Abstract

We measured the pulse profile and speed of femtosecond laser pulses propagating through scattering media by using a cross-correlation method. Multiple scattering in random media splits the pulse into two components: a coherent (ballistic) component and an incoherent (diffusive) component. We found the pulse profile of the coherent component to remain essentially unchanged while its speed was reduced. We found the delay in the time of arrival of the coherent component to be linearly proportional to the thickness of the media and the concentration of the scatterers. We composed the measured delay times with delay times derived from theoretical models. In one of the models coherent interference is neglected, and the increase in refractive index in the medium is purely a function of the volume fraction of the scatterers. This model underestimates the delay time. The theoretical model developed by M. Lax,¹ which accounts for the coherent interference of the forward scattered waves and the effective field acting on a scatterer, describes the experimental results better than does the former model. The change in refractive index for coherent pulses appears to be described by the coherent interference of waves scattered in the forward direction with the primary wave.

© 1990 Optical Society of America