Process control in microwave heating of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 179

been reported to work satisfactorily in conditions ranging from below
room temperature to above 300°C [25]. Finally, the last group of optical
fibre sensors, which are referred to as fibre Bragg grating (FBG) sensors, are
made by forming a distributed Bragg reflector within a short segment of an
optical fibre that reflects particular wavelengths and transmits all others.
FBG sensors are considered one of the most suitable temperature sensors
for temperature measurement in the presence of strong electromagnetic
fields. Because of their suitability for the applications discussed previously
in this topic, the working principles of this group of optical ibre sensors
are described in detail in the following sections.

5.2.3.1 Fibre Bragg grating sensors: Structure
and working principles
The FBGs have been used as temperature sensors in many civil engineer-
ing applications, mostly related to the monitoring of temperature of the
concrete subjected to elevated temperatures in the absence of strong elec-
tromagnetic fields [19,21,26,27]. The satisfactory employment of FBGs in
noncivil engineering applications involving the use of strong electromag-
netic fields has also been reported by a number of studies [7,9,17,28]. The
results of a recent study showed that certain types of FBG sensors can
be used as temperature sensors in the microwave-assisted applications dis-
cussed in Chapters 2 to 4. The findings of this study are reviewed and dis-
cussed in detail in the subsequent sections. However, prior to illustrating
the suitability and precision of FBGs as temperature sensors for microwave-
assisted applications, a brief introduction to the working principles of FBGs
is first provided to help readers appreciate what makes FBGs good tempera-
ture sensors in the presence of electromagnetic fields.
A Bragg grating is a periodic structure fabricated by exposing parts of
a photosensitised fibre core to ultraviolet light (Figure 5.6) [29]. Figure 5.7
shows a typical commercially available FBG sensor. When light propagating
along a single-mode fibre encounters the periodic variation in the refractive
index of the in-ibre grating, a small amount of light is reflected at each point
of the refractive index transition. If each of the reflections is in phase, they will
add coherently and produce a large net reflection from the grating. This phase
matching occurs typically at only one specific wavelength, and this wave-
length is called the Bragg wavelength. Any thermal expansion, strain, or pres-
sure change in the structure monitored by the FBG leads to a change in the
grating spacing and thus a change in the FBGs' refractive index, which results
in a shift in the Bragg wavelength [30]. The Bragg wavelength, λ B , is related to
the grating period and the effective refractive index of the fibre n eff by

λ B = 2Λ n (5.1)

eff