Instrumentation & Measurement Magazine 24-5 - 56

Spectral Reconstruction and
Bayesian Model Framework for
Characterization of Long
Period Fiber Gratings
Bernardo Dias, Paulo Santos, Pedro A. S. Jorge,
José M. M. M. de Almeida, and Luís C. C. Coelho
T
he use of Long-Period Fiber Gratings (LPFGs) as
sensors has been thoroughly researched, given the
multitude of parameters these structures can monitor
by themselves (such as temperature, strain, curvature) and
the potential for combination with other materials that allow
for monitoring of parameters such as humidity, pH and
chemical concentration, at a low price and with easy fabrication
processes available. This interest has increased the need
for the development of interrogation systems for these sensors,
particularly in the C-band spectral region. Given the
cost and physical limitations (such as size and weight) of traditional
solutions like Optical Spectrum Analyzers (OSA),
the development of low-cost approaches for LPFG spectral
analysis became an important topic that needed further
development. The development of a simple curve fitting
routine for LPFG spectra is reported in this article, along
with a framework for automatic detection of certain physical
phenomena such as corrosion and the presence of chemical
species, among others.
LPFG Spectral Reconstruction in Lowcost
Solutions
Although LPFG-based sensors have been thoroughly studied
and developed, the application of this technology in
extreme environmental conditions is still in development.
This is mainly due to the need for low-cost and easily portable
solutions for the measurement of LPFG spectra, which is
traditionally done with expensive optical equipment, such
as bench-top tunable laser sources, Amplified Spontaneous
Emission (ASE) sources and OSAs. An approach to the development
of this new technology was published recently [1],
based on the thermal modulation of three distributed feedback
lasers (DFBs) tuned at 1530, 1550 and 1570 nm that would
sweep a region of 4 nm around that central wavelength. Given
the data collected, a spectral reconstruction routine (based on
Gaussian/Lorentzian curve fitting) was done, allowing for the
determination of important parameters such as amplitude and
minimum wavelength of the attenuation band of the LPFG,
for the monitoring of sensors. This routine introduced a considerable
number of errors and uncertainty in the parameters
of interest, showing the need for the development of a better
routine.
Research of curve fitting routines in the case of LPFG is
still an unexplored topic. Besides the basic gaussian fitting
routine described in [1], another publication regarding this
topic was done by Guo et al. [2], in which a low-cost approach
for LPFG interrogation based on an arrayed-waveguide-grating
(AWG), was developed to provide spectral information
in the range of 1540 to 1560 nm along 25 channels. After data
acquisition, a curve fitting routine was applied to the data,
with the theoretical model consisting of a linear combination
of N gaussian functions. In practice, the number of gaussians
was reduced to two, which seemed to fit the acquired
data. The problem with this model is that it does not easily
fit LPFG spectra with shapes very different from a gaussian
function, requiring a linear combination of many gaussians
and thus, increasing the number of model parameters. In the
work presented here, a simpler model based on the generalized
logistic distribution is demonstrated, which seems to
fit a wide range of LPFG spectra with only four parameters,
two of which are direct quantities of interest in LPFG-based
sensing (attenuation spectrum amplitude and minimum
wavelength).
In the case of Fiber Bragg Gratings (FBGs), several peak
tracking and spectral reconstruction techniques have been
developed and reported [3]. Although some of these methods
can be used in the case of LPFG (such as gaussian and
polynomial methods), most require an almost complete
This work was supported by POCI-01-0145-FEDER-031220: " SolSensors-Development of Advanced Fiber Optic
Sensors for Monitoring the Durability of Concrete Structures; " Program Budget COMPETE 2020-Operational Program
Competitiveness and Internationalization; the Lisbon Regional Operational Program in its FEDER component;
and by the Foundation for Science and Technology (FCT).
56
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
August 2021

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