Scattering of femtosecond laser pulses by small droplets has been measured and compared with predictions, yielding some interesting new applications for time integrated detection of the scattered field. The scattering intensity of integrated detection becomes monotonic with droplet size over large regions of scattering angle and morphology dependent resonances are surpressed, opening the way for particle sizing using the scattered intensity. Furthermore, the ripple structure no longer appears in the rainbow region of scattering, simplifying rainbow refractometry significantly. These scattering proporties of femtosecond laser pulses have been demonstrated in the laboratory using a novel Paul trap for levitating single droplets.
The authors demonstrate within a metrology experiment the applicability of a recently proposed temporally incoherent semiconductor laser source which relies on nonlinear dynamics. The realized spectrally broadband emission with an output power of 110mW and a coherence length of only 120μm is used in a rainbow refractometry experiment for sizing of liquid droplets, representing an important problem in industrial processes. The observed emission characteristics are attractive for implementation of modern imaging and metrology techniques which are based on the properties of well-directed, temporally incoherent light.
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.
Rainbow refractometry has been investigated numerically for small droplets under ultrashort pulse illumination. Using Fourier Lorenz-Mie Theory, local maxima of the scattering function were investigated as a function of the scattering angle, particle size and refractive index. The primary rainbow was shown to be detectable for pulse lengths as short as 10 fs and free of interferences with other scattering orders for droplet diameters down to 5 lm. An extension of the Generalized Lorenz-Mie Theory was used to obtain an estimation of the influence of nonsphericity on refractive index detection. Overall, the possibility of improved in situ temperature measurements with Rainbow refractometry for small particles was shown to be feasible.
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