Fig. 1. Calculated absorption lines of CO₂ and H₂O at 296 K and 1.01×10⁵ N/m² from HITRAN2016

Fig. 2. Photo-thermal gas detection system. (a) Experimental setup; (b) SEM image of HC-ARF; (c) F-P gas cell

Fig. 3. Reflection spectra of the fabricated F-P cavities with low quality factors

Fig. 4. Effect of modulation depth and modulation frequency on the amplitude of the signal. (a) Relationship between the normalized amplitude and modulation depth with 10 kHz modulation frenquency; (b) frequency response of the normalized amplitude and noise with 0.14 cm^-1 modulation depth

Fig. 5. Influence of AR coating on the rear end face. (a) 2f spectra at V(CO₂)=10^-4; (b) noise with pure N₂

Fig. 6. Detection performance of CO₂ concentration. (a) 2f spectrum at V(CO₂)=10^-5; (b) 2f signal amplitude at different volume fractions

Fig. 7. Measuremental results of the CO₂ standard gas within 1 h at V(CO₂)=10^-5. (a) 2f spectrum; (b) peak-to-peak value

Fig. 8. Influence of pump power on the 2f signal amplitude/noise. (a) 2f signal spectra at V(CO₂)=10^-4; (b) relationship between the 2f signal amplitude/noise and the pump power

Fig. 9. Influence of intensity noise and phase noise. (a) Relationship between the noise and received power in 15-cm-long gas cell; (b) relationship between the noise and F-P cavity length with received power of 620 μW