Fig. 1. Simulation and characterization of micro-nano fiber. (a) Simulation of evanescent field and photothermal effect; (b) transmission spectrum of fabricated microfiber

Fig. 2. PTS gas detection system based on microfiber with sub-wavelength dimension. (a) Experimental setup; (b) microfiber guiding 660 nm visible laser

Fig. 3. Photothermal phase modulation and second harmonics. (a) Dynamic process when the pump light is off and on; (b) spectrum of phase modulation

Fig. 4. Evaluation of system performance by second harmonic signal. (a) Second harmonics corresponding to different NH₃ volume fractions; (b) linear fitting of V_pp; (c) second harmonic waveform within 180 s (36 cycles); (d) V_pp within 30 cycles

Fig. 5. Influence of pump power on SNR. (a) Second harmonics corresponding to different pump power; (b) comparison between signals and 1σ noises at different pump power

Fig. 6. Ability to resist ambient vibration noise