Concentration measurements by intracavity laser absorption spectroscopy for the case of strongly overlapped spectra

The feasibility of fiber laser intracavity absorption spectroscopy (FLICAS) for accurate measurement of pressure and concentration of CO and CO ₂ at elevated pressures (up to 6.5 atm) was studied. FLICAS is a very sensitive absorption spectroscopy method which allows for simultaneous measurement of several species even in harsh environment, since it is not sensitive to broadband optical losses caused by particle scattering and absorption, optical window contamination, etc. However, the wings of spectral lines can overlap at elevated pressures, resulting in broadband absorption which is not monitored by FLICAS. It makes the direct evaluation of baseline challenging. Nevertheless, fitting the lineshapes of individual lines using the HITRAN database allows for recovering of the baseline position and shape in some cases. The CO and CO ₂ spectra were studied under controlled conditions in the spectral range of 6350–6385 cm^{- 1} and processed by custom LabVIEW code to take into account the overlapping line effect. This procedure allows for accurate determination of the pressure and the CO concentration at least up to 6.5 atm. More dense and line-rich CO ₂ spectrum allows correct pressure evaluation up to 6.5 atm, albeit with higher uncertainty. Accurate measurements of the CO ₂ concentration are possible only up to about 4 atm.