Zhu-Qiang Zhong

Time-Delay Signature Suppression of Polarization-Resolved Chaos Outputs from Two Mutually Coupled VCSELs

Time-delay (TD) signature performances of polarization-resolved chaos outputs from two mutually coupled vertical-cavity surface-emitting lasers (MC VCSELs) are investigated through detailed simulations by means of self-correlation function (SF) and permutation entropy (PE). The results show that, under suitable coupling strength (η) and frequency detuning (Δf) between the two MC VCSELs, the TD signatures of two polarization modes from each VCSEL can be suppressed simultaneously. Furthermore, based on the map of TD signature evolution in the parameter space of Δf and η, the optimal parameter region of TD signature suppression has been determined.

Direct generation of broadband chaos by a monolithic integrated semiconductor laser chip

A solitary monolithic integrated semiconductor laser (MISL) chip with a size of 780 micrometer is designed and fabricated for broadband chaos generation. Such a MISL chip consists of a DFB section, a phase section and an amplification section. Test results indicate that under suitable operation conditions, this laser chip can be driven into broadband chaos. The generated chaos covers an RF frequency range, limited by our measurement device, of 26.5GHz, and possesses significant dimension and complexity. Moreover, the routes into and out of chaos are also characterized through extracting variety dynamical states of temporal waveforms, phase portraits, RF spectra and statistical indicators.

Tbits/s physical random bit generation based on mutually coupled semiconductor laser chaotic entropy source.

Using two mutually coupled semiconductor lasers (MC-SLs) outputs as chaotic entropy sources, a scheme for generating Tbits/s ultra-fast physical random bit (PRB) is demonstrated and analyzed experimentally. Firstly, two entropy sources originating from two chaotic outputs of MC-SLs are obtained in parallel. Secondly, by adopting multiple optimized post-processing methods, two PRB streams with the generation rate of 0.56 Tbits/s are extracted from the two entropy sources and their randomness are verified by using NIST Special Publication 800-22 statistical tests. Through merging the two sets of 0.56 Tbits/s PRB streams by an interleaving operation, a third set of 1.12 Tbits/s PRB stream, which meets all the quality criteria of NIST statistical tests, can be further acquired. Finally, after additionally taking into account the restriction of the min-entropy, the generation rate of two sets of PRB stream from the two entropy sources can still attain 0.48 Tbits/s, and then a third set of merging PRB stream is 0.96 Tbits/s. Moreover, for the sequence length of the order of 10 Gbits, the statistical bias and serial correlation coefficient of three sets of PRB streams are also analyzed.

Time-delay signature of chaos in 1550 nm VCSELs with variable-polarization FBG feedback.

Based on the framework of spin-flip model (SFM), the output characteristics of a 1550 nm vertical-cavity surface-emitting laser (VCSEL) subject to variable-polarization fiber Bragg grating (FBG) feedback (VPFBGF) have been investigated. With the aid of the self-correlation function (SF) and the permutation entropy (PE) function, the time-delay signature (TDS) of chaos in the VPFBGF-VCSEL is evaluated, and then the influences of the operation parameters on the TDS of chaos are analyzed. The results show that the TDS of chaos can be suppressed efficiently through selecting suitable coupling coefficient and feedback rate of the FBG, and is weaker than that of chaos generated by traditional variable-polarization mirror feedback VCSELs (VPMF-VCSELs) or polarization-preserved FBG feedback VCSELs (PPFBGF-VCSELs).

Polarization-resolved time-delay signatures of chaos induced by FBG-feedback in VCSEL

Polarization-resolved chaotic emission intensities from a vertical-cavity surface-emitting laser (VCSEL) subject to feedback from a fiber Bragg grating (FBG) are numerically investigated. Time-delay (TD) signatures of the feedback are examined through various means including self-correlations of intensity time-series of individual polarizations, cross-correlation of intensities time-series between both polarizations, and permutation entropies calculated for the individual polarizations. The results show that the TD signatures can be clearly suppressed by selecting suitable operation parameters such as the feedback strength, FBG bandwidth, and Bragg frequency. Also, in the operational parameter space, numerical maps of TD signatures and effective bandwidths are obtained, which show regions of chaotic signals with both wide bandwidths and weak TD signatures. Finally, by comparing with a VCSEL subject to feedback from a mirror, the VCSEL subject to feedback from the FBG generally shows better concealment of the TD signatures with similar, or even wider, bandwidths.

Polarization Dynamics of 1550-nm VCSELs Subject to Polarization-Preserved FBG Feedback

We experimentally investigated the polarization dynamics of a 1550-nm vertical-cavity surface-emitting laser (VCSEL) subject to polarization-preserved and wavelength selective feedback, which is provided by a feedback loop composed of a variable attenuator (VA), a polarization controller, and a fiber Bragg grating. First, for given bias currents, the evolutions of the polarization behaviors with the feedback strength are inspected through adjusting the attenuation rate of the VA, and several typical evolution routes are found for different bias currents. Second, through fixing the attenuation rate of VA at minimum and continuously increasing the bias current, multiple current regions for two-mode coexistence are found, and the dominant polarization mode is multiply switched between two orthogonal linear polarization (LP) modes. Finally, the polarization-resolved dynamical characteristics of the VCSEL operating at two-mode coexistence are studied in detail, and the results demonstrate that under suitable operation parameters, both two orthogonal LP modes can simultaneously oscillate at chaotic state.

Generation of polarization-resolved wideband unpredictability-enhanced chaotic signals based on vertical-cavity surface-emitting lasers subject to chaotic optical injection.

A system framework is proposed and analyzed for generating polarization-resolved wideband unpredictability-enhanced chaotic signals based on a slave vertical-cavity surface-emitting laser (S-VCSEL) driven by an injected optical chaos signal from a master VCSEL (M-VCSEL) under optical feedback. After calculating the time series outputs from the M-VCSEL under optical feedback and the S-VCSEL under chaotic optical injection by using the spin-flip model (SFM), the unpredictability degree (UD) is evaluated by permutation entropy (PE), and the bandwidth of the polarization-resolved outputs from the M-VCSEL and S-VCSEL are numerically investigated. The results show that, under suitable parameters, both the bandwidth and UD of two polarization components (PCs) outputs from the S-VCSEL can be enhanced significantly compared with that of the driving chaotic signals output from the M-VCSEL. By simulating the influences of the feedback and injection parameters on the bandwidth and UD of the polarization-resolved outputs from S-VCSEL, related operating parameters can be optimized.

Dynamics of a Monolithically Integrated Semiconductor Laser Under Optical Injection

Influences of external optical injection on the nonlinear dynamics of a three-section monolithically integrated semiconductor laser (MISL) are investigated experimentally. The results show that, for the solitary three-section MISL, diversely dynamical states including the stable state as well as the so-called period-one, period-two, multi-period, and chaotic states can be observed through adjusting the currents of the gain section (IG) and the phase section (IP). However, the chaotic operation region of the solitary MISL in the parameter space of IG and IP is very small and found to exist when 21.28 mA <; IG <; 26.40 mA and 31 mA <; IP <; 37 mA. After introducing an external optical injection, the MISL originally operating at other dynamical states can always be driven into chaotic state under suitable injection strength and frequency detuning, and a relatively large IG will be helpful for obtaining broad and continuous chaotic regions in the parameter space of injection strength and frequency detuning.

Experimental Observation of Current-Induced Bistability in a Semiconductor Laser With Positive Optoelectronic Feedback

Bistability in a single-mode distributed feedback semiconductor laser (DFB-SL) with positive optoelectronic feedback (POEF) is observed experimentally. For a given biased current, the output dynamical states of DFB-SL with POEF are critically dependent on variation routes of the laser-biased current. Different routes of biased current variation lead to a bistability characterized by states of different time series, power spectra, and phase portraits.

Tunable Broadband Chaotic Signal Synthesis From a WRC-FPLD Subject to Filtered Feedback

The synthesis of wavelength-tunable broadband chaotic signals is experimentally demonstrated by using a weak-resonant-cavity Fabry–Perot laser diode (WRC-FPLD) subject to filtered feedback. To perform the broadband tunability from 1544 to 1556 nm, a tunable optical filter is used for providing filtered feedback to drive the laser into chaos. With adjusting the central wavelength of the filter, about 20 longitudinal modes of the WRC-FPLD can individually realize the chaotic output within the tunable range of the filter. Moreover, by suitably selecting the feedback power, the chaos signal with a flat power spectrum and broad bandwidth up to ~30.0 GHz can be achieved.