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We propose and demonstrate a chaos generation scheme that conceals the time-delay signature (TDS) by injecting the output of an external-cavity semiconductor laser (ECSL) into a two-element phased array (ECSL-TPA). We consider the case of the ECSL output with obvious TDS, and evaluate the performance of the ECSL-TPA with four waveguide structures in TDS suppression. The numerical results demonstrate that the ECSL-TPA allows for desired TDS suppression in wide parameter space regardless of the waveguide structures considered. However, different TDS behavior exists for these structures, which provides guidance for future design and optimization of the proposed ECSL-TPA. Additionally, the influence of some key parameters including the injection rate, frequency detuning, laser separation ratio, pump rate, linewidth enhancement factor, and frequency offset on TDS is systematically studied. Appropriate settings of these key parameters are beneficial for TDS suppression and thus optimizing optical chaos. The phased array can be extended to include a large number of elements and thus generates multiple independent chaotic signals with indistinguishable TDS in parallel, paying the way for chaos-based applications that require independent, high-quality chaos simultaneously. [ABSTRACT FROM AUTHOR]

The important role of parameters in master laser with optical feedback for the elimination of time-delay (TD) signature in semiconductor laser subject to chaotic optical injection is investigated systemically. The experimental results show that TD signature suppressed chaotic signals can be credibly generated by increasing the feedback strength of the master laser, which is quite different from the trends observed in semiconductor laser (SL) with optical feedback. Systematically numerical analysis is also carried out as a validation, and it is shown that with low bias current and strong feedback strength, parameter regions contributing to successful TD suppression are much wider. Furthermore, it is shown that the influence of frequency detuning in TD concealment will change with the increase of feedback strength. All the numerical results are in perfect accordance with experimental observation. [ABSTRACT FROM AUTHOR]

High speed random number generation (RNG) utilizing a nonlinear optoelectronic oscillator (OEO) is explored experimentally. It has been found that by simply adjusting either the injected optical power or the gain of the modulator driver, low complexity dynamics such as square wave, and more complex dynamics including fully developed chaos can be experimentally achieved. More importantly, physical RNG based on high-speed-oscilloscope measurements and pseudo RNG based on post-processing are implemented in this paper. The generated bit sequences pass all the standard statistical random tests, indicating that fast physical and pseudo RNG could be achieved based on the same OEO entropy source. Our results could provide further insight into the implementation of RNG based on chaotic optical systems. [ABSTRACT FROM AUTHOR]

We study experimentally and theoretically the permutation entropy (PE) of the optical intensity $I(t)$ of an external-cavity semiconductor distributed feedback laser in the coherence collapse regime. Our PE analysis allows us to uncover the intrinsic dynamical complexity at multiple timescales of the delayed-feedback system, as well as to investigate how the experimental observations can be determined by modeling. An overall good agreement between experiment and theory corroborates the effectiveness of the Lang–Kobayashi model, though the model underestimates the entropy on the timescale of the relaxation oscillations and can lead to a time-delay signature that is less evident than in experiment, indicating a potential vulnerability of chaos encryption. This provides a critical test of the standard theoretical framework in which chaotic external-cavity semiconductor lasers are understood. [ABSTRACT FROM AUTHOR]

Abstract: We study the influence of statistical distribution properties on ultrafast random-number generators (RNGs) using chaotic laser system consisting of a semiconductor laser subject to dual-chaotic optical injections. Two completely different distributions are considered in this paper: one is a long-tailed distribution, and the other is a well-fitted Gaussian distribution. The numerical results show that, using minimum post-processing, symmetric distribution allows for the extraction of 4 least significant bits (LSBs) per sample; while for the asymmetric distribution the produced sequence based on the last LSB still exhibits certain bias. In other words, the important role of symmetric distribution in fast generation of random bits using multi-bit extraction scheme is demonstrated in numerical simulations. [Copyright &y& Elsevier]

Abstract: We have discussed numerically the characterization of time delay (TD) signature in a chaotic vertical-cavity surface-emitting laser (VCSEL) subject to optical feedback, where the results for polarization-preserved optical feedback (PPOF) and polarization-rotated optical feedback (PROF) are presented comparatively. It is found that, when the feedback strength is moderate, TD signature can be retrieved successfully by analyzing the total intensity, intensities of x-linearly polarized (x-LP) mode, and y-LP mode in a VCSEL with PPOF; while for PROF case, the intensity amplitude peaks in autocorrelation function (ACF)/delayed mutual information (DMI) of the two orthogonal LP modes are located at multiple values of TD. Furthermore, the influence of the feedback strength and time delay has been investigated. The results show that, a further increase of the feedback strength (roughly>40ns−1) leads to the successful identification of the TD signature in the two LP mode intensities of VCSELs with PROF. It is also observed that the estimation of TD depends significantly on the value of the spin–flip relaxation rate (γ s ). For low relaxation rates the TD signature could be much easier to be revealed by the two LP modes than that for large values of relaxation rate in VCSELs with PROF. Finally the influence of injection current and the device linear anisotropies is investigated, and the results of TD estimation are confirmed by calculating permutation entropy (PE). [Copyright &y& Elsevier]

A critical issue in the usage of permutation entropy (PE) for the complexity measure is the selection of the embedding time delay. It is known that PE evaluated with multiple delays can help us gain additional insight into the unpredictability degree at different timescales. In this paper, we investigate the unpredictability degree of the optical chaos in a special type of small networks of semiconductor lasers (SLs) using permutation entropy. These laser systems, such as SLs subject to delayed self-feedback, mutually delay-coupled SLs, and a ring configuration consisting of three unidirectionally delay-coupled SLs, exhibit similar correlation properties characterized by autocorrelation function. The results show that, for a given embedding time delay, the curves for the PE follow the same trends upon monotonically increasing the coupling rate, regardless of how complex the system structure actually is. Furthermore, the results corroborate the effectiveness of PE as an unpredictability measure for small networks consisting of SLs with a single coupling delay, which can be extracted from the intensity autocorrelation function. Finally, we show an example for generating unpredictability-enhanced optical chaos based on small networks of SLs. [ABSTRACT FROM AUTHOR]