This paper reports the experimental investigation of two different approaches to random bit generation based on the chaotic dynamics of a semiconductor laser with optical feedback. By computing high-order finite differences of the chaotic laser intensity time series, we obtain time series with symmetric statistical distributions that are more conducive to ultrafast random bit generation. The first approach is guided by information-theoretic considerations and could potentially reach random bit generation rates as high as 160 Gb/s by extracting 4 bits per sample. The second approach is based on pragmatic considerations and could lead to rates of 2.2 Tb/s by extracting 55 bits per sample. The randomness of the bit sequences obtained from the two approaches is tested against three standard randomness tests (ENT, Diehard, and NIST tests), as well as by calculating the statistical bias and the serial correlation coefficients on longer sequences of random bits than those used in the standard tests.

Two approaches for ultrafast random bit generation based on the chaotic dynamics of a semiconductor laser

Chaotic dynamics of a semiconductor laser subject to optical feedback from a frequency-detuned fiber Bragg grating (FBG) is investigated experimentally and numerically. Although the FBG is similar to a mirror in perturbing the laser into chaos, it is not the same as a mirror because it provides spatially distributed reflections. Such distributed reflections effectively suppress the undesirable time-delay signature (TDS) contained in the autocorrelation function of the chaotic intensity time-series. The investigation shows that the best suppression of TDS is attained when the FBG is positively detuned from the free-running laser frequency. The TDS suppression is due to dispersion at frequencies near an edge of the main lobe of the FBG reflectivity spectrum. The suppression prefers the FBG at a positive detuning frequency because the laser cavity is red-shifted by the antiguidance effect. Numerically, the dynamical behavior is mapped in the parameter space of detuning frequency and feedback strength, where wide regions of chaos are identified. Experimentally, the TDS suppression by FBG feedback is demonstrated for the first time. The positively detuned FBG suppresses the TDS by over ten times to below 0.04 in the experiments.

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Chaotic Time-Delay Signature Suppression in a Semiconductor Laser With Frequency-Detuned Grating Feedback

Both the time-delay signature and the bandwidth of chaos have been investigated in four schemes of chaos generation in vertical-cavity surface-emitting lasers (VCSELs). The results show that suppression of the TD signature and the enhanced bandwidth of chaos can be obtained simultaneously in two schemes - a chaotic beam unidirectionally injected into a CW VCSEL and a CW VCSEL mutually coupled to a chaotic VCSEL. However, suppression of the TD signature and the enhanced chaos bandwidth cannot be achieved simultaneously in two other schemes - a CW optical beam unidirectionally injected into a chaotic VCSEL and two mutually coupled CW VCSELs.

Wideband Chaos With Time-Delay Concealment in Vertical-Cavity Surface-Emitting Lasers With Optical Feedback and Injection

Time-delay signature of chaos in mutually coupled vertical-cavity surface-emitting lasers subject to polarization rotated optical injection has been investigated experimentally. Autocorrelation function and permutation entropy are used to quantitatively identify the time-delay signature of chaos. The experiment results show that the time-delay signature is sensitive to the polarization rotated angle. Minimum time-delay signature has been observed in the intermediate polarization rotated angle for the lower bias current. This is in good agreement with the theoretical prediction. At higher bias currents, the lower time-delay signature has been obtained with parallel optical injection.

Experimental study of time-delay signature of chaos in mutually coupled vertical-cavity surface-emitting lasers subject to polarization optical injection

The time-delay signature (TDS) of chaos output in a 1550 nm vertical-cavity surface-emitting laser (VCSEL) subject to fiber Bragg grating (FBG) feedback is investigated experimentally. Autocorrelation function (ACF) and mutual information (MI) are used for quantitatively identifying the TDS of chaos. For various bias currents, the TDS evolution with the feedback strength is different, as the FBG provides wavelength-selective feedback. Furthermore, based on the TDS map of the FBG feedback VCSEL (FBGF-VCSEL) in the parameter space of feedback strength and bias current, the optimal TDS suppression regions, where the dominant polarization mode of FBGF-VCSEL locates at the edge of the main lobe of FBG reflection spectrum, have been determined. Finally, for comparative purpose, the TDS of chaos in mirror feedback VCSEL (MF-VCSEL) also has been presented, and the results show that an FBGF-VCSEL possesses better TDS suppression performance than an MF-VCSEL.

Experimental investigation on the time-delay signature of chaotic output from a 1550 nm VCSEL subject to FBG feedback

Time-delay (TD) signature concealment in a chaotic vertical-cavity surface-emitting laser (VCSEL) is studied experimentally. The VCSEL is subject to variable optical polarization angle of feedback along with variable optical feedback strength and bias current. The TD signature concealment is determined through the use of autocorrelation and information theory-based permutation entropy functions. It is found that the TD signature is concealed at low feedback of strength of order -18 dB. At moderate feedback strength, the TD signature is sensitive to the rotation of optical polarization angle of feedback with TD concealment being observed for polarization angles in the range from 45° to 90°.

Experimental Investigations of Time-Delay Signature Concealment in Chaotic External Cavity VCSELs Subject to Variable Optical Polarization Angle of Feedback

K. K. Shore

Papers

Experimental Investigations of Time-Delay Signature Concealment in Chaotic External Cavity VCSELs Subject to Variable Optical Polarization Angle of Feedback