When electromagnetic radiation is emitted, scattered, or reflected from an object moving toward or away from an observer, the observed wavelength is shifted (the Doppler effect). The molecules of a gas will scatter electromagnetic radiation (Rayleigh scattering). Thus, radiation that is Rayleigh-scattered from a gas may be shifted in wavelength because of the velocity of the scattering molecules.

The distribution of velocities will vary with the composition of the gas, so when light of a single frequency is scattered from a gas, the frequency spectrum of the scattered light is broadened (Doppler broadening). The degree of broadening will depend upon the velocities of the molecules, which in turn depend upon the composition of the gas. Therefore, it is possible to determine the composition of a gas by measuring the Doppler broadening of Rayleigh-scattered light.

A theoretical model was developed that predicts the spectrum of the Doppler-broadened light and the signal-to-noise ratio for this technique as a function of the key design parameters. This model includes both direct detection and heterodyne detection approaches. An example of predicted spectrum is shown in figure 1.

Five approaches to implementation of this technique were considered singly and in combination as candidates for a proof-of-principle experiment:

• Homodyne detection.
• Offset homodyne detection.
• Heterodyne detection.
• Vapor absorption filter.
• Fabry Perot interferometer.

A survey of the key technologies for the laser, detector, modulator, filter, and frequency discriminator indicated that two approaches may be currently feasible for a proof-of-principle experiment: homodyne and a combination of vapor absorption filter and Fabry Perot. Analyses using the model developed in this phase indicate that both of these techniques are expected to provide signal-to-noise ratios suitable for proof-of-principle experiments. Designs for these two experiments are outlined in figures 2 and 3. The next step in development after proof of principle will be to add a scanning system to enable acquisition of a two-dimensional image of the distribution of the leaking gas.