Detection of Low Hydrostatic Pressure Using Fiber Bragg Grating Sensor
Corresponding email: retno.wigajatri@ui.ac.id
Published at : 07 Dec 2023
Volume : IJtech
Vol 14, No 7 (2023)
DOI : https://doi.org/10.14716/ijtech.v14i7.6714
Madani, N.A., Purnamaningsih, R.W., Poespawati, N.R., Hamidah, M., Rahardjo, S., Wibowo, D.K., 2023. Detection of Low Hydrostatic Pressure Using Fiber Bragg Grating Sensor. International Journal of Technology. Volume 14(7), pp. 1527-1536
| Naufal Aiman Madani | Department of Electrical Engineering, Universitas Indonesia |
| Retno Wigajatri Purnamaningsih | Department of Electrical Engineering, Universitas Indonesia |
| Nji Raden Poespawati | Department of Electrical Engineering, Universitas Indonesia |
| Maratul Hamidah | Research Center for Electronics, National Research and Innovation Agency |
| Sasono Rahardjo | Research Center for Electronics, National Research and Innovation Agency |
| Dena Karunianto Wibowo | Department of Applied Physics, KTH Royal Institute of Technology |
The aim of this study was to investigate the detection of low hydrostatic pressure, serving as the foundation for developing an underwater pressure sensor. This included creating a single-mode, uniformly structured Fiber Bragg Grating (FBG) with a stainless-steel coating. The experiment included loading the sensor with different volumes of fresh water ranging from 0 ml to 6 ml, in increments of 0.25 ml, in the perpendicular vertical direction. This volume range corresponded to hydrostatic pressures ranging from 0 Pa to 40.55 Pa. The experimental results showed a consistent linear relationship between low hydrostatic pressure and Bragg wavelength, implying a sensitivity of 0.8092 pm/Pa, according to theoretical expectations. Subsequently, mathematical simulations were conducted based on the results to predict the sensor performance under various potential seabed temperatures. The simulation results indicated that as the temperature rose, there was a corresponding increase in the reflected wavelength difference by 5.3754 ×10-9 nm/Pa for every 1°C increase in seawater temperature.
Bragg wavelength; Fiber bragg grating; Low hydrostatic pressure
Pressure is a crucial parameter in analyzing environmental conditions
and has diverse applications, which include underwater environmental monitoring
(Matsumoto et al., 2018), as well as
earthquake and tsunami detection (Tanioka, 2018).
Underwater pressure measurements typically comprise the use of a Bottom
Pressure Recorder (BPR) system and a buoy (Irfan et
al., 2021). BPR is deployed in the Ocean Bottom Unit (OBU) module
designed to detect underwater pressure and transmit the collected data to a
buoy. Subsequently, data from both OBU and buoy are sent to a ground station
for further analysis (Winarno et al., 2021; Suastika et al.,
2019).
The use of OBU and buoy for underwater pressure detection presents
certain limitations. These limitations include limitations to near real-time
monitoring capability and high production, operational, and maintenance costs.
The identified shortcomings of buoy-based pressure detection techniques have
prompted study efforts and the implementation of fiber optic-based sensors for
underwater measurements (Yang, Zhu, and
An., 2022; Ramirez et al., 2022; Lei, 2020).
Fiber optic
sensors offer several advantages, such as applicability in hard-to-reach
locations and higher immunity to electromagnetic interference (EMI) (Rong
and Qiao, 2019). These advantages, among others, drive
numerous developments in optical-based sensors, including accelerometers (Zhang et al., 2017),
which find applications in various electronics and diverse fields (Zainudin et al., 2017; Dwiyantoro, Nugraha, and Choi, 2016).
An essential aspect of underwater
measurements includes detecting pressure changes developing from the movement
of objects around the sensor (Nishida et al., 2018). In addition, it is crucial to understand the sensor
threshold, resolution, and sensitivity. Numerous studies explored hydrostatic
pressure in water. (Her and Weng, 2021) developed a Fiber Bragg Grating (FBG) sensor
with an integrated epoxy diaphragm for measuring water pressure and depth. The
study showed sensitivities ranging from 175.5 pm/kPa to 43.7 pm/kPa for FBG in
the pressure range of 491 – 2943 Pa. In another study, (Bhowmik et al., 2015) investigated the sensitivity of hydrostatic
pressure on polymer-type FBG with different diameters to assess the impact of
etching on sensitivity. The results showed that pressure in the range of 0-1000
kPa, increasing by 100 kPa, led to a sensitivity of 0.2 pm/kPa for unetched FBG
and 0.75 pm/kPa for etched FBG. Additionally, (Nguyen, Chiang, and Tsai, 2022) examined a bare FBG pressure sensor based on an oval-shaped
3D-printed structure. In the range of 0 - 0.45 MPa, with increments of 0.05
MPa, a sensitivity of 6.834 nm/MPa was observed. Furthermore, (Amos, Prabhu, and Nuguna, 2017) proposed a theoretical design of a hybrid FBG/EFPI sensor for
temperature and pressure measurements in subsea underwater applications. Simulations
showed an increase in temperature sensitivity from 13.95 pm/°C to 23.89 pm/°C.
There was no
recent experiment performed with stainless steel-coated FBG pressure sensors to
detect small changes in water pressure, particularly under 50 Pa. In this study, experiments were
conducted within the range of 0 to 40.55 Pa, with an increase of 1.69 Pa. The
aim was to observe the performance of a stainless steel-coated,
uniform-structured, and single-mode Fiber Bragg Grating (FBG) pressure sensor
in detecting very low hydrostatic pressure. Furthermore, based
on the results, mathematical simulations were conducted to predict the sensor
performance in detecting subtle pressure changes as experienced while immersed
in underwater conditions. In Section 2, the operational concept of FBG is
explained and the experimental setup and procedures are described in Section 3.
Results and predictions of sensor output in the form of mathematical
simulations are described in Section 4, while Section 5 contains the conclusions.
It is crucial to be aware that all experiments were carried out at the
Optoelectronics Laboratory, Electronics Research Center (PRE) of the National
Research and Innovation Agency (BRIN). Consequently, the results are expected
to be the basis for the development of underwater pressure sensors in
Indonesia.
2. Literature Review
2.1. Operating Principle of Fiber Bragg Grating
(FBG)
FBG functions by selectively filtering specific wavelengths of light through partial reflection and transmission. This mechanism relies on tracking the reflected Bragg wavelength (Werneck, Allil, and Nazare, 2017). When light enters, only the one that matches the FBG Bragg wavelength will be reflected and the rest will pass through to the output. This can be called the resonant condition of a grating which is expressed as follows (Singh et al., 2023).
Where 
signifies FBG Bragg wavelength, 
signifies the effective refractive index of
the core, and 
signifies the grating
period.
2.2. Fiber
Bragg Grating (FBG) as a Pressure Sensor
Equation (1) clearly shows that Bragg wavelength, maximally reflected, is particularly influenced by the effective refractive index of the fiber core and the grating period. FBG shows heightened sensitivity to changes in both strain and temperature. The effects of strain and temperature on FBG Bragg wavelength are described as follows (Zhang, Tian, and Fu 2017).
In Equation (2), 
represents the change in temperature, and
represents the change in strain.
The strain change in Equation (2) arises from two sources: mechanical strain 
and temperature strain 
The relationship between these two strains is defined as follows (Tanguy, Li, and
Mengxi, 2021).
is defined as the product of the material
coefficient of thermal expansion 
and the change in temperature 
as expressed by (Leal-Junior et al., 2019). 
By substituting the value of
In this context, 
denotes the photo-elastic
coefficient, 
signifies the total strain arising from both mechanical strain 
and temperature strain , 
represents the change in the
refractive index due to temperature, and
If FBG is used as a pressure
sensor on an object experiencing no temperature changes, the temperature change
value can be considered zero (
Figure 1 showed the schematic setup of the Bragg wavelength measurement system, designed to simulate hydrostatic pressure through varying water volume conditions. In the schematic setup shown in Figure 1, a light source, represented by SLED DL-BP1-1501A with a wavelength range of 1535 - 1565 nm, was connected to a 3-port circulator for directing light. The light wave entered through port 1 and exited at port 2, directed toward the stainless steel-coated single-mode uniform FBG positioned on the measurement object. The reflected light wave from FBG re-entered the circulator through port 2 and exited at port 3, directed toward the optical interrogator. Port 3 lacked an input circulator, making it an optical null. The central reflected wavelength of the single-mode uniform grating FBG used in the measurement was observed in an optical spectrometer integrated into the software
Figure 1 Schematic of FBG sensor
setup
The outcome of this test included comparing the central reflected wavelength with the applied pressure on FBG, specifically the pressure exerted by the water introduced into the container. Figure 2 showed a visual representation of the setup used in this experiment.
Figure 2 Experimental setup for FBG
experiment under low hydrostatic pressure
In the experiment, a stainless steel-coated was
placed on a single-mode uniform FBG at the bottom of a plastic measuring glass
with a diameter of 4.3 cm. FBG was installed by drilling holes on both sides of
the measuring glass bottom, and silicone sealant covered the holes.
Subsequently, the experimental setup experienced low hydrostatic pressure by
incrementally adding water droplets with a volume of 0.25 ml. Water volume
consistency was maintained using a syringe pump into the measuring glass, as
shown in Figure 3. The addition of water droplets
occurred gradually, ranging from 0 ml to 6 ml with increments of 0.25 ml.
Similar to prior tests, data were collected at a frequency of 60 samples per
minute or 1 sample per second. The data from the optical spectrum analyzer
(OSA) were later averaged per minute to reduce the ripple effect occurring
during the initial introduction of water droplets into the measuring glass as
seen in Figure 3.
Figure 3 Procedure for conducting an experiment on low
hydrostatic pressure using an FBG sensor
Figure 4 showed one of the FBG experiment outcomes under low hydrostatic pressure in the vertical direction. The measurements stabilized when the volume reached 0.75 ml, equivalent to 5.07 Pa of hydrostatic pressure. Below the volume of 0.75 ml, the measurements were unstable due to water pressure from various directions, but this instability was quickly eliminated as the pressure on FBG was mainly from the vertical direction. Another factor that mitigated the effect of water pressure from different directions was the amount of water pressing on FBG at a given time. Selecting a smaller measurement range reduced the ripple that occurred when filling water into the container, thus diminishing the influence of pressure from various directions.
Figure 4 Graph of experimental results for low hydrostatic pressure using
FBG sensor at 25°C
The
experiment results, according to Figure 4, showed a threshold point at
0.75 ml, corresponding to a hydrostatic pressure of 5.07 Pa. It was evident
from the results that an increase in volume by 0.25 ml, equivalent to a
hydrostatic pressure of 1.69 Pa, resulted in a distinguished difference in the
reflected wavelength. This difference was particularly visible in the range of
0.75 ml (5.07 Pa hydrostatic pressure) to 4 ml (27.03 Pa hydrostatic pressure),
showing a sensitivity of 0.0056 nm/ml or 0.8284 pm/Pa. Figure 5 visually showed the linear range
of the experiment.
The established linear relationship signified the consistency of central reflected wavelength changes concerning variations in hydrostatic pressure. The consistency showed that the experiments were affiliated with theoretical expectations. Consequently, further data analysis could be undertaken to predict the sensor performance in underwater environments.
Figure 5 Graph showing the results of the experiment on low hydrostatic
pressure using an FBG sensor at 25°C
A theoretical review was conducted to establish the relationship between the applied hydrostatic pressure and the reflected wavelength. In this experiment, silicone sealant was used to install and hold FBG sensor in the cylindrical container. Additionally, Equation (6) could be further derived as outlined by (Her and Weng, 2021).
In the given expressions,
According to Equation (7), an increase in applied pressure on FBG was directly correlated
with the rise in Bragg wavelength. In this experiment, FBG
was used to measure hydrostatic pressure, which referred to the pressure
exerted by fluids. Therefore, Equation (7) could be further
derived to ascertain the relationship between the volume of the fluid and FBG Bragg
wavelength.
In this context,
Per Equation (8), it was observed that the increase in Bragg wavelength showed a linear relationship besides volume (V). This arrangement suggested that the conducted measurements were associated with the theoretical foundation. Furthermore, the consistency between theoretical derivation and measurement results implied the ability to predict FBG performance in an underwater environment.
Figure 6 showed the comparison between experimental measurements and
calculated results using Equation (8), with 
=1547 nm, V ranging from 0.75 ml to 4 ml, 
=1000 kg/m3, 
=0.22, v=0.5, g=9.8 m/s2 , E=2.03 MPa, and t=10 mm.
Figure 6 Graph compared the low hydrostatic pressure experiment with its
mathematical simulation
The data showed a direct proportionality
between the increase in Bragg wavelength and the volume of fresh water. The
sensitivity, calculated theoretically, was set upright at 1.3063 pm/0.25 ml or
0.7734 pm/Pa, while the experimental measurements yielded a sensitivity of
1.3667 pm/0.25 ml or 0.8092 pm/Pa. The mean error between experimental and
theoretical calculations was 4.6%. The result was comparable to those (Her and Weng, 2021) where sensitivity values around 0.1755 pm/Pa were obtained for
measurements ranging from 491 to 2943 Pa. In addition, there was an increase in
sensitivity from 0.1755 pm/Pa to 0.8092 pm/Pa. It was important to note
variations in the range and step of the measurements taken.
To prepare the sensor for ocean use, where
the density differs from fresh water, simulations were conducted. These
simulations aimed to anticipate the shift in Bragg wavelength and sensor
sensitivity in two distinct fluids, fresh water with a density of (Webb, 2019) and seawater with a density of (Romeo, Albo, and Lago, 2021).
Figure 7 to Figure 10 showed the comparison of simulation results for both types of fluids at temperatures of 4°C, 15°C, 25°C, and 50°C. The chosen temperatures in the simulations aimed to represent the conditions of deep-sea waters in Indonesia. This range covered normal seawater conditions at a depth of 2000 m, which was approximately 4°C (Odumodu and Mode, 2016), to seawater conditions influenced by underwater volcanic activities, reaching temperatures well above 50°C.
The
simulation results from Figure 7 to Figure 10 showed that as the temperature
rose, there was a noticeable increase in the reflected wavelength shift. This
increase was more prominently evident in the Table below.
Table 1 Reflected central wavelength shift at various temperature
|
T (°C) |
freshwater (pm) |
seawater (pm) |
|
4 |
1.3131 |
1.3498 |
|
15 |
1.3132 |
1.3499 |
|
25 |
1.3133 |
1.3500 |
|
50 |
1.3135 |
1.3503 |
* for every 0.25 ml increase
Using the
data provided in the table above, the average increase in the reflected central
wavelength shift was nm/0.25 ml
or nm/Pa for every 1°C increase in freshwater. In
seawater, this increase was nm/0.25 ml or nm/Pa for every 1°C rise.
In
conclusion, from the laboratory-scale experiment and subsequent data analysis,
several key results were obtained. For example, in low hydrostatic pressure
measurements using FBG, a threshold of 5.07 Pa with a resolution of 1.69 Pa was
established. The results showed a linear relationship between low hydrostatic
pressure and the reflected wavelength in the range of 0.75 ml – 4 ml,
equivalent to 5.07 Pa - 27.3 Pa hydrostatic pressures. Consequently, the
observed sensitivity was 0.8092 pm/Pa, associating with the theoretical basis,
and a 4.6% margin of error. Simulations also showed that with an increase in
temperature, there was a noticeable increment in the reflected wavelength
shift. On average, there was an increase of 5.22898 × 10-9 nm/Pa for
each 1°C increment in freshwater and 5.3754 × 10-9 nm/Pa for each
1°C increment in seawater.
The authors are grateful to the Faculty
of Engineering, University of Indonesia, 2022-2023 for providing support
through the Seed Funding Grant No: NKB-1957/UN2.F4.D/ PPM.00.00/2022 and RIIM
Grant 2022-02 (G3) No: RIIM-30141557602.
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