Optical heterodyning study of the propagation dynamics of IR femtosecond laser pulses in a strongly scattering porous medium

A system is devised for optical heterodyning based on a femtosecond Cr:forsterite laser using a balance scheme for the laser noise compensation. The dependence of a heterodyne signal on the time delay is measured by detecting backscattered laser radiation from a strongly scattering porous object (a sheet of paper). It is found that the backscattered signal contains 'a long tail' with an exponential decay caused by multiple scattering. The exponent of the exponential is determined by the lifetime of photons in a scattering layer. The absorption and scattering coefficients for different types of paper are measured by the photon lifetime.