A single mode fibre core is laterally exposed to a periodic pattern of powerful UV laser light to create an optical sensor called an FBG. The exposure results in a long-lasting rise in the fiber\'s core\'s refractive index (ncore), producing fixed index modulation known as grating (?). A specific wavelength of input light, known as the Bragg wavelength or Bragg related to grating period, must be reflected by the grating inside the fibre optic core while transmitting all other wavelengths. FBG operates as intended by using the regular variations in the single mode fibre core\'s refractive index, a Bragg reflector can be constructed on an optical fibre. A specific wavelength of light will be reflected and all others will be transmitted as it passes through the FBG. A shift in the wavelength of the light reflected when the temperature or strain surrounding the grating changes is seen.
Due to their prominent characteristics, such as their extremely small size, light weight, immunity to electromagnetic interference (EMI), electrical neutrality, and ability to be easily embedded into a structure without having any effects on the mechanical properties of the structure of the object under investigation, Fibre Bragg Grating (FBG) have demonstrated a great potential advantage in biomedical application over the past ten years .Due to its capability of converting the absorbed energy completely into heat without creating PA signals brought on by scattering particles, Fibre Bragg Grating was employed as a photoacoustic (PA)detection tool to identify the presence of tumors. The photoacoustic technique is distinct because it combines ultrasonic resolution with light contrast. Due to its advantages of being noninvasive, having a high detection sensitivity, and being able to detect small element size, this approach is employed in the detection of tumors,.
Optical fibre that has been specially developed can be used as a sensor. The core refractive index of the Optical Fibre for sensor applications is different from the typical fibre core and cladding refractive index in a small area of the fibre . Normally, that small section of the Optical Fibre core is where a periodic structure is introduced. This area of the fibre core is known as Fibre Bragg Gratings (FBG) because it reflects light of a particular wavelength. Where DBR is a structure made of numerous, alternate layers of materials with varying refractive indices, or by periodically changing a dielectric waveguide's characteristics and leading to periodic changes in the guide's effective refractive index ,. FBG-based sensors are those whose sensitivity is determined by the Bragg wavelength shift of Fibre Bragg Gratings. The FBG is a periodic wavelength scale modification of the refractive index that is encoded in the fibre core segment. The light at a specific wavelength that satisfies the Bragg condition is reflected by Bragg gratings. When forward and back propagation modes at a particular wavelength couple, this reflection in a grating happens . When the particular requirement e.g Bragg condition between the light wave vectors and the grating's vector number is met, the coupling coefficient of the modes is at its maximum:
the grating period, the effective refractive index of the core, the effective wavelength of light known as the Bragg wavelength, and the diffraction order m. Figure 1 depicts the fibre Bragg grating's operating principle.
Theoretically, there are an infinite number of Bragg wavelengths for a single FBG. It is derived from equation (1) that the diffraction order Bragg wavelength varies for different values of m. Since there is a significant spectral gap between both Bragg wavelengths, only one sometimes two Bragg resonance wavelength are actually used in practise. For instance, if the grating's initial Bragg wavelength , m=1 is 1550 nm, the second one will be twice as short, at 750 nm. While the sources used for fibre typically have a spectral range of no more than 100 nm. If the modulation of the refractive index in FBG is not sinusoidal which is typically the case, additional Bragg peaks may appear. For instance, the Fourier spectrum of a rectangular grating includes many modulation frequencies, which can lead to a number of Bragg peaks. Despite the fact that the majority of fiber-based gratings have essentially sinusoidal index modulation. There are several FBG structures; however, in this study, a uniform FBG was used for the experiment and analysis to examine how well an FBG functions as a sensor.
II. THE FUNDAMENTALS OF FBG SENSING PRINCIPLE
According to figure 1, the Fibre Bragg Grating (FBG) is a single mode fibre with a periodic refractive index , n modulation along its core. A fixed index modulation known as a grating is produced when a single mode optical fibre is exposed to strong UV radiation, which increases the reflective index of the fibre core. The Bragg's wavelength which is the maximum reflectivity is the wavelength that is reflected when the FBG is exposed to a specific wavelength since the grating area's period is roughly half that of the input light's wavelength, as indicated in equation (2) .
A high voltage and high electromagnetic interference environment might cause an electronic sensor to malfunction in the electrical and electronics industries. Under these circumstances, it is almost hard for a standard sensor to measure. The measuring of parameters by an FBG sensor built on an optical fibre is a good solution to this issue. Lightweight, simple to install, and capable of multiplexing, fibre bragg grating (FBG) sensors may sense a variety of characteristics, including temperature, strain, load, pressure, and others, at numerous locations along the same sensor cable. Traditional sensors require electricity to function. Because optical fibre sensors are inert, they can be installed passively for many hundred kilometres on transmission lines, gas pipelines, etc. without a power source.
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