temperature monitoring is impractical, when there is no prior
knowledge of sensor placement and when a large number of
sensors is required, due to their immunity to electromagnetic
interference, high sensitivity, compactness and simplicity of
fabrication. Among the approaches, fibre Bragg gratings
(FBGs) have received much attention for use as sensors in
measuring strain, temperature and other physical quantities.
In most sensing schemes [1–6], the FBGs act as point sensors
where prior knowledge of placement is essential and a large
number of individual sensors may be required to cover an
extended area.
In this paper, we demonstrate a distributed temperature
sensor using a linearly chirped apodized fibre grating as the
sensing element. Changes in the local Bragg wavelength
caused by localized temperature changes along the grating
induce changes in the reflected spectrum of the grating [7]. By
measuring the reflected spectrum and fitting it to a numerical
model based on the transfer matrix method with the effects of
the applied temperature profile, we are able to determine the
temperature change, position and width of a localized region
of heating. This sensing technique is potentially useful for
medical applications and structural monitoring.
2. Introduction
Practically, there are several calculation techniques for
determining the optical spectrum of a fibre Bragg grating given
an arbitrary (subject to constraints due to the approximations
used) structure. Among these, Rouard’s method [8] (also often
called the transfer matrix technique) is extensively used as it
is fast and sufficiently accurate for most structures. We use
this technique in this work to calculate the expected spectrum
given the structure of a known grating with a known applied
temperature profile modifying its internal structure.
In our work on distributed temperature sensing using
an apodized chirped fibre grating, the position-z-dependent
coupling constant, κ(z), and Bragg wavelength, λ
B
(z), of the
grating are defined as
κ(z) = κ exp
−
z
l/2
2n
log 2
(1)
λ
B
(z) = λ
c
+ γz + g(z, T , L
p
,C
p
) (2)
where κ is the coupling constant and the exponential function
in κ(z) is an nth-order hyper-Gaussian apodized profile applied
during grating fabrication. l is the physical length of grating at
3 dB bandwidth, λ
c
is the central wavelength of grating, γ is the
effective chirp rate and g(z, T , L
p
,C
p
) is the temperature
profile along the grating under test with T being the applied
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