An optical fiber solution-concentration sensor based on whispering gallery mode (WGM) is proposed in this paper. The WGM solution-concentration sensors were used to measure salt solutions, in which the concentrations ranged from 1% to 25% and the wavelength drifted from the left to the right. The experimental results showed an average sensitivity of approximately 0.372 nm/% and an
The whispering gallery mode (WGM), also known as whispering gallery wave, was first discovered and proposed in 1912 by the British physicist John William Strutt (Raleigh) [
In 1990, Morgan et al. [
In 2002, the U-shaped fiber-optic pH of sensors based on evanescent wave absorption was reported by Gupta and Sharma [
In 2009, Wang et al. [
In 2009, a WGM refractive index sensor was reported [
In 2010, a similar bending interferometric fiber-optic sensor was proposed [
This study describes a macrobending-induced WGM fiber-optic sensor. Based on the analysis of the WGM spectrum, wavelength variations in optical fibers with differing bend radii were observed. The sensitivity and linearity of the sensors were also analyzed and calculated.
The diameter of the optical fiber plays an important role in macrobending. As the fiber’s diameter decreases, its flexibility increases. Therefore, the life of the bending fiber also increases. The optical fibers in this study were wet-etched using buffered oxide etch (BOE) to alter their diameters. A stripper was used to remove 3 cm of the external protective layer of the optical fiber. Fifty stripped optical fibers were adhered to a plastic holder. The holder with the fibers was placed inside a plastic box filled with BOE for etching. Fiber diameters were altered using various etching durations.
This study successfully developed a WGM sensor which is of low cost, can be mass-produced, and has the ability to accurately control the bending radius of the optical fiber. Additionally, this process increases the accuracy of measuring differing concentration/refractive indices of liquids.
Figure
The manufacturing process of the WGM concentration sensor.
Experimental setup of the WGM solution-concentration sensing system.
The objective of this study is to macrobend the etched optical fiber to form a WGM character for sensing. The spectra of WGM concentration sensors were affected as the surrounding refractive index changed with different concentrations. Figure
The spectra of the macrobending fiber with different bending radii.
The relationship between the WGM wavelength and the bend radius of optical fiber.
In the solution-concentration measurement experiment, the WGM spectra were observed to be closely related to the concentration of the solution. This phenomenon shows that the difference between the refractive indices of two solution mediums affected the results of the WGM spectra. Figure
Interference spectra drifts with the concentration of salt solution.
Figure
Relationship between the concentration of salt solution and wavelength.
The spectral analysis indicated that higher concentrations of salt solutions caused the resonant wavelength to drift toward the long wavelength side (red shift). On the contrary, when the media had a low refractive index, the interference spectra changed accordingly, and the resonant wavelength drifted toward the short wavelength side (blue shift). In short, the experiments proved the WGM concentration sensors to be extremely stable and highly reproducible.
This paper demonstrated the manufacturing process of the WGM concentration sensors. The results show the influences of refractive indices on the spectral characteristics of the fiber-optic WGM sensor. In addition, WGM solution-concentration sensors were used to monitor the concentration of the salt solutions. The resonant dip spectrum (in air) shows a significant interference loss, and the interference curves of the solutions with a concentration of 1–25% drifted toward the right gradually. The results of salt-solution concentration monitoring showed an average sensitivity of 0.372 nm/% and an
This work is supported by the National Science Council, Taiwan (Grant no. NSC 100-2628-E-151-002-MY3).