Fig. 1. Quadrature phase lock-in principle block diagram

Fig. 2. Structure of the FPGA-based WM-TDLAS CO₂ concentration detection system

Fig. 3. Block diagram of FPGA internal logic structure

Fig. 4. Detection system linearity experiment. (a) Amplitude extraction result of sinusoidal signal； (b) Linear fitting of extracted amplitude with actual value

Fig. 5. Anti-noise and Q-value experiment. (a) Harmonic extraction results with different signal-to-noise ratio； (b) Frequency amplitude response of DLIA

Fig. 6. Experimental platform for CO₂ concentration detection

Fig. 7. Experimental results of carbon dioxide concentration detection. (a) Second harmonic waveforms; (b) Carbon dioxide concentration inversion curve

Fig. 8. Response time test of carbon dioxide detection system

Fig. 9. System stability experiment. (a) Carbon dioxide concentration fluctuates; (b) Allan variance