EXPRESS: Development of dynamic time-over-threshold method for signal processing in cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) enables highly sensitive detection of trace gases but typically requires high-speed, high-resolution digitizers for exponential fitting of the measured "ring-down" decay signals. This work investigates a simplified signal processing approach based on the Time-over-Threshold (ToT) method, using only threshold crossing times instead of full waveform acquisition. In contrast to the ToT method with static thresholds, dynamic threshold signals are employed to improve linearity and robustness with respect to signal amplitude variations. The proposed dynamic ToT method is implemented using a compact analog circuit and evaluated using a mid-infrared CRDS setup for CO₂ measurement. Experimental results demonstrate good agreement with conventional fitting, with improved reliability compared to the ToT method with static thresholds and comparable linearity over a broad range of ring-down decay rates. These findings establish the dynamic ToT method as an accessible and scalable alternative for acquiring decay data in CRDS, particularly suited for simultaneous multi-channel or multi-wavelength measurements.