A Molecule-Specific Filter Modulator

An optical Fabry-Perot filter has been devised to aid in the detection of weak infrared emission from hot molecules. The comb-like transmission spectrum of the filter was matched to the comb-like infrared emission spectrum of CO. Other small molecules are also suitable. The filter can be made to perform rapid background subtraction by slightly detuning the filter spectrum from the molecular emission spectrum. This invention is called spectral dithering and can be performed at rates up to several kilohertz using a piezoelectrically driven Fabry-Perot. In cases where atmospheric fluctuations are a significant noise source, background subtraction at kilohertz rates can essentially freeze the fluctuations. This improves the effectiveness of the background subtraction. Estimates of the signalbackground and signalnoise improvements for the filter are given. The spectral dithering capability was demonstrated in a laboratory experiment using CO emission from a flame.