Programmable FPGA-based TDLAS-WMS hygrometer (PFTWH) for broad-range water vapour detection: Quantitative assessment of digital lock-in amplifier (DLIA) performance via integrated WMS system (I-WMS)

This study demonstrates the capability of wavelength modulation spectroscopy (WMS) based on tunable diode laser absorption spectroscopy (TDLAS) using a hand-held field-programmable gate array (FPGA) system. Programmable FPGA-based TDLAS-WMS hygrometer (PFTWH) enables measurement of water vapour concentrations over a broad range (70–5800 ppm) for applications in climate science. This system incorporates a custom-designed digital lock-in amplifier (DLIA), optimized for water vapour detection. For the DLIA, a 100 Hz cut-off frequency and a finite impulse response filter with a windowed-Hamming design and a 200-tap coefficient were used. The PFTWH demonstrated simultaneous processing capability of 1 f and 2 f signals by programming the DLIA on a commercial FPGA board. To quantitatively evaluate DLIA performance, we introduced an integrated WMS system that enables parallel operation of a commercial FPGA-based TDLAS-WMS hygrometer (CFTWH), the PFTWH, and direct TDLAS (dTDLAS). The 1 f and 2 f signals are extracted from both the CFTWH and PFTWH, while the dTDLAS provides reference concentration information for performance assessment. The linearity and accuracy of the designed PFTWH were validated by comparison with the CFTWH. Additionally, long-term measurements of water vapour concentrations across the broad range (70–5800 ppm) were successfully performed with a time resolution of 0.1 s, maintaining stable performance over 1000 s with 30 s averaging. Furthermore, both PFTWH and CFTWH were demonstrated to meet uncertainty, accuracy, and precision requirements for airborne measurement.