Fig. 1. Basic principle of LaVS. The left top panel shows the main configuration based on FMCW interferometry, and other parts show the procedure for simultaneously retrieving absorption and dispersion spectra of gases of different concentrations with different absorption path lengths.

Fig. 2. Experimental setup of LaVS. The inset shows absorption line intensities of acetylene around 1520 nm. PC, polarization controller; FRM, Faraday rotation mirror; ATT, optical attenuator; BPD, balanced photodetector; DAQ, data acquisition.

Fig. 3. Procedure for simultaneous retrieval of absorption and dispersion spectra of 8440-ppm acetylene filled in the MPC. (a) Recorded beat signals. (b) FT results of the beat signals shown in (a). (c) Retrieved absorption and dispersion spectra (each with 20 results averaged) by applying IFT to three reflection peaks (#5, #11, #17) indicated in (b). The inset in (c) is the enlargement of the end part of the spectrum, showing details of the oscillating noise due to spectral leakage in the FT process.

Fig. 4. Measured and fitted absorption and dispersion spectra of acetylene of different concentrations with different absorption path lengths. (a), (b) For low concentrations of 102 ppm and 1090 ppm, with absorption path lengths of 19.386 m and 3.949 m, respectively. (c)–(e) For high concentrations of 8440 ppm, 9.3%, and 99.3%, with absorption path lengths of 6.103 m, 1.064 m, and 0.359 m, respectively. Each spectrum is an average of 20 scans.

Fig. 5. Results of absorption spectra in the transmission mode. (a) Measured and fitted absorption spectra of 1.2-ppm and 8-ppm acetylene with an absorption path length of 41.961 m. Each spectrum is averaged over 20 scans. (b) Allan-Werle deviation plots yielded by continuous measurement results of 1.2-ppm acetylene during 240 s (shown in the inset).

Fig. 6. Plots of measured acetylene concentrations versus assumed ones. Each error bar shows the standard error of the mean of 20 measurement results and is magnified 20 times for clarity.