Open Type Solid Photoacoustic Trace Gas Sensor with Multi-Pass Absorption Enhancement

A wavelength modulation solid photoacoustic spectroscopic (WM-SPAS) sensor enhanced with an open-type multi-pass cell (OMPC) is reported for highly sensitive detection of trace gases, especially suitable for highly corrosive and long optical-to-thermal (non-radiative) relaxation gaseous species. Such open configuration is quite different from traditional trace gas detection methods in that the separation design of the acoustic signal detector and gas absorption cavity avoids the adversely corrosive effect and reduces signal fluctuations caused by high flow rates. The modulated beam after the optical absorption by the target gas in the designed open-type multi-pass path is directed into a self-designed solid chamber, filled with carbon powder while the photoacoustic (PA) pressure signal is analyzed to yield the target gas concentration. By optimizing the incident beam angle, the OMPC achieves 96 reflections, yielding a 9.6 m optical path length enhancement. Using acetylene (C₂H₂) as a test sample and a DFB laser as the excitation source, this WM-SPAS sensor achieves sensitivity of 80 ppb and corresponding normalized noise equivalent absorption coefficient equal to 2.42 × 10^-9 cm^-1 W/Hz^-1/2 with 1 s time constant and modulation frequency as low as 39 Hz, which enables the sensor to detect gases with slow non-radiative relaxation. An Allan deviation analysis indicated the minimum detection limit could be further improved to 7 ppb at 100 s integration time. The response deviation of the PA signal under different flow rates was characterized by a coefficient of variation of 0.71‰. With its separate structure design, this newly developed PAS trace gas sensor offers unique advantages for open trace gas detection in high-flow and corrosive environments.