Chaos, popular as butterfly effect, describes the irregular phenomenon of deterministic systems. Based on the unique features of sensitivity to initial conditions and unpredictability to future evolutions, chaos from laser diodes have found applications in private optical communication links, high-rate random number generations, Lidar systems, as well as recent optical computing networks. However, most light chaos are operated in the near-infrared regime, and hence the applications are limited to this spectral range as well.
A Chinese team of scientists, led by Professor Cheng Wang from ShanghaiTech University, China recently reported a mid-infrared hyperchaos source in Light: Science & Applications. The chaos generation relies on interband cascade lasers, one major type of laser sources in the mid-infrared regime. The lasers without any external perturbation usually produce continuous-wave output.
In order to trigger the chaos production, the team applied an external perturbation named as optical feedback to the lasers. Optical feedback is a technique that the laser output is reflected back to the laser cavity through a reflection mirror. The delayed light interacts nonlinearly with the laser device and generates chaos under certain operation conditions.
In the 1980s, mid-infrared chaos has been produced from gas lasers including CO2 lasers and He-Xe lasers. However, the gas lasers are rather bulky and the chaos bandwidth is limited up to the megahertz range. In contrast, the chaos in interband cascade lasers, explored by the Chinese team, reaches the frequency coverage of gigahertz range. This broadband chaos is desirable for high-speed information processing and transmission. In particular, their analysis proved that the chaos owned several positive Lyapunov exponents rather than one.
This suggests the chaos is the type of hyperchaos, which is of high-dimensional complexity. This high-order complexity is an essential requirement to implement secure communications in practice. In addition, the team pointed out that interband cascade lasers with optical feedback could produce hyperchaos within wide controlling parameter spaces, including the bias current condition, the feedback condition, as well as single or multimode lasing condition. "The broadband mid-infrared hyperchaos may enable applications in secure free-space communication links and remote chaotic Lidar systems," the scientists anticipate, "This is because the atmosphere has low-loss transmission windows (3 to 5 μm and 8 to 12 μm) in the mid-infrared regime."
Source: http://english.ciomp.cas.cn/