Fig. 1. Absorption spectrum of H₂CO (1 mL·L^-1) around 3.6 μm

Fig. 2. Tuning characteristics of the ICL

Fig. 3. Structure of the cavity enhanced H₂CO detection system
EOM: Electro-optic phase modulator; PBS: Polarizing beam splitter; PZT: Piezoelectric transducer; PD: Photodiode detector

Fig. 4. The error signal and the cavity transmitted signal

Fig. 5. The cavity transmission signal with PDH dynamic locking

Fig. 6. (a) The measured signal of H₂CO (10 mL·L^-1); (b) Absorption spectrum of H₂CO (1 mL·L^-1)

Fig. 7. Relation between the amplitude of the cavity transmission signal and the concentration

Fig. 8. The Allan variance analysis