Signal-to-noise ratio analysis in laser absorption spectrometers using optical multipass cells

In high resolution absorption spectrometers with conventional light sources, the signal-to-noise ratio (SNR) is usually limited by the thermal noise level of the detector-preamplifier combination, which is independent of the light source power. However, the noise in many laser absorption spectrometers is dominated by the excess or shot noise which is dependent on the transmitted laser power, and which in turn is dependent on the number of reflections in a multipass cell. The optimum absorption path length for a high frequency modulated (FM) and a conventional wavelength modulated (WM) diode laser absorption spectrometer is investigated in this paper. The major result is that, due to the power attenuation by the multipass cell, the best SNR of a shot noise limited FM spectrometer is achieved at substantially shorter absorption paths, when compared with the excess noise limited WM spectrometer. This finding implies that the implementation of the FM technique in absorption spectrometers with multipasscells can improve the SNR only by 1 order of magnitude. Although desirable, this is substantially less than the improvement of 2 orders of magnitude expected in quantum limited conditions with a single pass cell.