The Doppler Limit

The requirement for specificity in mixtures helped drive an important design criterion, a desire to operate well into the Doppler limit so that signal processing with narrow, well-defined lineshapes would be possible. If the system were not in the Doppler limit, then the pressure broadening among the gases in the mixture would have made the linewidth problem intractable; the collision broadening cross sections for different pairs of gases can vary by a factor of five [1]. However, in the Doppler limit, the lineshape in each gas’ reference library was the same as in the mixture. While the complexity of the 32 gas mixture considered here did not require this level of analytical power, we wanted to develop a system that could deal with as complex a mixture as possible, so we preserved this requirement.

This choice had three immediate consequences: (1) a SMM/THz technology was chosen whose spectral purity and absolute frequency calibration are much better than defined by the Doppler width, (2) because of the smaller number density, the lower pressure reduces the absorption strengths of the gases, and (3) it also causes the gases to saturate at relatively low power levels. The first of these leads to the choice of a cw electronic source technology and the latter two reduce the ppx sensitivity of the system.

An important design consequence of this decision is that we chose to use a heterodyne detection system because its sensitivity at the low power levels dictated by the low pressure, small volume cells is better than that of square law detectors, although both are limited in principle by Townes noise [1].