Fig. 2: Experimental setup and results.
a Experimental setup for the photo-acoustic detection of gaseous acetylene (C2H2). A tunable continuous-wave (CW) laser with optical frequency νCW is amplified by an erbium-doped fiber amplifier (EDFA) and used as a common seed for the generation of two optical frequency combs with repetition rates of \({f}_{{\rm{rep}}}^{(1)}\) and \({f}_{{\rm{rep}}}^{(2)}\) via electro-optic modulation (EOM). Acousto-optic modulation (AOM) of the CW laser with \({f}_{0}^{(1)}\) and \({f}_{0}^{(2)}\) controls the relative offset between both combs. (COL: free space collimator; PD: reference photo-detector MIC: low-noise MEMS microphone, see Methods for more details). b Acoustic multi-heterodyne signal recorded by the microphone (5 interferograms; after high-pass filtering) for an acetylene filled cell. c Spectrum of the acoustic multi-heterodyne signal for 80 ms and 800 ms long acquisitions. Inset: Multi-heterodyne reference spectrum as recorded by the reference photo-detector (over the same span). d Acetylene absorption signature obtained after normalizing the acoustic multi-heterodyne spectrum by the reference spectrum for an 800 ms acquisition duration (blue dots); shaded areas (gray, yellow, blue) represent the standard-error intervals for different acquisition durations (8 ms, 80 ms and 800 ms). The red line shows the HITRAN model for comparison.
