Abstract
A graphene oxide-coated in-fiber Mach-Zehnder interferometer (MZI) formed with a multimode fiber-thin core fiber-multimode fiber (MMF-TCF-MMF) is proposed and experimentally demonstrated for ammonia gas (NH3) sensing. The MZI structure is composed of two segments of MMF of length 2 mm, with a flame-tapered TCF between them as the sensing arm. The MMFs act as mode couplers to split and recombine light owing to the core diameter mismatch with the other fibers. A tapered TCF is formed by the flame melting taper method, resulting in evanescent wave leakage. A layer of graphene oxide (GO) is applied to the tapered region of the TCF to achieve gas adsorption. The sensor operates on the principle of changing the effective refractive index of the cladding mode of a fiber through changing the conductivity of the GO coating by adsorbed NH3 molecules, which gives rise to a phase shift and shows as the resonant dip shifts in the transmission spectrum. So the concentration of the ammonia gas can be obtained by measuring the dip shift. A wavelength-shift sensitivity of 4.97 pm/ppm with a linear fit coefficient of 98.9% is achieved for ammonia gas concentrations in the range of 0 to 151 ppm. In addition, we performed a repetitive dynamic response test on the sensor by charging/releasing NH3 at concentration of 200 ppm and a relative humidity test in a relative humidity range of 35% to 70%, which demonstrates the reusability and stability of the sensor.
Highlights
The absorption-related properties of graphene and graphene oxide on ammonia gas molecules have been studied [37,38], in which the results indicate that the GO showed highly distinguished selectivity towards NH3 in comparison with H2 and CH4 gases
The sensor was placed in a sealed gas chamber
This paper proposes a GO-coated Mach-Zehnder fiber interferometer for measurement of ammonia gas concentration
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. In view of the high sensitivity and response speed of evanescent field caused by microfiber to environmental changes, more and more researchers are paying attention to the microfiber sensor for ammonia gas sensing [20,21,22,23]. The sensing mechanism of graphene materials is mainly due to its two-dimensional structure at the micro-nano level and its large surface area, which is very beneficial to the atomic or molecular level In full contact, these contacts are essentially charge exchanges between molecules or atoms, which will change the local carrier concentration in the graphene material. Based on the Mach-Zehnder interferometer structure, which mainly forms stable and clear interference, we introduce a tapered region to enhance the evanescent field, which is used to improve the perception of the external environment changes in the sensing arm region.
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