Secure communication based on chaotic lasers has gained significant attention in recent years due to its high speed and compatibility with existing fiber-optic networks. Vertical-cavity surface-emitting lasers (VCSELs) are promising candidates, offering a compact structure and low power consumption. However, VCSELs are constrained by chaotic bandwidth limitations induced by relaxation oscillations, which reduce the secure transmission rate. Existing methods for broadening the chaos bandwidth often rely on complex external perturbations, which may hinder system synchronization. We propose a simplified method for generating broadband chaos by utilizing the relaxation-oscillation-free polarization mode of VCSELs.

We present a novel approach for generating broadband chaos by utilizing the relaxation-oscillation-free polarization mode of VCSELs. The experimental setup (Fig. 1) employs a custom single-mode VCSEL with a wavelength of 1550 nm and no internal isolator. The laser is powered by a low-noise current source and is equipped with precise temperature control to ensure stable operation. Optical feedback is provided via a fiber mirror, with its intensity regulated by a variable optical attenuator. Polarization controllers are used to align the feedback polarization, directing X mode feedback to the X mode and Y mode feedback to the Y mode. The output signals are separated into X+Y, X, and Y modes using polarization beam splitters. The spectral, temporal, and dynamic properties of the VCSEL are analyzed using an optical spectrum analyzer, a high-speed oscilloscope, and an electrical spectrum analyzer. Key parameters, including bias current and feedback strength, are systematically varied and optimized to investigate their effects on chaos generation, with the aim of identifying the optimal conditions for achieving broadband chaos with the desired characteristics.

We begin with an in-depth analysis of the free-running VCSEL to examine its intrinsic characteristics. The output power versus bias current curve reveals significant relaxation oscillations in both the X+Y and X modes, as indicated by the spectral side lobes in Fig. 2. These oscillations are typically associated with the laser’s inherent dynamics and may limit the bandwidth of the chaotic signal. However, the Y mode does not exhibit relaxation oscillations due to the asymmetric gain distribution of the single-mode oxide-confined structure, which suppresses oscillations in this mode. This unique characteristic of the Y mode makes it particularly suitable for generating broadband chaos, free from the constraints imposed by relaxation oscillations. Following this, the introduction of optical feedback leads all polarization modes to transition from the steady state (S) to quasiperiodic (QP) and chaotic (C) states as the feedback strength increases, as shown in Fig. 3. The parameter mapping in Fig. 3 indicates that higher bias currents require stronger feedback to induce chaos. The Y mode achieves an 80% energy bandwidth of 13.49 GHz with ±3 dB flatness, significantly outperforming both the X+Y mode (8.14 GHz) and the X mode (7.47 GHz), as shown in Fig. 4. This result emphasizes the superior performance of the Y mode in broadband chaos generation. Furthermore, the chaotic behavior of the Y mode exhibits weaker time-delay signature (TDS), as illustrated in Fig. 7. Additional suppression of TDS can be achieved using chirped fiber Bragg grating feedback, potentially further improving system performance. Bandwidth saturation is observed at a feedback strength of 27% (Fig. 5). This suggests an optimal feedback range for achieving the desired broadband chaos, beyond which the bandwidth does not significantly increase. The parameter-dependent mapping in Fig. 6 highlights the Y mode’s superior performance, with a bandwidth exceeding 10 GHz across a broad operational range (I=13.4 mA, K_f=24%).

We propose a method for generating broadband chaos based on the unrelaxed oscillating polarization modes of the VCSEL. The signal characteristics of the free-running VCSEL are demonstrated experimentally. The path through which optical feedback causes the VCSEL to enter chaos is analyzed. The parameter range for generating broadband chaos is determined. Through parameter optimization, Y-polarization mode chaos with an 80% spectral energy bandwidth of 13.49 GHz and a spectral flatness of ±3 dB is successfully achieved. TDS analysis demonstrates that the characteristic peaks of Y-mode chaos maintain significantly lower values compared to those in both the X+Y and X modes. These results offer a simple broadband chaotic signal source for applications in chaotic secure optical communication.