Ballistic and diffuse ultrafast laser pulses propagation in model random media and biological tissues.
Item
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Title
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Ballistic and diffuse ultrafast laser pulses propagation in model random media and biological tissues.
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Identifier
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AAI9325122
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identifier
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9325122
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Creator
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Liu, Feng.
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Contributor
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Advisers: R. R. Alfano | K. M. Yoo
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Date
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1993
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Language
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English
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Publisher
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City University of New York.
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Subject
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Physics, Optics
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Abstract
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The goal of this thesis is to study light scattering and ultrashort laser pulse propagation in the model discrete random media and biological tissues. The temporal profiles of the scattered ultrashort laser pulses were measured by using ultrashort laser pulses, streak camera with 10 ps time response and the femtosecond cross correlation technique.;The intensity of the diffuse pulse is found to be preferentially reduced by the absorption with respect to the intensity of the ballistic light because the diffuse light on the average travel through a longer path than the ballistic light. A simple experiment was performed to demonstrate that one can image through a highly scattering medium by increasing the absorption in the medium. The speed of the ballistic pulse was measured by the femtosecond second harmonic cross correlation method and was found to be reduced by the scattering. This speed reduction can be accounted for by the volume fraction combination of the indices of refraction of the scatterers and the water or by the coherent interference between the scattered waves and the primary wave.;No distinct ballistic pulse was observed when the laser pulse transmitted through a thin tissue. The continuous random variation of the dielectric constant in tissue may account for the lack of a distinct ballistic pulse. Nevertheless, the early arriving portion of the transmitted pulse was shown to carry image information. The earlier arriving photons were found to produce image of the hidden object with better quality. The intensity of the early arriving transmitted photons was found to decay exponentially with increasing tissue thickness. The earlier the photons arrive, the quicker their intensity decay. Diffusion theory was found to describe the overall transmitted pulse profiles well, but it underestimates the intensity of the early arriving photons. Compressing tissue was found not to change the key optical parameters of the tissue.;The scattered laser pulse profile in an infinite and semi-infinite random medium were measured and analyzed with the photon density and the photon flux of the diffusion theory. The photon density are found to describe the scattered pulse profile measured inside the scattering medium. Both photon density and the photon flux can be used to describe the scattered pulse profile from the surface of the medium.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
