Simulation of Fiber Bragg Grating (FBG) as A Strain Sensor

Abstract

In this study, the Fibre Bragg grating (FBG) is modelled, simulated, and characterised with respect to maximum reflectivity, bandwidth, the impact of applied strain on the wavelength shift, ?B, and the wavelength shift sensitivity with strain for an optical sensing system. This study measures the spectral response of FBG to strain using a commercial FBG with a centre wavelength of 1550 nm. The parameters employed in these simulations include the effective refractive index (1.46), the grating period (?) for 530 nm in the FBG performance, the variations in refractive index (?n) from 0.0002 to 0.0020, and the fibre grating length (L) from 1 to 10 mm. The analysis of the refractive index and grating length variation yields the bandwidth and spectrum reflectivity. OriginPro Software and Microsoft Excel are used to perform simulations on the FBG. Data are generated using the Excel sheet, and visualisations are produced using OriginPro Software. The obtained results show that the bandwidth and spectral reflectivity are impacted by variations in the refractive index and grating length. Furthermore, the obtained results indicate that variations in the Bragg wavelength can be attributed to an elongation of the grating zone caused by the applied strain.

Introduction

I. INTRODUCTION

Fibre Bragg Grating (FBG) have shown a great potential advantage in biomedical application over the past ten years [1] due to their prominent characteristics, which include 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 object under investigation[2][3].Fibre Bragg Grating was used as a photoacoustic (PA)detection method to detect the existence of tumours because of its capacity to transform the absorbed energy entirely into heat without producing PA signals caused by scattering particles[4]. Because it combines light contrast and ultrasonic resolution, the photoacoustic approach is unique[5]. This technique is used in tumour diagnosis because of its benefits, which include noninvasiveness, high detection sensitivity, and the ability to identify small element sizes[6],[7].

A sensor can be made out of specifically constructed optical fibre. In a tiny portion of the fibre, the core refractive index of the optical fibre intended for sensor applications differs from that of the conventional fibre core and cladding [8]. Usually, a periodic structure is introduced in that tiny portion of the optical fibre core. Fibre Bragg Gratings (FBG) are the name given to this region of the fibre core because it reflects specific wavelengths of light. When a dielectric waveguide's properties are regularly altered, the effective refractive index of the guide is also periodically altered [9],[10]. Alternatively, when a DBR is a structure composed of multiple, alternating layers of materials with variable refractive indices. The Bragg wavelength shift of Fibre Bragg Gratings determines the sensitivity of FBG-based sensors. Encoded in the fibre core segment, the FBG is a periodic wavelength scale alteration of the refractive index. Bragg gratings reflect light at a particular wavelength that meets the Bragg condition. This reflection in a grating occurs when forward and back propagation modes couple at a specific wavelength [11]. The coupling coefficient of the modes is highest when the specific condition, such as the Bragg condition, between the vectors of the light waves and the vector number of the grating, is satisfied:

For a single FBG, there are theoretically an infinite number of Bragg wavelengths. The diffraction order Bragg wavelength changes for different values of m, as may be obtained from equation (1). In actuality, only one or occasionally two Bragg resonance wavelengths are used because there is a large spectral gap between the two. The second Bragg wavelength of the grating will be twice as short, at 750 nm, assuming the first one, m=1550 nm, is 1550 nm. However, the spectral range of the sources utilised for fibre is usually limited to 100 nm.

Additional Bragg peaks may show up if the refractive index modulation in FBG is not sinusoidal, as it usually is. For example, a rectangular grating's Fourier spectrum contains a large number of modulation frequencies, which can result in several Bragg peaks. Even though the index modulation of most fiber-based gratings is essentially sinusoidal. There are several FBG structures; however, in order to test how effectively an FBG works as a sensor, this study's experiment and analysis employed a uniform FBG.

A. The Fundamentals of FBG Sensing Principle.

The Fibre Bragg Grating (FBG) is a single mode fibre having periodic refractive index modulation along its core, as seen in figure 1. When a single mode optical fibre is subjected to intense UV radiation, the reflective index of the fibre core rises, creating a fixed index modulation known as a grating[9]. Since the period of the grating area is approximately 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 subjected to a particular wavelength is known as the Bragg's wavelength, or maximum reflectivity.

Conclusion

In conclusion, the refractive index variation and grating length modifications were used to get the bandwidth and reflectivity spectrum. Additionally, the FBG\'s effectiveness as a strain sensor was ascertained. According to the simulation results, reflectance rises with increasing grating length and refractive index. When the grating length is increased, the bandwidth of FBG drops, and when the refractive index change is increased, it increases. With OriginPro software, the apodization of the Gaussian profile technique can be used to suppress the sidelobes of spectral reflectivity.

References

[1] E. Al-Fakih, N. A. Abu Osman, and F. R. Mahamd Adikan, “The Use of Fiber Bragg Grating Sensors in Biomechanics and Rehabilitation Applications: The State-of-the-Art and Ongoing Research Topics,” Sensors, vol. 12, no. 12, pp. 12890–12926, 2012. [2] T. Fink, Q. Zhang, W. Ahrens, and M. Han, “Study of ?-phase-shifted, fiber Bragg gratings for ultrasonic detection,” Fiber Opt. Sensors Appl. IX, vol. 8370, no. 7, pp. 1–7, 2012. [3] G. Wild and S. Hinckley, “Optical Fibre Bragg Gratings for Acoustic Sensors,” Int. Congr. Acoust., no. August, pp. 1–7, 2010. [4] A. Yarai and T. Nakanishi, “Fiber Bragg grating applied pulsed photoacoustic detection technique for online monitoring concentration of liquid,” J. Acoust. Soc. Am., vol. 123, no. 5, p. 3285, 2008. [5] C. Li and L. V Wang, “Photoacoustic tomography and sensing in biomedicine,” Phys. Med. Biol., vol. 54, no. 19, pp. R59–R97, 2009. [6] M. Xu and L. V. Wang, “Photoacoustic imaging in biomedicine,” Rev. Sci. Instrum., vol. 77, no. 4, p. 041101, 2006. [7] J. Xia, J. Yao, and L. V Wang, “Photoacoustic Tomography?: Principles and Advances,” Prog. eletromagnic Res., vol. 147, no. May, pp. 1–22, 2014. [8] E.Udd, Fiber Optic Sensors: An Introduction for Engineerings and Scientists (John Wiley and Sons, New York, 1991). [9] A.Cusano, A. Cutolo and M. Giordano, “Fiber Bragg Gratings Evanescent Wave Sensors: A View Back and RecentAdvancements”, Sensors, Springer-Verlag Berlin Heidelberg, 2008. [10] K.O.Hill and G. Meltz, “Fiber Bragg Grating Technology Fundamentals and Overview”, Journal of Lightwave Technology, Vol. 15,No. 8, August 1997 [11] G.P. Agrawal, Fiber-Optic Communication Systems, (John Wiley & Sons, 2002). [12] P. Fomitchov and S. Krishnaswamy, “Response of a fiber Bragg grating ultrasonic sensor,” Opt. Eng., vol. 42, no. 4, pp. 956–963, 2003. [13] M. A. Othman, M. M. Ismail, H. A. Sulaiman, et.al., “An Analysis of 10 Gbits / s Optical Transmission System using Fiber Bragg Grating ( FBG ),” IOSR J. Eng., vol. 2, no. 7, pp. 55–61, 2012. [14] https://www.fiberoptics4sale.com/blogs/archive-posts/95046406-what-is-fiber-bragg-grating [15] https://tempsens.com/blog/fiber-bragg-grating-based-sensors

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.