Gain analysis of an optical multipass cell for spectroscopic measurements in luminous environments

The signal improvement obtained for a spectroscopic measurement with a multipass cell can be expressed as the gain over a single pass measurement. We develop a unified analysis of amplitude gain and signal-to-noise ratio (SNR) gain for both cw and pulsed systems with application to Rayleigh/Raman spectroscopy, fluorescence, and similar techniques. Our analysis reveals that the SNR gain is dependent on the single pass efficiency, number of passes, and temporal overlap. Increasing the single pass efficiency of a multipass system will increase both the amplitude and SNR gains. While increasing the number of passes always improves the amplitude gain, the SNR gain actually has an optimal number of passes. In particular, what has not been recognized for pulsed systems is that increasing the pulse overlap fraction is equally or even more significant than the number of pass or efficiency increases. For cw systems, the amplitude gain and SNR gain are identical.