Fast random number generation using optically mutually injected semiconductor lasers

Tzu-Yu Lo^1*, Chin-Hao Tseng³, Sheng-Kwang Hwang^1,2

¹Department of Photonics, National Cheng Kung University, Tainan, Taiwan
²Meta-nano Photonics Center, National Cheng Kung University, Tainan, Taiwan
³Department of Information and Computer Sciences, Saitama University, Saitama, Japan

* Presenter:Tzu-Yu Lo, email:jennyhaha99@gmail.com

With increasing research interest in chaotic dynamics induced by laser nonlinearity, numerous approaches have been developed to generate broadband chaotic signals using optical injection, optical feedback, or mutual optical injection systems. As the spectral bandwidth expands, the achievable sampling rate and consequently the random number generation rate increase. However, as research has primarily focused on enhancing bandwidth and speed, the extent of randomness inherently possessed by chaotic signals has often been neglected. The inherent feedback periods in optical feedback and mutual injection systems frequently introduce residual periodicity that degrades the randomness of chaotic signals, leading to the widespread use of post-processing techniques, most commonly XOR operation, to suppress correlations and pass statistical randomness tests. Nevertheless, even when post-processed numbers meet these criteria, they do not fully guarantee the unpredictability of the numbers, as insufficient inherent randomness can still cause partial predictability or periodic recurrence. Moreover, the reliance on XOR operations or other post-processing steps presents additional challenges for large-scale commercial integration. These limitations highlight the need for a physical entropy source capable of directly generating highly random signals without post-processing. In this work, we demonstrate a high-speed chaos generation system using two mutually injected semiconductor lasers for physical random number generation. The chaotic output exhibits a broadband electrical spectrum extending up to nearly 50 GHz, significantly enhancing the achievable sampling rate and, consequently, the random number generation rate. Under optimized operating conditions, broadband chaos is excited with entropy exceeding 4 bits per sample at a sampling rate of 128 GSamples/s without any post-processing. By ensuring negligible correlation between the outputs of the two lasers, the system achieves a random number generation rate exceeding 1 Tbit/s.

Keywords: semiconductor lasers, physical random number generation, laser dynamic