Chaotic unpredictability properties of small network mutually-coupled laser diodes

The chaotic unpredictability properties of mutually-coupled laser diodes (LDs) are investigated numerically. The unpredictability degree is evaluated quantitatively via the permutation entropy (PE). The effects of coupling strength, frequency detuning, feedback strength, as well as time delays are considered. It is shown that, compared with the unidirectional coupling case, two unpredictability-enhanced chaotic signals can be simultaneously obtained for the mutual coupling case, and the parameters regions contributing to unpredictability-enhanced chaos are also broadened. Besides, the PE values for two mutually-coupled LDs are close to each other, with the exact relationship being related to frequency detuning, due to the leader-laggard relationship in terms of injection locking effect. We also consider small network of mutually-coupled LDs, where the effects of connection topologies and frequency detuning are mainly examined. The small network of mutually-coupled LDs can generate several independent unpredictability-enhanced chaotic signals in parallel, which is extremely useful to substantially increase the bit rate and improve the randomness of random number generators based on chaotic LDs.