Fig. 1. Diagram of experimental setup

Fig. 2. Absorption spectra of NH₃ near 2239 nm

Fig. 3. Influence of pressure (a) and temperature (b) on absorption coefficient of NH₃

Fig. 4. Emission power spectrum of broadband light source

Fig. 5. Light source driver module. (a) The physical image; (b) current-driver mode; (c) voltage-driver mode

Fig. 6. Relationship between driver current (a), driver voltage (b) and optical power

Fig. 7. Frequency response curve of photoacoustic cell

Fig. 8. Relationship between NH₃ concentration and photoacoustic signal

Fig. 9. The assessment of system. (a) Continuous measurement for 5000 s; (b) Allan variance analysis of the system