Theoretical modeling of multimode laser frequency-modulation spectroscopy

A report is given on the extension of the theory of frequency-modulation spectroscopy (FMS) to include multimode laser sources. First, the theory is developed analytically, including the effects of the laser mode intermodulation on the FMS signal. The theory is then investigated in the limit of weak absorption. Finally, the analytical results are evaluated in a number of cases through the use of a computer model that is able to simulate the FMS spectrum expected for arbitrary molecular and atomic absorption lines. The model demonstrates that absorption features that are narrow compared with the overall laser linewidth may still be discernible through the application of multimode laser FMS.