Abstract
A trace ammonia (NH3) detection system based on the near-infrared fiber-optic cantilever-enhanced photoacoustic spectroscopy (CEPAS) is proposed. A fiber-optic extrinsic Fabry-Perot interferometer (EFPI) based cantilever microphone has been designed to detect the photoacoustic pressure signal. The microphone has many advantages, such as small size and high sensitivity. A near-infrared tunable erbium-doped fiber laser (EDFL) amplified by an erbium-doped fiber amplifier (EDFA) is used as a photoacoustic excitation light source. To improve the sensitivity, the photoacoustic signal is enhanced by a photoacoustic cell with a resonant frequency of 1624 Hz. When the wavelength modulation spectroscopy (WMS) technique is applied, the weak photoacoustic signal is detected by the second-harmonic detection technique. Trace NH3 measurement experiments demonstrate that the designed fiber-optic CEPAS system has a linear response to concentrations in the range of 0 ppm – 20 ppm at the wavelength of 1522.448 nm. Moreover, the detection limit is estimated to be 3.2 ppb for a lock-in integration time of 30 s.
Highlights
Trace ammonia (NH3) detection is important for environment monitoring and medical diagnosis [1,2,3]
We present a fiber-optic cantilever-enhanced photoacoustic spectroscopy (CEPAS) for trace NH3 gas detection, which combines a fiber-optic extrinsic Fabry-Perot interferometer (EFPI) based cantilever microphone and a first-order longitudinal resonant PA cell
A near-infrared erbium-doped fiber laser (EDFL) combined with an erbium-doped fiber amplifier (EDFA) is applied as the PA excitation light
Summary
Trace ammonia (NH3) detection is important for environment monitoring and medical diagnosis [1,2,3]. Among the proposed trace gas detection methods, laser photoacoustic (PA) spectroscopy (PAS) shows many technical advantages, such as small sample volume, high detection sensitivity, and continuous measurement ability [4,5,6]. When the modulated excitation laser is tuned to one of the vibrational-rotational transitions of targeted gas molecules, a portion of the gas molecules move up to the excited state. The PA pressure wave can be detected by an acoustic sensor, such as an electronic microphone. Since the concentration of the target gas is proportional to the amplitude of the PA pressure wave, the target gas can be quantitatively detected [7]. Webber et al reported a PAS based NH3 gas detection system by interrogating a transition near 1532 nm with a 500 mW power from an erbium-doped fiber amplifier (EDFA) [8]. A detection limit of 9 ppb was achieved with an Article type: Regular
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