Speckle experiments in random lasers.

We present measurements of speckle in a random laser. We analyze its first-order statistics and show that, contrary to what might be expected for passive systems, analyses of the intensity distribution P(I) and the speckle spot size do provide information about light transport inside the system. P(I) is used to determine the degree to which an incident probe is amplified by the random laser. The shrinking speckle spot size reflects the change in path length distribution P(Lambda); we deduce that the average path length in the studied random laser is two times longer above threshold than in a passive diffusive system.