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

We report a new addressing mechanism for quasi-disrtributed absorption sens ors based on the frequency modulated continuous wave (FMCW) method. The sen sor units consist of open-path microoptic cells constructed from gradient i ndex (GRIN) lenses, each of differing lengths. Coherence addressing of the cells using FMCW is achieved by the interferometric mixing of two signals o riginating from each cell (from the glass/air interfaces). The time delay b etween the two reflections, along with the linear frequency ramp of the sou rce, gives rise to beat frequencies in the mixed output which are different for each cell. The connecting fiber length between two successive sensor c ells is chosen to be much greater than the coherence length of the source s o that the reflections from different cells do not interfere. The interfere nce patterns of all sensor cells add up at the detector whereby each indivi dual sensing cell is identified by its power spectrum in the frequency doma in. We show theoretically and experimentally how individual cells can be ad dressed and the measured signals obtained by suitable choice of cell length , proper modulation of the source and appropriate signal processing.