Open-Hardware Analog Lock-In amplifier for wavelength modulation spectroscopy gas sensing

• Fully analog lock-in amplifier designed specifically for wavelength modulation spectroscopy. • Achieved 5.1 ppb methane detection limit with 15-second integration time. • Best noise-equivalent absorption coefficient among 15 compared literature systems. • Demonstrated stable 10-day continuous operation in ambient conditions. • Open-hardware design enables cost-effective deployment for gas sensing applications. This paper presents the design, optimization, and characterization of a cost-effective electronic circuit for sensitive and selective gas detection using wavelength modulation spectroscopy (WMS). The system was specifically developed for remote monitoring of methane emissions. Our custom lock-in amplifier (LIA) architecture was evaluated against commercial instrumentation, demonstrating comparable performance at significantly reduced cost. The detection system achieved a methane detection limit of 5.1 ppb at 15 seconds integration time, with excellent linearity (R 2 = 0.99987) across a wide concentration range. Long-term testing confirmed stable operation during continuous ambient methane monitoring, showing potential for future implementation in commercial apparatus.