Ultrahigh Sensitive Trace Gas Sensing System with Dual Fiber-Optic Cantilever Multiplexing-Based Differential Photoacoustic Detection

Abstract

An ultrahigh sensitive trace gas sensing system was presented with dual cantilever-based differential photoacoustic detection. By combining the double enhancement of multipass absorption and optical differential detection, the gas detection sensitivity was significantly improved. The dualchannel synchronous photoacoustic detection was realized by fiber-optic Fabry−Perot interference spectrum multiplexing. The photoacoustic signals detected by two fiber-optic cantilever microphones installed in a differential photoacoustic cell (DPAC) were out of phase, while the detected gas flow noises were in phase. The optical differential detection method achieved both highly sensitive optical interference measurement and differential noise suppression. In the multipass configuration, the interaction path between excitation light and target gas achieved 4.1 m, which improved the photoacoustic signal by an order of magnitude compared with a single reflection. The maximum gas flow allowed by the system based on the DPAC was 250 sccm, which realized the dynamic monitoring of H 2 S in the SF 6 background. The detection limit for H 2 S in SF 6 background was 5.1 ppb, which corresponds to the normalized noise equivalent absorption coefficient of 9 × 10 −10 cm −1 W Hz −1/2 .

Differential Cantilever Enhanced Fiber-Optic Photoacoustic Sensor for Diffusion Gas Detection

“…The length changes of the two F–P cavities are simultaneously obtained using a spectral demodulation algorithm based on WLI. For two low fineness extrinsic fiber-optic F–P cavities, the interference light ( I I ( k )) received by a spectrometer can be written as false[ I I ( k ) = 2 I 1 ( k ) [ 1 + υ 1 cos nobreak0em0.25emfalse⁡ false( 4 π ( d 10 + Δ d 1 ) k + π ] + 2 I 2 ( k ) [ 1 + υ 2 cos nobreak0em0.25em⁡ false( 4 π ( d 20 + Δ d 2 ) k + π ] where I 1 and I 2 are the light intensity incident on two F–P cavities, which are a function of wavenumber ( k ). d 10 and d 20 are the static cavity lengths of the two F–P cavities, respectively.…”

“…By performing wavenumber interpolation at equal intervals and fast Fourier transform (FFT), the frequency information on the superimposed interference spectrum can be obtained. The relationship between F–P cavities and frequency and phase information on the spectrum can be expressed as d i = π n i k 1 − k 0 φ ( n i ) = 2 d i k 0 + φ 0 where d i is the F–P cavity length of the sensor i . n i is the peak location corresponding to the i -th cavity length in the amplitude spectrum.…”

High-Precision Multipass Fiber-Optic Photoacoustic Gas Analyzer Based on 2f/1f Wavelength Modulation Spectroscopy

“…At present, wavelength modulation spectroscopy (WMS) is widely used in PA gas sensors because it can improve detection sensitivity. , The second harmonic signal is dependent on the gas concentration, modulation coefficient, and laser power . During the continuous use of the laser, the output power will gradually decay, resulting in weakening of the excited PA signal.…”

Rapid Photoacoustic Exhaust Gas Analyzer for Simultaneous Measurement of Nitrogen Dioxide and Sulfur Dioxide

“…Photoacoustic (PA) spectroscopy (PAS) is one of the most promising trace gas detection methods. − Compared with the normal detection methods, PAS has the advantage of fast response, low interference, high sensitivity, and portability. − At present, many PAS systems have been proposed for the detection of SO 2 and NO 2 . The usual exciting sources of PA signal are quantum cascade laser (QCL), − distributed feedback (DFB) laser, − light-emitting diode (LED), , laser diode (LD) − and infrared thermal radiation source. − For SO 2 detection, QCL can operate at the fundamental absorption bands of the SO 2 molecule. Because of the strong absorption of the molecules and the high power of QCL, a brilliant minimum detection limit can be achieved .…”