Coherence addressing of quasi-distributed absorption sensors by the FMCW method

We report a new addressing mechanism for quasi-distributed absorption sensors based on the frequency modulated continuous wave (FMCW) method. The sensor units consist of open-path microoptic cells constructed from gradient index (GRIN) lenses, each of differing lengths. Coherence addressing of the cells using FMCW is achieved by the interferometric mixing of two signals originating from each cell (from the glass/air interfaces). The time delay between the two reflections, along with the linear frequency ramp of the source, gives rise to beat frequencies in the mixed output which are different for each cell. The connecting fiber length between two successive sensor cells is chosen to be much greater than the coherence length of the source so that the reflections from different cells do not interfere. 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 theoretically and experimentally how individual cells can be addressed and the measured signals obtained by suitable choice of cell length, proper modulation of the source and appropriate signal processing.