Instrumentation & Measurement Magazine 24-5 - 12

Precisely Designed and Modeled
Double-Clad Fiber Optic
Pressure Sensor
Mohammad Davoud Talebzadeh and Marzieh Khademalrasool
I
n this article, we investigated and designed a double-clad
fiber optic pressure (DFOP) sensor, which can acceptably
calculate the pressure values using the input and output
intensities/powers of light. In order to know the received
power, the overlap integral of the emitted light from the core
of a double-clad fiber with its reflection from a mirrored diaphragm
onto either itself or the inner clad was considered.
Also, the diaphragm deflection was approximated as a spherical
convex mirror which diverged the received light cone and
increased the receiving area albeit for the light Gaussian distribution.
In comparison with similar existing setups, this sensor,
with its precise calibration, provided a wider range of output
normalized power of about 0.3 W (watt). Thus, the sensor can
be made more simply and inexpensively since there is no need
for very high precision optical time-domain reflectometers
(OTDR) or power-meters.
Fiber Optic Displacement Sensors
Fiber optic sensors (FOSs) are better than very similar electronic
sensors due to their high sensitivity, responding speed,
and compactness. They are lightweight, offer electromagnetic
wave isolation, are compatible with environments, and
are sustainable in various environments, even harsh ones.
From the spatial measurement classification point of view,
FOSs are divided into point sensors and distributed sensors.
A point sensor measures one or multiple quantities in a single
point, while a distributed sensor comprehends multiple discrete
or continuous measurements along a fiber optic line [1].
Optical fiber tip sensors and fiber Bragg-grating sensors are
examples of the mentioned classes [2], [3]. Point sensors are
generally classified into intensity sensors and interferometric
sensors, and the intensity sensors are the first implemented
ones [4], [5]. Simplicity, reliability, inexpensive cost, high sensitivity
and ease of production are the most important virtues
of point-intensity-based fiber optic sensors. Usually, their
noncoherent light sources propagate through an emitter multimode
fiber, then couple to another (other) receiver one(s) as
sensing head(s). Therefore, the output intensity is a function
of observed parameters, and special calibration or referencing
12
methods are needed in order to make sure that undesired
sources do not affect measurement/observed parameters [1].
Among these groups of sensors, the Intensity-based fiber optic
displacement (ISFOD) sensors are simpler and more often
used [6], [7]. One reason for their importance is that many
other kinds of sensors, as well as pressure sensors, can be made
by ISFOD sensors [8]-[10].
The optical fiber displacement sensors with various setups
were experimentally and theoretically studied by many
researchers [11]-[19]. " Double-fiber reflective plate setups " is
an important group of ISFOD sensors. In these arrangements,
an emitter fiber is adjusted in the vicinity of the receiver fiber
in a way that they are parallel or angled to each other [7], [12],
[20]. The other simpler sensor setups are single-fiber ones [21]-
[23], for which there is no comprehensive analysis, and many
of such articles are generally experimental reports.
On the other hand, there are common electrical and electro-mechanical
capacitor sensors which are used in factories
and industries to measure the pressure. They generally consist
of two parallel plates, and one of them is a flexible diaphragm
[24]-[26]. When a pressure inserts a deflection to the diaphragm,
the capacitor geometry changes, and therefore the
related capacitance varies. In these common sensors, the capacitance
can be calculated as a function of the deflection
pattern.
Combining a deflectable diaphragm as a reflective plate
and a simple double-clad fiber probe, we have designed a high
sensitivity and very simple pressure sensor. Its important virtue
is to omit the electron (electric charge or voltage) from
measuring processes where it guarantees the safety considerations,
especially in explosive or inflammable mediums such
as gas or alcohol tanks. We employed Mathematica9 to solve
the multiple mathematical models for the optical and mechanical
components of the DFOP sensor, simultaneously. Based on
this analysis, a specialized calibration and a relative function
can be proposed for any set of parameters. Also, the proposed
arrangement provides a wider range and consequently more
precision of measurements in comparison with the reported
common similar sensors.
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
August 2021
Instrumentation & Measurement Magazine 24-5

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