Measurements Of Femtosecond Pulse Propagation Property In Random Media

The study on probing and imaging of random media by using laser, especially on biological tissue, has attracted widespread attention. Optical technology, with the advantages of non-contact, non-ionizing, micro-trauma even no trauma and high flexibility, has great application value in the biomedical field. However, light scattering, caused by the inhomogeneity of tissues, makes the effective information submerged by scattered light and affects the image quality seriously. One of the aproaches to solve the problem is to time-resolve the transmitted light signal.In this paper, we described several theories about propagation of light in random media, including the radiation transfer theory and Mie scattering theory. Several gating methods are overviewed, such as spatial filtering, coherent and incoherent time gating. A time gating experimental device is built and light signals transmitted from several different random samples are measured with this device. The main contents of this paper are as follows:1. Random media and its three types are summarized: discrete random medium, continuous random media and random rough surface. The mechanisms of the interaction between light and random media are explained.2. The radiation transfer theory and the Mie theory are discussed. Variations of extinction cross-section, scattering cross-section and absorption cross-section with particle diameter, refractive index and incident wavelength are calculated.3. The mechanism of spatial filtering, coherent and incoherent time gating are analysed. The principles of several nonlinear optical switchs are explained.4. Measuring device is built. Femtosecond pulse laser generated by Ti-sapphire regenerative amplifier system is the light source, and second harmonic generation (SHG) optical switch as the gating method. The representative particle suspension media is chosen as random sample, and light signals transmitted from these media are measured.