The Study on the Spectral Analysis of Fiber Bragg Grating (FBG) as a Strain Sensor

Abstract

By adjusting the grating length and refractive index change, parameters of the Fibre Bragg grating which are the effective refractive index, Bragg wavelength, grating period, and strain-optic constant are provided and discussed, along with the characterization of the grating, including strain, Bragg wavelength shift, spectral response, and bandwidth. The coupled mode equations form the basis for data analysis and acquisition. This research uses a high-level computer language, such as GNU Octave software, to simulate FBG spectral responses.

Introduction

I. INTRODUCTION

Fibre Bragg Grating (FBG) have shown over the past ten years [1] a great potential advantage in biomedical application 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 was used as a photoacoustic (PA)detection method to detect the existence of tumours because it has the ability to transform the absorbed energy totally into heat without producing PA signals brought on by scattering particles[4]. The photoacoustic approach stands out because it blends light contrast with ultrasonic resolution[5]. Given the benefits of being non-invasive, having a high detection sensitivity, and this method is used in the diagnosis of tumours since it can identify small element sizes[6],[7].

A sensor can be made out of specifically created optical fibre. In a tiny portion of the fibre, the optical fiber's core refractive index differs from the standard fibre core and cladding refractive index for sensor applications [8]. Usually, a periodic structure is introduced in that little area of the optical fibre core. Due to its ability to reflect light of a specific wavelength, this region of the fibre core is referred to as the Fibre Bragg Gratings (FBG). When a dielectric waveguide's properties are periodically changed, this causes periodic variations in the waveguide's effective refractive index [9],[10]. Alternatively, DBR can be created by stacking several, alternative layers of materials with different refractive indices on top of each other.

The sensitivity of FBG-based sensors is based on the Bragg wavelength shift of the Fibre Bragg Gratings. The FBG is a periodic adjustment of the refractive index on a wavelength scale that is stored in the fibre core segment. Bragg gratings reflect light that meets the Bragg condition at a particular wavelength. This reflection in a grating occurs when forward and back propagation modes at a specific wavelength pair [11]. The coupling coefficient of the modes is at its highest when the specific criteria, such as the Bragg condition between the light wave vectors and the grating's vector number, is satisfied:

The grating period, core's actual refractive index, effective light wavelength (sometimes called Bragg wavelength), and diffraction order m. The workings of the fibre Bragg grating are shown in Figure 1.

For a single FBG, there are theoretically an infinite number of Bragg wavelengths. Equation (1) makes clear that for different values of m, the diffraction order Bragg wavelength varies. Only one or perhaps two of the Bragg resonance wavelengths are actually employed in practise since there is a large spectral gap between them. For instance, the second Bragg wavelength will be twice as short, at 750 nm, if the grating's first Bragg wavelength, m=1, is 1550 nm. While the spectral range of sources utilised for fibre is normally limited to 100 nm. Additional Bragg peaks could show up if the modulation of the refractive index in FBG is not sinusoidal, which is frequently the case. For instance, a rectangular grating's Fourier spectrum contains a large number of modulation frequencies, which can result in numerous Bragg peaks. Despite the fact that the index modulation of the vast majority of fiber-based gratings is essentially sinusoidal. There are several FBG structures; however, in this work, an experiment and analysis were conducted using a uniform FBG to determine how well an FBG performs as a sensor.

II. FIBER BRAGG GRATING SENSING PRINCIPLE

The Fibre Bragg Grating (FBG) is a single mode fibre with periodic refractive index, n modulation along its core, as seen in figure 1. When a single mode optical fibre is exposed to intense UV radiation, which raises the fibre core's reflective index, the result is a fixed index modulation known as a grating[9]. Since the period of the grating area is roughly half that of the wavelength of the input light, as shown in equation (2) [1][3][8], the wavelength that is reflected when the FBG is exposed to a particular wavelength is the Bragg's wavelength, which is the wavelength with the maximum reflectivity.

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Conclusion

Changes in fibre length are used to simulate strain based on the outcomes of the simulation. When the strain increases, the Bragg wavelength shift increases. Therefore, grating length also increases which causes the FBG reflectivity to increase. At grating lengths greater than 9.0 mm, the reflectivity of FBG achieves its maximum point, also known as full reflection. As the refractive index change increases, so does FBG reflectivity. When the refractive index change is greater than ?n = 0.0020, 100% of reflection will occur. Additionally, when the refractive index changes and the grating length increases, the FBG sensor bandwidth decreases.

References

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Copyright

Copyright © 2023 Muhammad Arif Bin Jalil. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.