  The photon statistics and bunching of a semiconductor laser with external
optical feedback are investigated experimentally and theoretically. In a
chaotic regime, the photon number distribution is measured and undergoes a
transition from Bose-Einstein distribution to Poisson distribution with
increasing the mean photon number. The second order degree of coherence
decreases gradually from 2 to 1. Based on Hanbury Brown-Twiss scheme,
pronounced photon bunching is observed experimentally for various injection
currents and feedback strengths, which indicates the randomness of the
associated emission light. Near-threshold injection currents and strong
feedback strengths modify exactly the laser performance to be more bunched. The
macroscopic chaotic dynamics is confirmed simultaneously by high-speed analog
detection. The theoretical results qualitatively agree with the experimental
results. It is potentially useful to extract randomness and achieve desired
entropy source for random number generator and imaging science by quantifying
the control parameters.
