We introduce a novel light-scattering technique for investigating the dynamics of random media with a broad range of optical densities. By use of the spatial coherence properties of a single-mode optical fiber and the temporal coherence of a broadband source, the measurement volume is isolated at the end of the optical waveguide. Optical mixing between the fluctuating scattered light and the Fresnel-reflected field at the fiber-medium interface is analyzed directly in the frequency domain. The unique characteristics of this new technique are discussed in the context of simultaneous measurement of average scatterer size and concentration in dense colloidal suspensions.