Simultaneous measurement of gas absorption spectra and optical path lengths in a multipass cell by FMCW interferometry

We present a novel method based on optical frequencymodulated continuous-wave interferometry that can realize simultaneous measurement of gas absorption spectra and optical path lengths (OPLs) in the widely used multipass cells (MPCs). This method involves a Fourier transform (FT) and an inverse FT, by which the gas absorption spectrum and the OPL are retrieved in frequency and time domains, respectively. Various OPLs are achieved by retrieving gas absorption spectra associated with different reflection positions in the MPC. As a demonstration, absorption spectra of acetylene around 1520 nm are measured using a commercial White type MPC, achieving a noise equivalent absorbance of 0.01 and a spatial resolution of 290 μm over 22.5 m OPL. Our proposed method shows advantages for MPC-based gas sensing applications in the significant relaxation of OPL calibration demands and flexible extension of measurement dynamic range.