Laser chaos has wide spectrum, noise-like, and synchronizable features, with great application potential in many fields, thus catching much research attention. Semiconductor lasers can produce broadband and high-complexity chaos under external light feedback and are the most widely studied and employed chaos source. The chaos mechanism produced by semiconductor lasers is that the inherent relaxation oscillation is unstable and the power spectra are broadened under external optical feedback disturbance. This leads to the phenomenon that the generated power spectrum energy of the chaos is mainly distributed around the relaxation oscillation frequency, the chaos bandwidth is usually less than 10 GHz, and the power spectra are not flat enough, which is unable to satisfy the practical application requirements. Meanwhile, researchers have proposed many schemes to increase the chaos bandwidth. On the one hand, the introduction of complex external optical paths is a kind of method, but this method will not only result in large size, high cost, and poor stability of the chaos source but also make the chaos synchronization extremely difficult, which is not conducive to the development of high-speed chaos secure communication technology. Another way to improve the chaos bandwidth is to start with the semiconductor laser itself, but a problem with this method is that the generated broadband chaos cannot be controlled at high speed, thereby becoming another obstacle to the development of high-speed chaos secure communication technology. Since the distributed reflection (DR) laser is an ultrafast semiconductor laser and has broken through the bandwidth limitation of previous direct-tuned lasers in the last five years, we propose a three-section DR laser structure with phase sections, which can be realized by changing the injection currents in the phase and DBR sections for state modulation of chaos.

We propose and simulate a three-section DR laser with phase sections for broadband laser chaos generation. Firstly, the simulation software VPIcomponentMaker is utilized to build the DR laser simulation model, and the internal parameters of the three-section DR laser are given, based on which the modulation response curve of the DR laser is studied. Secondly, under external optical feedback, we investigate the dynamical state of the DR laser in the chaos process with the increase in feedback intensity. Additionally, we explore the effects of photon-photon resonance (PPR) frequency and intensity on the generation of laser chaos by adjusting the injection currents in the phase section and the DBR section.

We build a simulation model of a three-section DR laser, verify the rationality and correctness of the parameter selection by the modulation response curve of the laser (Fig. 2), and enhance the -3 dB bandwidth of the DR laser to 37.16 GHz due to the enhanced modulation response caused by PPR. Then, the dynamic state evolution of the three-section DR laser under external optical feedback is studied. As the external feedback strength K_f increases from the steady state to the chaos state under K_f=0.0501, as shown in Fig. 3, the DR laser behaves in an intermittent oscillation state, and the oscillation duration in the time series gradually becomes longer. Meanwhile, we investigate the relationship between the variation of the quasi-periodic oscillation duty cycle of the DR laser and the feedback strength as K_f increases (Fig. 4). When the DR laser enters into chaos and continues to increase K_f, the bandwidth of the DR laser continues to increase up to more than 45 GHz. Furthermore, we explore the effects of PPR frequency and PPR strength on the chaos bandwidth of the laser by changing the phase and DBR section currents to regulate the PPR (Fig. 6). The results show that the smaller PPR frequency leads to greater bandwidth of the chaos, while the PPR intensity has little effect on the bandwidth of the generated chaos.

The proposed three-section DR laser can produce chaos states with significantly enhanced bandwidth due to the PPR effect under the action of external optical feedback, and the chaos bandwidth is increased to more than 2.25 times in the same conditions compared with the traditional optical feedback DFB laser. With the increasing optical feedback strength, the DR laser enters the broadband chaos state from the steady state through the intermittent path. Additionally, we can also observe the intermittent periodic state under the combined action of external cavity feedback and PPR effect, and the intermittent quasi-periodic state under the combined action of external cavity feedback, relaxation oscillation, and PPR effect. It is also found that the PPR frequency is the main factor affecting the chaos bandwidth of the laser, which points out the direction for optimizing the chaos bandwidth. Changing the phase section and DBR section current of the three-section DR laser can not only optimize the laser chaos bandwidth but also quickly regulate the laser chaos state, which is expected to become a key device for the development of high-speed secure optical communication technology.