Instrumentation & Measurement Magazine 24-5 - 57

acquisition of data in the attenuation band region, which
cannot be guaranteed in the reported low-cost system,
because it relies on the use of DFBs in specific spectral regions
that may not match the attenuation band minimum.
This lack of spectral reconstruction methods and their importance
for the low-cost platform (to prevent errors in
the measurements) justified the development of a new
framework.
Low-cost Approaches for LPFG
Interrogation
The complete experimental setup to use with LPFGs requires
an interrogation system capable of illuminating the
wavelength range where the attenuation band is located
and some measuring system to determine the power output
of the sensor. In previous publications [1], our group
reported the development of a low-cost system based on
three thermally modulated DFBs that could provide spectral
data of the LPFG's attenuation band in very narrow intervals
(approximately 4 nm around a central wavelength). The
possible absence of data in critical regions of the spectrum
(such as the attenuation band's peak) introduces considerable
uncertainty in the determination of crucial parameters
for LPFG sensing, such as the attenuation amplitude and
central wavelength.
In recent developments, the low-cost solution was
reprogramed to be controlled
from a Raspberry
Pi 4, combining the computational
power of an
advanced single board
computer, and full
integration of lasers, photodetectors,
and other
peripherals, which makes
it possible to perform
LPFG-based sensing at
a lower cost and at realtime.
This should provide
a high-level control of the
system and a graphical interface
integrated with a
touchscreen, offering the
user an easier experience
for LPFG sensing and system
control, as seen in
Fig. 1.
Using the Raspberry
Pi's integrated UART
modules, the communication
with the laser
board microcontroller
(Model STM32F103) is
bidirectional, allowing
instructions to be sent (using
the UART Tx terminal
August 2021
in the Raspberry Pi's pins) to the laser from the user interface.
The LPFG spectral power is then read by the photodetectors
board that is connected via SPI to the laser board, allowing
for wavelength and optical power information to be sent
to the Raspberry Pi (using the UART Rx terminal) where it
can be processed and plotted. The Raspberry Pi is also programmed
to handle errors in the laser board and display it
to the user.
An important feature regarding LPFGs is the variety of
shapes of the attenuation band. Multiple factors can influence
the overall shape, such as the number of markings done to the
fiber (directly related to the attenuation intensity), the spatial
period (directly related to the wavelength of the band's minimum),
and the fabrication process (with less band symmetry
and presenting side-lobe artifacts) [4]. While using traditional
equipment, this variety of shapes will not generally impose
a problem due to the availability of data in high resolution
and large spectral range, but with scarce data points (such
as in the low-cost interrogation system) this can be problematic,
causing errors and uncertainties in the measurements.
To compensate for the lack of spectral data when comparing
with these traditional solutions, a good spectral reconstruction
framework is required. This set of tools aims to minimize
the error in the determination of the sensing parameters, as
well as provide information regarding the uncertainty of said
parameter.
Fig. 1. Setup of the low-cost LPFG interrogation system. A Raspberry Pi 4 controls the laser's action, which along with
the photodetector board, enables the measurements of the LPFG sensor attenuation band. The Raspberry Pi module also
processes the data and provides for a Graphical User Interface via touchscreen.
IEEE Instrumentation & Measurement Magazine
57

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