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Keywords:

  • laser sheet dropsizing;
  • stimulated Raman scattering;
  • droplet sensing

We show that the threshold of stimulated Raman scattering (SRS) from single micro-droplets increases with the bandwidth of the pulsed excitation laser radiation. SRS thresholds were experimentally investigated for two droplet sizes and two excitation bandwidths. For the narrowband excitation, a frequency doubled Nd:YAG laser with a central wavelength of 532 nm, a full width half maximum (FWHM) bandwidth of 0.05 nm and a repetition rate of 10 Hz was used. For the comparison with the broadband excitation, a dye resonator containing Rhodamine 6G being pumped by the Nd:YAG laser was utilized with a central wavelength of 566 nm and a FWHM bandwidth of 4 nm. Droplets of 89 and 116 µm diameters, respectively, were illuminated by a vertical light sheet. If the broadband excitation is applied instead of the narrowband excitation - for both droplet sizes - the pulsed excitation energies can be increased by a factor of approximately 5 before the SRS threshold is reached. The multiplication register of an emCCD detector was used to detect low signals. The gain factor of the detector was calibrated and adapted to different excitation energies and signal intensities. Copyright © 2012 John Wiley & Sons, Ltd.