Fig. 1. Schematic of the principle of PAS gas sensing

Fig. 2. (a) Typical schematic of EDFA; (b) basic schematic of power-amplified PAS gas sensor

Fig. 3. Schematic of amultipass photoacoustic resonator[27]

Fig. 4. Several typical external optical build-up photoacoustic cells. (a) Feedback-controlled linear resonator, locking the cavity mode to the central wavelength of laser; (b) feedback-controlled bow-tie resonator, locking the cavity mode to the central wavelength of the laser; (c) optical-feedback V-shaped build-up cavity, locking the laser to the cavity mode; (d) optical-feedback linear build-up cavity, locking the laser wavelength to the cavity mode

Fig. 5. Schematics of several typical laser-intracavity PAS sensors. (a) Based on 3.39 μm HeNe gas laser; (b) based on 5.5 μm tunable external cavity diode laser; (c) based on 1.5 μm tunable fiber ring laser

Fig. 6. Absorption spectra of different gas molecules in C band

Fig. 7. (a) Wavelength modulator based on FBG[55]; (b) intensity modulator combing FBG and AOM[63]

Fig. 8. Three typical acoustic sensors. (a) Condenser microphone; (b) quartz tuning fork; (c) fiber-tip acoustic sensor based on polymer diaphragm