Theoretical analysis of a quasi-distributed optical sensor system using FMCW for application to trace gas measurement

We report on the performance of a quasi-distributed sensor system for narrow band absorbers using the frequency-modulated continuous wave (FMCW) method, including signal-to-noise analysis. The sensor units consist of open-path micro-optic cells constructed from GRIN lenses, each of differing lengths. For addressing of the cells interferometric mixing of two signals originating from each cell (from the glass/air interfaces) is employed. Due to the linear frequency ramping of the source, different beat frequencies arise in the output for each cell. The interference patterns of all sensor cells add up at the detector whereby each individual sensing cell is identified by its power spectrum in the frequency domain. We show system performance for narrow band absorbers and the limitations on the number of cells in the system.