Wavelength modulation technique-based photoacoustic spectroscopy for multipoint gas sensing

Ppb-level methane detection sensitivity based on a homemade Raman fiber amplifier and differential photoacoustic technology

A high-power near-infrared wavelength-modulated differential photoacoustic spectroscopy sensor for parts-per-billion (ppb) level methane detection is reported by using a homemade Raman fiber optical amplifier. A commercial 1653.7 nm continuous wave distributed feedback laser is employed as a seed source to excite a high light power of ∼550m W, which greatly improves sensor performance. Wavelength modulation spectroscopy and differential techniques are applied to further improve the signal-to-noise ratio of the photoacoustic signal. A 1σ minimum detection limit of ∼10p p b for methane detection is achieved with an integration time of 10 s.

Compact photoacoustic spectrophone for simultaneously monitoring the concentrations of dichloromethane and trichloromethane with a single acoustic resonator

Chlorinated hydrocarbons are frequently used as reagents and organic solvents in different industrial processes. Real-time detection of chlorinated hydrocarbons, as toxic air pollutants and carcinogenic species, is an important requirement for various environmental and industrial applications. In this study, a compact photoacoustic (PA) spectrophone based on a single acoustic resonator for simultaneous detection of trichloromethane (CHCl₃) and dichloromethane (CH₂Cl₂) is first reported by employing a low-cost distributed feedback (DFB) laser emitting at 1684 nm. In consideration of the significant overlapping of absorption spectral from trichloromethane and dichloromethane, the multi-linear regression method was used to calculate the concentrations of CHCl₃ and CH₂Cl₂ with special characterization of the absorption profile. The current modulation amplitude and detection phase in the developed PA spectrophone was optimized for high sensitivity of individual components. The measurement interference of CHCl₃ and CH₂Cl₂ on each other was investigated for accurate detection, respectively. For field measurements, all optical elements were integrated into a 40 cm × 40 cm × 20 cm chassis. This paper provides an experimental verification which strongly recommends this sensor as a compact photoacoustic field sensor system for chlorinated hydrocarbon detection in different applications.

Near-infrared laser photoacoustic gas sensor for simultaneous detection of CO and H2S

A ppb-level H₂S and CO photoacoustic spectroscopy (PAS) gas sensor was developed by using a two-stage commercial optical fiber amplifier with a full output power of 10 W. Two near-infrared diode lasers with the central wavenumbers of 6320.6 cm^-1 and 6377.4 cm^-1 were employed as the excitation laser source. A time-division multiplexing method was used to simultaneously detect CO and H₂S with an optical switch. A dual-resonator structural photoacoustic cell (PAC) was theoretically simulated and designed with a finite element analysis. A µV level background noise was achieved with the differential and symmetrical PAC. The performance of the multi-component sensor was evaluated after the optimization of frequency, pressure and modulation depth. The minimum detection limits of 31.7 ppb and 342.7 ppb were obtained for H₂S and CO at atmospheric pressure.

Multicomponent gas detection technology of FDM and TDM based on photoacoustic spectroscopy

In this paper, a multicomponent gas detection system based on photoacoustic spectroscopy (PAS) is proposed with a combination of frequency division multiplexing (FDM) and time division multiplexing (TDM), combining a resonance photoacoustic cell and broadband microphone. A PAS gas cell with a wide frequency response bandwidth was used to achieve the FDM by selecting a specific modulation frequency of each component gas. The sawtooth wave driver current of each laser was output at a constant time interval for achieving the TDM. Compared with the laser channel control using a photoswitch, the driver current control was a simpler and more convenient means to implement TDM. The four gas components of methane (CH₄), water (H₂O) vapor, carbon dioxide (CO₂), and acetylene (C₂H₂) were selected as sample gases for testing the feasibility of the method. The experimental results showed that the gas detection limits of CH₄, H₂O vapor, CO₂, and C₂H₂ were 75.435, 2.502, 341.960, and 4.284 ppm, respectively. In addition, the linear fittings of gas concentration were 0.99386, 0.99772, 0.98995, and 0.98955, respectively.