Optimizing semiconductor laser chaos with precise control of double optical feedback

We study a semiconductor laser coupled to two mirrors at a distance. Understanding such a delayed feedback system and how it can make the laser behave chaotically could lead to better random number generators, safer communications, and more widespread use of these devices. Based on previous reports, the two delays are positioned in the long cavity regime and differ in the order of half the relaxation oscillation period (ROP) to limit the appearance of the so-called time-delay signature (TDS). We show theoretically that a change of the feedback phase has a crucial impact on the TDS and chaos bandwidth (CBW). At intermediate values, a change in the feedback phase will either suppress or enhance the TDS. For high feedback rates, where the chaotic bandwidth is much higher, the system can switch rapidly between stable and chaotic states due to small variations of the feedback phase. We show experimentally that the CBW is increased by increasing the feedback strength if the feedback phase is controlled. In summary, with two feedback loops, one can further suppress the TDS and increase the CBW, given that the feedback phases can be controlled accurately. Our results contrast with the one-delay system, for which the feedback phase has only limited impact if the feedback mirror is far from the laser.