Power-law fluctuations near critical point in semiconductor lasers with delayed feedback.

Since the analogy between laser oscillation and second-order phase transition was indicated in the 1970s, dynamical fluctuations on lasing threshold inherent in critical phenomena have gained significant interest. Here, we numerically and experimentally demonstrate that a semiconductor laser subject to delayed optical feedback can exhibit unusual large intensity fluctuations characterized by power-law distributions. Such an intensity fluctuation consists of distinct intermittent bursts of light intensity, whose peak values attain tens of times the intensity of the maximum gain mode. This burst behavior emerges when a laser with a long time delay (over 100 ns) and an optimal feedback strength operates around the lasing threshold. The intensity and waiting time statistics follow power-law-like distributions. This implies the emergence of nonequilibrium critical phenomena, namely self-organized criticality. In addition to numerical results, we report experimental results that suggest the power-law intensity dynamics in a semiconductor laser with delayed feedback.