Fig. 6: Gas sensing performance.

a Experimental set-up of the SWE-PTS. EDFA: erbium-doped fiber amplifier, Cir: optical circulator, FC: fiber coupler, PD: photodetector. b Measured PTS signal amplitude, standard deviation (std) of the noise and SNR as a function of pump modulation frequencies. c Detected 2 f signal amplitude at f = 1 kHz and std. of the noise as functions of the pump power. Inset are the 2 f signals when the pump wavelength is tuned across the C₂H₂ lines. d Allan-Werle plot based on the noise data over a period of 2 h, which is shown in the inset. For the noise measurement, the lock-in time constant is 100 ms, corresponding to 0.94 Hz detection bandwidth. e Dynamic range evaluation with peak-to-peak value of the 2 f signal as a function of gas concentrations. The inset shows the 2 f signals for N₂, 100 ppm, 1000 ppm, 0.5%, and 1% C₂H₂ when the pump power is set at 8 mW. The detection bandwidth is 0.094 Hz. f Response time evaluation. The PTS signal amplitude rises exponentially to a flat level following 1−exp(−t/τ_rise) with the rise time τ_rise, and then follows an exponential decay in the form of exp(−t/τ_fall) with the rise time τ_fall. At ∼4 s, 330 ppm C₂H₂ gas is loaded into the gas cell at a flow rate of 200 sccm, which is prefilled with N₂. At ∼19 s, N₂ is refilled into the gas cell at the same flow rate