High sensitivity overtone absorption spectroscopy with intracavity laser technique

Enhancing the detection sensitivity of absorption measurements from gases has been achieved via a laser methodology which incorporates the gas sample to be monitored into the optical resonator cavity of a multimode laser. Since the properties of the laser itself are modified by the intracavity absorber (gas), the absorption information is contained on the output laser beam. The absorption spectrum of the gas(es) appearing within the laser resonator is measured by spectrally dispersing the laser output. Under appropriate measurement conditions, the concentration of the intracavity gaseous species can also be quantitatively determined. Termed intracavity laser spectroscopy (ILS), these techniques have been reported to enhance detection sensitivity of absorption by a factor of 10⁶ to 10⁹. To illustrate this enhanced detection sensitivity, the effective absorption pathlength of an ILS measurement compared to that required from a conventional absorption measurement can be compared. The effective ILS absorption pathlength can reach several hundred kilometers meaning that this optical pathlength would be required to obtain the same change in absorbance for the same ground-state concentration of the gaseous species, in a conventional absorption measurement. Such enhancement can be obtained while maintaining the other advantages traditionally associated with absorption spectroscopy