Abstract
An optical fibre ammonia gas sensor utilising a functionalised four-leaf-clover-shaped suspended-core fibre (SCF) is demonstrated. The fibre is functionalised by depositing a thin layer of an ammonia sensitive dye (tetraphenylporphyrin tetrasulfonic acid hydrate) (TPPS) on the wall of the inner cavities of the SCF through capillary action. An in-line fibre sensing structure is designed for light transmission through the SCF and also allows gas exchange with the atmosphere through two porous polyethylene housings. The sensing structure exhibits a temperature-dependent transmission due to the thermal expansion of the housings. A ratiometric method is applied for signal processing which is demonstrated to reduce significantly the effect of temperature change in the tested range (20-30 °C). The sensor is tested in ammonia concentration ranges from 0-10 ppm and demonstrates capability of detecting ammonia at ppb levels. The minimum tested concentration is 150 ppb in the experiment with a calculated limit of detection of 20 ppb. The sensor response time (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10-90%</sub> ) is 160 s and it is demonstrated to be reusable after treatment with hydrogen chloride vapour. Numerical simulation of evanescent absorption features of such an SCF indicates that approximately 0.02% of the optical power exists in the air holes for the fundamental propagation mode. Distinctive absorption bands observed in the transmission spectrum of the fabricated sensor can also be observed in the simulated model after adding the TPPS absorption layer in the holes.
Highlights
G AS sensing with optical fibre sensors is becoming a popular approach in many fields of application such as environmental monitoring [1], industry [2] and healthcare [3]–[5] due to their comparable sensing performance to traditional electrochemical and semiconductor gas sensors, and distinctive advantages such as compact size, remote sensing, multiplexing and immunity to electromagnetic interference
The transmission spectrum calculated from Eq(3) exhibits two attenuation bands for the TPPS functionalised suspended-core fibre (SCF) sensor of different lengths whereas the bare SCF shows no loss in its transmission value (Fig. 4f)
A new optical fibre ammonia sensor has been demonstrated by depositing a functional layer (TPPS) inside the holes of an SCF for spectroscopic analysis
Summary
G AS sensing with optical fibre sensors is becoming a popular approach in many fields of application such as environmental monitoring [1], industry [2] and healthcare [3]–[5] due to their comparable sensing performance to traditional electrochemical and semiconductor gas sensors, and distinctive advantages such as compact size, remote sensing, multiplexing and immunity to electromagnetic interference. Manuscript received April 19, 2021; revised May 17, 2021; accepted May 20, 2021. Date of publication May 24, 2021; date of current version August 2, 2021. The manufacture of the suspended-core fibre is supported in part by the National Science Foundation of China under Grant 61775014.
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