Diode laser spectroscopy using a calibration free phasor decomposition approach with RAM nulling

A limiting factor of tunable diode laser spectroscopy (TDLS) analysis is the large unwanted residual amplitude modulation (RAM) background present on the recovered 1st harmonic signal. A novel approach to remove the background RAM will be presented. This new technique will be used alongside the phasor decomposition method (PDM) [1], a calibration free technique for recovery of the absolute gas absorption line-shape. The RAM nulling method developed, successfully removes the background by optical cancellation. This is
achieved by placing a fibre delay line, which introduces a π phase change in the modulation signal, in parallel with the gas cell line. When these lines are coupled together the background signals are at anti-phase and hence cancel each other. The main benefit being that measurement sensitivity can be increased. Results illustrating RAM nulling whilst using the PDM technique for absorption line-shape recovery will be presented. Figure 1 shows a recorded PDM measurement illustrating the difference with and without RAM Nulling.