Abstract
We demonstrate a new refractive index (RI) and salinity sensor based on a lossy mode resonance (LMR) effect which combines fiber-optic side-polishing and radio-frequency (RF) sputtering techniques. The side-polished fiber can enhance optical fibers to generate an evanescent field in sensing applications. Gallium-doped zinc oxide (GZO) thin films produce a high attenuation lossy mode resonance effect that permits a highly sensitive refractive index and salinity fiber sensor. GZO thin film was prepared by an RF magnetron sputtering method. The thickness of the D-shaped fiber sensing device was 74.7 μm, and a GZO film thickness of 67 nm was deposited on the polished surface of the D-shaped fiber to fabricate LMR type liquid salinity sensors. The sensitivity of 3637.8 nm/RIU was achieved in the RI range of 1.333 to 1.392. To investigate the sensitivities of LMR salinity sensors, the NaCl solution salinities of 0%, 50%, 100%, 150%, 200%, and 250% were measured in this work. The experimental result shows that the sensitivity of the salinity sensor is 0.964 nm per salinity unit (SU).
Highlights
Detection of the liquid refractive index (RI) is an important issue in chemical, biological, and environmental engineering fields
We investigated Gallium-doped zinc oxide (GZO) thin films deposited on the D-shaped fibers with different thicknesses to compare the sensitivity of lossy mode resonance (LMR) sensors
Gallium-doped zinc oxide (GZO) thin films were prepared by radio-frequency (RF) magnetron sputtering technique
Summary
Detection of the liquid refractive index (RI) is an important issue in chemical, biological, and environmental engineering fields. There are many approaches used to measure the liquid refractive index, such as fiber Bragg grating [3], surface plasmon resonance (SPR) [4], interferometric technology [5], etc. The sensitivity and dynamic detection range of the refractive index measuring methods are limited [6]. We propose a cost-effective approach for sensing changes in refractive indices by using lossy mode resonance (LMR). Salinity is a key parameter to determine the density of seawater in determining many aspects of the chemistry of waters and soils. The density and absolute salinity can be evaluated by a direct measurement of the refractive index, but the absolute salinity of seawater is difficult to measure directly.
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