Abstract
Ammonia is an important indicator among environmental monitoring parameters. In this work, thin-core fiber Mach-Zehnder interferometer deposited with poly (acrylic acid) (PAA), poly (allyamine hydrochloride) (PAH) and single-walled carbon nanotubes (SWCNTs-COOH) sensing film for the detection of ammonia gas has been presented. The thin-core fiber modal interferometer was made by fusion splicing a small section of thin-core fiber (TCF) between two standard single mode fibers (SMF). A beam propagation method (BPM) is employed for the design of proposed interferometer and numerical simulation. Based on the simulation results, interferometer with a length of 2 cm of thin-core fiber is fabricated and experimentally studied. (PAH/PAA)2 + [PAH/(PAA + SWCNTs-COOH)]8 film is deposited on the outer surface of thin-core fiber via layer-by-layer (LbL) self-assembly technique. The gas sensor coated with (PAH/PAA)2 + [PAH/(PAA + SWCNTs-COOH)]8 film towards NH3 gas exposure at concentrations range from 1 to 960 ppm are analyzed and the sensing capability is demonstrated by optical spectrum analyzer (OSA). Experimental results show that the characteristic wavelength shift has an approximately linear relationship in the range 1–20 ppm, which is in accordance with the numerical simulation. Thus, this paper reveals the potential application of this sensor in monitoring low concentration NH3 gas.
Highlights
Among various kinds of optical fiber sensors, those based on the interference theory account for a large proportion
When the light transmitted from the single mode fibers (SMF) into the thin-core fiber (TCF) will excite cladding modes propagating within the cladding of the interferometer due to the core diameter of SMF is larger than TCF
Interference occurring within the cladding of the TCFMI will indicate the spectral response to the output SMF, which in turn can be affected by the RI of the external environment
Summary
Among various kinds of optical fiber sensors, those based on the interference theory account for a large proportion. Mach-Zehnder interferometer is based on double beam interference structure with advantages of high precision, low cost, and easy to adjust, which is widely used in the distributed sensing[16], and applied to point measurement sensor[17,18]. In 1979, optical fiber pressure and temperature sensor based on the Mach-Zehnder interference was first proposed by G. More and more interferometer fabricated with thin-core fiber and multimode fiber is applied to measure the pH value[21,22], relative humidity[23,24], temperature[25,26], pressure[27], refractive index[28,29] and so on.
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