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Research on evanescent field ammonia detection with gold-nanosphere coated microfibers
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Abstract
In this paper, based on TDLAS technology, an all-fiber NH3 concentration detection system was built by using the designed microfiber gas absorption cell. The core part of the NH3 detection system was sensed by a 1.51 μm microfiber. The test results of the system indicate that there is a good linear relationship between the second harmonic amplitude and the corresponding concentration for NH3 in the concentration range of 20000 ppm~100000 ppm (correlation coefficient of fitting formula R=0.9962). To improve the detection performance of NH3 concentration, the gold-nanosphere (GNS) coated microfiber is used to enhance the evanescent field effect. According to the experimental results, the sensitivity of the microfiber coated GNSs NH3 concentration detection system has been greatly improved and the lower detection limit of NH3 concentration can reach 260 ppm. Repeated monitoring of different concentrations of NH3 shows that the detection system is stable with a maximum relative error of 5.38%, which makes it suitable for long-term stable NH3 monitoring and has wide application prospects. -
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References
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Overview
Overview: Tunable diode laser absorption spectroscopy (TDLAS) is a highly sensitive laser absorption spectrum measurement technology, which determines the information of target gas by detecting the absorption intensity of the spectrum. With high selectivity, high sensitivity, rapid detection and high precision, it has been widely used in the real-time and online detection of atmospheric trace and polluted gases. However, most of gas absorption cells used in the existing detection system are based on the reflected spatial light structure, which requires the cooperation of the collimator with high precision. The structure of gas absorption cells is complex, expensive and bulky. On the other side, the optical fiber gas sensor based on fiber evanescent field is a neoteric kind of technology, and thus it takes light to measure the signal of the carrier and can adapt to various environments. Moreover, the evanescent field based optical fiber by coating various nanomaterials can improve the sensitivity of gas sensing, which has attracted wide attention.
In this paper, combined with TDLAS technology and gas sensing based on the evanescent field fiber, a set of all-fiber NH3 concentration detection system was built. The light source is a distributed feedback laser at around 1512 nm and the laser controller is modulated to select the unique absorption spectrum of NH3 to reduce the interference of carbon dioxide, vapor and other trace gases. A small gas absorption cell consists of a tapered microfiber with a 1.51 μm diameter. The microfiber was pulled into a cone by hydrogen and oxygen flame technology, and the second harmonic signal was finally extracted by a lock-in amplifier. The experimental results verified the feasibility of the evanescent field gas absorption cell, which is applied in the TDLAS system to detect ammonia gas. There was a good linear relationship between the amplitude of the second harmonic and the corresponding concentration in the range of 20000 ppm~100000 ppm of NH3. To improve the detection performance of NH3 concentration, gold-nanosphere (GNS) coated microfiber was used to enhance the effect of fiber evanescent fields. Compared with the measurement results of the evanescent field and GNSs coated microfiber, the detection sensitivity of the coated evanescent field significantly improved. Meanwhile, the TDLAS detection system based on GNSs coated microfibers as the evanescent-field based fiber gas absorption cell has good stability with the maximum relative error of 5.38%, and the detection limit of NH3 concentration can reach 260 ppm. The system has a wide application prospect in the field of ammonia detection.
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Figure 1.
Absorption peaks of CO2, H2O and NH3 nearly 1512 nm
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Figure 2.
The schematic diagram of the TDLAS ammonia detection system with a microfiber gas absorption cell
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Figure 3.
The characteristic of laser output. (a) The output spectrum of the laser when the driving current is adjusted from 30 mA to 70 mA, and the operating temperature is 25 ℃; (b) The output spectrum of laser with the change of working temperature, and the driving current is 50 mA; (c) The output wavelength corresponds to the curve at different temperature and current; (d) The output power and loading voltage of laser with different driving current
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Figure 4.
(a) The structure chart of a microfiber; (b) The relationship between drawing time, diameter of uniform region and transmission loss in the microfiber; (c) Diameter of the microfiber
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Figure 5.
The evanescent field radiation of the microfiber before (a) and after (b) GNSs deposition; (c) SEM image of the GNSs deposited on the surface of the microfiber. The insertion is a segment of the microfiber deposited by GNSs
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Figure 6.
(a) 2f signals at different NH3 concentrations; (b) The measured data and linear fitting curve of the 2f signal's amplitude versus NH3 concentration
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Figure 7.
(a) 2f signals at different NH3 concentrations in the microfiber gas absorption pool after GNSs coating; (b) The measured data and linear fitting curve of the 2f signal's amplitude versus NH3 concentration; (c) The absorption waveform of NH3 with concentration of 6000 ppm and its corresponding second harmonic wave (the small figure is the noise without absorption)
