Since the 1990s, optical fiber has been employed for strain sensing since it has been shown that the behavior of the spectra shift under strain is comparable to fiber Bragg gratings (FBGs). Dispersed light in the optical fibers is brought on by changes in the fiber's refractive index (RI). In backscatter reflectometry, wavelength shift is determined by employing cross-correlation of the spectra before and after strain changes and auto-correlation with the spectra before strain changes. Based on the sensors' responsiveness, this wavelength shift may subsequently be utilized to determine temperature or strain information.
Rayleigh Scattering-Based Technique
Rayleigh scattering is a commercial optical fiber's main source of light scattering. This Rayleigh scattering-based distributed fiber optic sensing has been used extensively in various fields, including civil engineering and aerospace engineering for structural health monitoring (SHM). Its benefits include immunity to electromagnetic interference, low weight, and long-distance sensing. Commercial communication optical fibers are often used as the sensing components in distributed fiber optic sensing based on Rayleigh scattering.
Communication Optical Fibers
Optical fibers in the communication wavelength bands are typically low-loss and low-dispersion fibers.
Communication optical fibers have low attenuation and can detect signals over great distances of several kilometers. They are also reasonably priced due to mass manufacture for the communication sector.
As a result, they are often used as sensors in distributed fiber optic sensing. However, the Rayleigh scattering-based technique has limitations since the backscattered light signal in the communication optical fiber is often relatively low.
Techniques to Enhance Backscattered Light Signal's Strength
Several techniques boost the backscattered light signal's strength in the optical fibers. For example, the dispersed signal's strength may be significantly increased by the ultraviolet (UV) light irradiation technique by around 20 dB, which may be related to the increased numerical aperture after exposure. Surgery applications have also made use of this technique.
Optical fiber cores may produce microstructures due to femtosecond lasers. The backscattered signal is significantly enhanced by the random microstructures, which also enhances the dispersed fiber optic sensing's signal-to-noise ratio. Both of these techniques rely on lasers' destruction of the optical fibers.
In optical fibers, nanoparticles have been frequently employed to pump the light signal. However, backscattered light in the optical fiber also results from the nanoparticles' refractive index difference from the fiber's core. Therefore, doping nanoparticles into the fiber's core is also a technique to enhance backscattered light.
How the Research was Conducted
This work investigates the nanoparticle (NP) doping of optical fibers using backscattering spectral characteristics of gold nanoparticle-doped optical fiber. First, the characteristic of the spectra shift under strain was theoretically examined, and then the examples of fiber Bragg gratings and commercial fibers were contrasted. Next, analysis was carried out on the nonlinear relationship between strain and the wavelength shift brought on by adding nanoparticles. The scattering spectra of gold nanoparticles with various sizes and volume ratios were examined in the wavenumber domain using a gold nanoparticle optical fiber as an example.
Using Mach-Zehnder Interferometer
A Mach-Zehnder interferometer was connected with light produced by a controllable laser. A photodetector was used to detect the interference between the light transmitted in the reference arm and the backscattered light from the nanoparticles in the sensing fiber. Local strain detection with millimeter precision was accomplished by combining the beat signal produced by the reference signal and the light backscattered at various locations along the optical fiber.
Gold nanoparticles of 400 nm, 300 nm, 200 nm, and 100 nm were selected as the usual big-size gold NPs employed for simulations. The volume ratios ranged from around 10-10 to approximately 10-2, with various gauge lengths. The backscattered light spectrum alters as the optical fiber's refractive index and axial strain are altered. Therefore, the values for strain were determined by demodulating the spectral shift.
Significant Findings of the Study
Compared to fiber Bragg gratings or dispersed strain sensing based on Rayleigh scattering, nanoparticles doped optical fibers exhibit a similar spectrum shift behavior under strain. The spectra properties, including the spectra' fluctuation and the strength of the backscatter, were analyzed. The period number of the spectra is significantly influenced by the imaginary portion of the effective refractive index. A link between gauge length and the period number was also evident. However, there was no correlation between the size of the nanoparticles and the range of gold nanoparticles from 100 to 400 nm.
The single scattering volume ratio limits the backscattered light. However, the absorbing quality of the material limits the tendency of the intensity to rise, even if raising the volume ratio further may increase the backscattered light intensity. These theoretical findings could encourage the development of nanoparticle-doped fiber sensors in the future.
Xiang Wang, Rinze Benedictus & Roger M. Groves (2022) Spectral characteristics of gold nanoparticle doped optical fibre under axial strain. Scientific Reports. https://www.nature.com/articles/s41598-022-20726-2