Knowledge Bank

A new method of acquiring intracavity laser absorption spectra (ILS) has been developed that enables spectra to be obtained at much higher spectral resolution than has been possible $previously1$. Using the usual method for acquiring ILS spectra, the resolution obtained is determined by the resolving power of the associated spectrograph. Even with multi-pass, echelle grating spectrographs, the maximum resolution reported in practice is about 800,000. The new technique enables considerably better resolution to be achieved. It involves use of an intracavity etalon and the observation that the intensity of an etalon fringe is reduced if the fringe occurs in vicinity of an absorption feature. In addition, the position of the etalon fringe does not change. The fringe positions can be determined very accurately. The etalon is tuned across its free spectral range and the absorption spectrum recovered by summing the series of intracavity etalon spectral profiles. The intensities of several water lines occurring around 720 nm were examined using the new approach. A standing wave dye laser was employed and the etalon was tuned using a high resolution stepping motor. Results obtained over a series of water pressures show the applicability of the Beer-Lambert relationship for ILS. This study demonstrates that quantitative data at very high sensitivity and ultra-high resolution can be obtained using the new approach.