Adjacent-resonance etalon cancellation in ring-down spectroscopy

Adjacent-resonance etalon cancellation provides a means of significantly reducing both systematic and random errors introduced in cavity ring-down spectroscopy by unwanted etalons. By stretching the ring-down cavity symmetrically about its center point and collecting two data sets at cavity lengths separated by λ/2, two fringing components offset in phase by 180 deg are obtained. When these two data sets are averaged, oscillations in effective mirror reflectivities due to fringing are dramatically reduced. The technique is demonstrated in a 12 cm monolithic ring-down spectrometer. Long-term time constant measurements show a decrease in noise-equivalent absorption and an increase in maximum effective averaging time due to a reduction in noise coupled in by etalons; trace water spectra demonstrate how removing the systematic fringing components eases absorption peak identification.