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Application of femtosecond lasers to the polarization ratio technique for droplet sizing

Ultra-short laser pulses (tpulse = 220 fs) were applied to a monodisperse droplet stream, demonstrating the feasibility of the polarization ratio technique for instantaneous droplet size measurements with high spatial and temporal resolution. The polarization ratio technique is based on the signal ratio of perpendicularly and parallel polarized components of Mie scattered light. When applied to sprays this technique yields signal ratios proportional to the diameter D21. Using a highly reproducible monodisperse droplet stream the drop sizing of individual droplets allows the comparison to well accepted methods such as the phase Doppler technique or direct imaging. Droplet sizes were varied over a wide range, including droplet sizes relevant for practical combustion devices. The application of ultra-short laser pulses avoids oscillations in the scattered signal caused by interference between different scattering orders. This effect is limited to diameters larger than a cut-off diameter that depends on the coherence length of the light source. These oscillations are a major drawback when using CW laser sources operating at long coherence lengths, as they lead to an ambiguous relation between droplet diameter and polarization ratio and necessitate spatial or temporal averaging. It is shown that the application of femtosecond lasers significantly reduces the number and magnitude of oscillations in the polarization ratio and therefore improves the precision of the polarization ratio technique for droplet sizing by a factor of 3 on average depending on the mean droplet diameter.