Time-delay signature suppression in a chaotic semiconductor laser by fiber random grating induced random distributed feedback.

The isochronous cluster synchronization with time delay (TD) signature suppression in delay-coupled vertical-cavity surface-emitting laser (VCSEL) networks subject to variable-polarization optical injection (VPOI) is theoretically and numerically studied. Based on the inherent symmetries of network topology, parameter spaces for stable cluster synchronization are presented, and zero-lag synchronization are achieved for VCSELs in same clusters. Additionally, the TD signature reduction for the dynamics of VCSELs in the stable clusters are systematically discussed. It is shown that both moderate polarizer angle and frequency detuning between different clusters have strengthen the effect of TD signature suppression. Moreover, the isochronous cluster synchronization with TD signature concealment is also verified in another VPOI-VCSEL network with different topology, indicating the generality of proposed results. Our results shed a new light on the research of chaos synchronization and chaos-based secure communications in VCSEL networks.

Aiming at the problems of limited bandwidth and periodicity of chaotic laser, a monolithically integrated chaotic semiconductor laser subject to random feedback and mutual injection is proposed. The chaotic laser consists of two distributed feedback laser diodes (DFB-LD), two semiconductor optical amplifiers (SOA) and a passive optical waveguide. Optical injection of two coupled DFB-LDs achieves chaotic signal shaping and bandwidth enhancement. Random grating along the passive waveguide provides stochastic feedback to suppress the time delay signature (TDS). A theoretical model is established based on the Lang-Kobayashi model to explore the dynamics of the proposed chaotic laser. A chaotic signal with TDS of 0.06 and bandwidth of 13.12 GHz is obtained by simulation. Furthermore, the effects of parameters including feedback ratio, mutual injection ratio, and detuning frequency on the TDS and bandwidth of chaotic signal are investigated. The results show that the broadband chaotic laser signal with TDS suppression can be obtained by controlling parameters reasonably.

A wavelength tunable chaotic external cavity semiconductor laser (ECSL) composed by a distributed-feedback laser diode (DFB-LD) and a broadband chirped fiber Bragg grating (CFBG), is proposed and experimentally demonstrated for generating tunable chaos signals with time delay signature (TDS) suppression. The experimental results show that by using a CFBG with a 3-dB reflection bandwidth of approximately 30 nm and a low dispersion of approximately 1.25 ps/nm, the chaotic ECSL can be tuned over a wide wavelength range, and the generated chaos possesses a low TDS of about 0.04. With the increase of the bias current of the DFB-LD, the chaotic ECSL is tuned towards long-wavelength direction, the effective bandwidth of the chaotic signal is extended, and the TDS of the chaotic signal remains effectively suppressed. To highlight the advantages of the proposed scheme, relevant experiments for the DFB-LD under fiber mirror feedback and fiber Bragg grating feedback are also performed and compared.

Concealment of time delay signature of chaotic output in a slave semiconductor laser with chaos laser injection

A simple schemefor generating chaotic signals with concealed time-delay signature (TDS) is proposed and experimentally demonstrated. The architecture of the system is based on a semiconductor laser (SL) under multi-path optical feedback (MPOF) provided by a 2 × 2 fiber coupler (FC) and a fiber mirror (FM). The results show that SLs with MPOF are more beneficial for achieving TDS suppressed chaotic output than those with single optical feedback (SOF) systems. In addition, the TDS of the chaotic signal generated by the SL under MPOF is insensitive to the length deviation of the feedback cavities, and therefore such a scheme is convenient for practical application. Furthermore, the influence of FC coupling ratio in the MPOF module, feedback strength, and bias current of the SL on the TDS is comprehensively studied. By selecting suitable parameters, the TDS of the chaotic signal generated by the SL under MPOF can be completely concealed. Finally, permutation entropy (PE) is adopted to evaluate the complexity of the generated chaotic signal, and the result demonstrates that the suppression of TDS does not lead to the degradation of complexity for the proposed scheme.

In this paper, we investigate the possibility of using optical injection to efficiently suppress the time-delay (TD) signatures of chaotic signals in a large experimentally accessible parameter range of semiconductor ring lasers (SRLs). We also study how this optical injection can improve the signal bandwidths. The injection signal is obtained from a master SRL with either optical self- or cross-feedback. For optical self-feedback configurations, it is found that the suppression of TD signatures is similar to what has been found in injected Fabry–Perot semiconductor lasers, i.e., narrow range of parameters with respect to the detuning and injection strengths. For cross-feedback configurations, however, the TD signatures can be suppressed in a wide range of parameters; meanwhile, the bandwidths are significantly improved for the same range of parameters. This is particularly useful for the security in chaos-based communications and also for generating random bits with improved properties.

We numerically investigate the time delay signature (TDS) properties of a chaotic semiconductor laser subject to nonlinearly-modulated feedback. In the proposed scheme, two semiconductor lasers that play the role of a built-in nonlinear modulator is positioned in the external cavity of the chaos generator laser, which forms a structure of semiconductor laser ring. The simulation results show that the TDS of the chaos generator in the laser ring is quite smaller than that of the chaos induced by the conventional optical feedback. Moreover, the TDS embedded in the intensity and phase time-series of the chaotic signal can be further concealed, by introducing an additional optical feedback into the built-in nonlinear modulator.

Conceal time-delay signature of chaotic vertical-cavity surface-emitting lasers by variable-polarization optical feedback

Aiming at the problems of limited bandwidth and obvious time-delay signature (TDS) of monolithically integrated chaotic semiconductor lasers, this paper proposes a four-segment InP-based monolithically integrated chaotic semiconductor laser. The laser structure is composed of a distributed feedback (DFB1) laser region, a semiconductor optical amplifier (SOA) region, a distributed Bragg reflector (DBR) grating region and a distributed feedback (DFB2) laser region. Mutual injection between the two DFB laser regions effectively enhances the bandwidth of power spectrum. The DBR grating region is used to form a complex multi-feedback cavity to suppress the TDS caused by the fixed external cavity structure. The simulation results show that the chaotic signal with the standard bandwidth of 29.2 GHz and the TDS value of 0.094 is generated. The research in this paper can further improve the performance of monolithically integrated chaotic laser, and provide a high-quality chaotic laser source for secure optical communication, chaotic lidar and distributed optical fiber sensing.

To quantitatively evaluate the time-delay (TD) signatures of chaotic signals generated by vertical-cavity surface-emitting lasers (VCSELs) with polarization-rotated optical feedback (PROF), we propose four cases of resolution coefficients R based on correlation functions. The resolution coefficient characteristics for the x-polarization (XP) mode, y-polarization (YP) mode and the total output are considered. The dependences of R on the feedback strength and feedback delay are discussed and compared carefully. The two-dimensional maps of R show that the TD signatures for the single polarization mode (i.e., XP or YP mode) are much more difficult to retrieve than those for the total output in the entire parameter space. Thus, by using single polarization mode as a chaotic carrier, the TD signatures are extremely difficult to be identified, which contributes a lot in the security-enhanced VCSELs-based chaotic optical communication systems.

The effect of periodic phase modulation of light on a chaotic external-cavity semiconductor laser working in a regime of low-frequency fluctuations (LFFs) is studied numerically. It is observed that the phase modulation changes the time period between consecutive dropouts in the emitted laser intensity. A new variable ΦL is defined as the phase of the laser's LFFs, which increases in time with 2π after each power dropout event. The phase ΦPM of the periodic phase modulator is unfolded on the real axis and increases linearly at a rate given by the modulating frequency. It is shown that the phase difference between the laser and the modulator ΔΦ(t)=mΦL(t)−nΦPM(t), where m and n are integers, remains constant in time, leading to phase-synchronized states, for specific values of the modulating frequency and amplitude.

Chaotic laser generated by semiconductor lasers has been attached great importance in areas of secure communication, random number generation, and optical sensing. This article mainly reviews the research progress in the performance improvement of chaotic laser, including bandwidth enhancement and time‐delay signature suppression. First, the methods of chaotic laser generation and the existing problems are briefly introduced. Then, innovative methods for the optimization of chaotic characteristics are analyzed and discussed. The integrated chaotic semiconductor lasers are also discussed. At last, avenues for the development trends and future applications of chaotic semiconductor lasers are prospected.

The concealment of time delay signature is one of the most critical issues in delayed-feedback optical chaotic systems. In this paper, we present a method to address this problem without significantly complicating the original architecture. By properly introducing a dispersion module into the feedback loop, a different frequency component of chaotic spectrum will be subject to different time delay in feedback loop, thus destroying the generation mechanism of time delay signature (TDS). The concealment performance is verified by both time-domain and spectrum analysis methods. The relation between the amplitude of TDS and cavity dispersion value is evaluated in detail. Since the dispersion module has no filtering effect, the chaotic bandwidth will not be affected during the TDS concealment process. This proposal solves the security problem of feedback chaotic systems without introducing much extra effort in implementation, and the underlying principle is not restricted to all-optical chaotic systems but applies for other optelectrical chaotic dynamics, which include a feedback cavity.

Optical chaotic communication using correlation demodulation between two synchronized chaos lasers

The random time delay brings great challenges to the controller design for the Internet-based teleoperation system, the worst case is that it can destabilize the Internet-based teleoperation system. This paper first gives a brief historical preview of bilateral passive control theory, and then a direct passive scattering transform method, extending the passive bilateral control method to the Internet-based random time delay teleoperation system, is advanced to guarantee the teleoperation system's stability with any non-symmetric random network time delay. Finally, a virtual master-slave manipulator bilateral control scheme is designed for mobile robot teleoperation based on the passive scattering transformation method, and simulations are carried out to verify the results at the end of this paper. © 2012 IEEE.