Software-defined lock-in detection for photoacoustic gas sensing

Abstract

Photoacoustic spectroscopy enables compact and selective gas sensing, but its weak acoustic signals require effective phase-sensitive detection. Conventional PAS systems commonly rely on standalone lock-in amplifiers, which separate the demodulation function from excitation generation, sensor control, and data management, limiting system integration and reconfigurability. This work presents a real-time software-defined digital lock-in amplifier that unifies modulation-waveform generation, phase-coherent signal acquisition, quadrature demodulation, configurable filtering, real-time visualization, and data logging within a single LabVIEW application interfaced with a National Instruments USB-6361 data-acquisition module. The S-DLIA was integrated into a CO 2 PAS platform employing a distributed-feedback diode laser at 4992.5157 cm -1 , a differential resonant photoacoustic cell, and second-harmonic wavelength modulation. It was benchmarked against a commercial SR830 lock-in amplifier processing the same photoacoustic signal under matched conditions. The sensor responded linearly to CO 2 concentrations from 100 to 1,000 ppm ( R 2 = 0.999). Allan deviation analysis at an integration time of 220 s yielded minimum detection limits of 0.505 ppm for the S-DLIA and 8.909 ppm for the SR830 under the tested configuration, corresponding to a 17.6-fold lower detection limit compared with the SR830. Continuous operation over 24 h at a fixed laboratory location tracked indoor CO 2 variations without apparent drift. These results validate the S-DLIA as a reconfigurable and flexible alternative to dedicated lock-in instruments in PAS, demonstrating the potential of software-defined synchronous detection for integrated gas-sensing systems.