Mode-Locked Laser Raman Spectroscopy.A New Technique for the Rejection of Interfering Background Luminescence Signals

A new technique for the rejection of interfering luminescence background signals employing a mode-locked argon ion laser and single photon timing detection electronics is described. Advantage is taken of the disparity between the lifetime of Raman scattering and the lifetime of luminescence emission. Only those photons emitted or scattered by the sample during the mode-locked laser pulse are passed on to the recording circuitry by the single photon timing detection system. Essentially all of the Raman signal can be recorded and a large fraction of the luminescence background is rejected. A detailed discussion of several alternative schemes for implementing this concept is given, along with a theoretical treatment of the appropriate signal-to-noise considerations. The fluorescence rejection capabilities of one of these configurations has been tested on samples consisting of a nonfluorescent Raman scatterer doped with a highly fluorescent dye impurity. The spectra obtained via the mode-locked technique show a substantial background suppression. Background slope is also reduced and the signal-to-noise ratio shows an improvement consistent with our theoretical calculations based on fluorescence lifetime, laser pulse shape, laser pulse repetition rate, and average mode-locked power.