Fiber-Optic Photoacoustic Dual-Gas Analyzer Based on a Symmetric Four-Cone Converging Cell with Dual-Frequency Resonance

Abstract A symmetric four-cone converging photoacoustic (PA) cell with dual-frequency resonance is proposed for the simultaneous measurement of two gases. The resonance tubes are designed as conical frustums rather than conventional cylinders reducing cell volume. Finite element optimization equalizes the amplitudes of the two resonance peaks at 2850 and 4220 Hz. A high-sensitivity cantilever transducer is used to transmit the PA signals to a fiber-optic demodulator. The operating frequencies of the two lasers are each locked to half of their respective resonant frequencies. A dual-channel digital lock-in amplifier is used to measure the PA signals of acetylene and methane in two distinct frequency channels. Due to the special tube geometry, conventional circular spots based on matrix transmission theory are inapplicable. A linear light spot matching the tube shape is designed using a 4Lf imaging system and spherical reflection, achieving 25 reflections and a 9.93 m optical path length. Compared with double-pass absorption, the signal is amplified by 19.69 times. Crosstalk analysis showed signal crosstalk of −104.9 dB and frequency crosstalk of −110.1 dB. For trace gas analysis, two distributed feedback (DFB) lasers with operating wavelengths of 1532.8 nm for C2H2 at 50.2 mW and 1653.7 nm for CH4 at 51.3 mW are employed. The laser models are DFB-1532.8 and DFB-1653.7 from Xiao Xiao Photonics, respectively. At 1 s integration time, the 1σ minimum detection limits (MDLs) for the two gases reach 3.94 and 20.3 ppb, respectively. Meanwhile, the noise-equivalent absorption (NNEA) coefficients are 2.26 × 10–10 W·cm–1·Hz–1/2 and 6.47 × 10–10 W·cm–1·Hz–1/2, respectively.