Lasing with coherent feedback in weak scattering media

We present detailed experimental and numerical studies of lasing with coherent feedback in weakly scattering systems. The interference effects, which are weak in the passive systems, are strongly enhanced by coherent amplification in the active systems. The lasing modes are confined in the vicinity of the excited volume due to absorption of emitted light outside it. In the ballistic regime where the size of gain volume is less than the scattering mean free path, lasing oscillation occurs along the direction in which the gain volume is most extended, producing directional laser output. The feedback for lasing originates mainly from backscattering of particles near the boundaries of pumped region, because its longest length of one path across the gain volume leads to most amplification. It results in regular frequency spacing of lasing modes, which scales inversely with the maximum dimension of the gain volume