Instrumentation & Measurement Magazine 24-5 - 42

applications and healthcare systems, due to the high flexibility
of the optical fibers and their capability of being embedded in
different structures [12]. In addition, the electromagnetic field
immunity enables their use in conjunction with assistive robots
(which generally have electric actuators) and with devices
that emit electromagnetic waves [13].
In this way, from such advantages the photonic-integrated
textiles field has had a strong impact, appearing first
in clothing accessories and signaling devices. The exceptional
predominance of optical fiber sensors, the photonics textiles as
they are called, has also allowed their use for breath and heart
rates or body temperature sensing [14].
Such optical fiber sensors were originally appreciated as
pointwise sensors, which transduce environmental parameters
from one individual location along the fiber length.
Numerous types of single-point fiber sensors include fiber
Bragg gratings (FBG), long period gratings, in-line fiber interferometers,
and other sensing configurations that feature
simple interrogation and low cost. The promise of fruitful
multiplexing schemes was among the major advantages
promoted by researchers on this field over competing technologies.
There are numerous common multiplexing schemes
that can be used such as optical wavelength-division, time-division,
spatial-division multiplexing, and coherence-domain
multiplexing. Although the multiplexing schemes and signal
processing methods bring more complexity in the interrogation
process when compared to conventional pointwise fiber
sensing devices, there are substantial advantages in creating
such systems like reducing cost and increasing quality of results
measurement for many applications. Single-point fiber
sensors, even when multiplexed to form quasi-distributed
fiber sensors, do not intrinsically deliver the amount of information
that can be provided from inherently distributed
optical fiber-based schemes that play along the entire fiber
length. The optical components' cost has been gradually reduced
due to progress in the optoelectronics field, and in this
way, an extensive availability of low cost and long length fibers
enables their use in remote and distributed sensing based on
optical scattering [15]. Local external perturbations along the
sensing fiber such as temperature and strain can be detected
by variations in amplitude, frequency, polarization, or phase
of the backscattered sensing light.
Most of such sensors
employ silica optical fibers,
which present low optical
loss; however, they have a
brittle nature with low impact
resistance and strain
limits [12]. These drawbacks
can be mitigated
by using polymer optical
fiber (POF) technology.
Advances in the polymer
processing, preparation
and fabrication enable the
development of different
42
types of POFs, which present higher strain limits, flexible features
and impact toughness [12]. Its easy incorporation into
textiles is due to its rough surface characteristic, as demonstrated
in many reports for wearable sensors for human health
assessment [13]. Also, related to flexibility of such smart textiles,
POFs have Young's modulus one order of magnitude lower
than silica fibers (leading to higher flexibility), which can be
even higher with specialty POFs in which highly flexible materials
are used [13]. Thus, the textile integration of POFs with high
flexibility leads to a higher flexibility and freedom of movement
of the clothing. Additionally, smart textiles that incorporate POF
sensors are generally based on the intensity variation sensing
principle, with the key points of portability and low cost.
Multiplexing Technique and Optical
Fiber Integration
As an important drawback that can limit the development of
distributed sensing arrays with intensity variation-based sensors
is their lack of multiplexing capabilities (especially when
compared with FBGs) and their sensitivity to light source
power deviations that can lead to inaccuracies in the measurements,
thus, a multiplexing technique for such sensors
which can also provide a self-referencing system is proposed
in [16]. In the multiplexing technique for intensity variationbased
sensors, two photodetectors are positioned on each
end facet of the fiber, whereas the light sources are transversally
positioned along the fiber, as presented in Fig. 1. In order
to side-couple the light emitting diodes (LEDs) with the fiber
core, lateral sections were made in the fiber to expose its core.
The multiplexing is achieved by time decoupling of the system,
where the LEDs are activated in a predefined sequence
in which each LED is activated at a time, without simultaneous
activation of two (or more) LEDs (Fig. 1). The optical
power measured by each photodetector is acquired when each
LED is activated, resulting in an acquisition matrix in which
the lines are the number of samples (i.e., the signals acquired
throughout the time) and the number of columns is the product
between the number of LEDs and photodetectors, e.g., if
there are n LEDs and two photodetectors, the acquisition matrix
has 2n columns.
For wearable applications (especially smart textiles) and
optical fiber-embedded structures, LED belts in flexible
Fig. 1. Schematic representation of the multiplexing technique for intensity variation-based optical fiber sensors.
IEEE Instrumentation & Measurement Magazine
August 2021

Instrumentation & Measurement Magazine 24-5

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