Sensitivity and resolving power of a Fabry–Perot interferometer and of optical mixing spectroscopy

The absolute sensitivity of various very-high-resolution spectroscopic methods is discussed in the specific case of light scattering studies. The methods considered are the plane-parallel Fabry-Perot interferometer, confocal Fabry–Perot interferometer, and heterodyne and homodyne recording methods. It is shown that the plane-parallel interferometer is to be preferred in the recording of spectra of width $\Delta\omega\gtrsim 10^8-10^9$ rad/sec whereas the confocal interferometer is more sensitive in the range $\Delta\omega\lesssim10^8-10^9$ rad/sec. The sensitivity of the optical mixing (heterodyne and homodyne) methods increases with decreasing spectral width $\Delta\omega$ and the methods themselves are the only that can be used in the range $\Delta\omega\lesssim10^7$ rad/sec. The minimum detectable scattering coefficient $dR/do$ (cm$^{-1}\cdot$ sr$^{-1}$) is plotted as a function of the spectral width $Ao$) for all the methods under discussion and it is assumed that modern photodetectors and laser sources are used.