Chaotic Vertical-cavity Surface-emitting Lasers Research Articles

Observation of additional delayed-time in chaos synchronization of uni-directionally coupled VCSELs.

We report an experimental and numerical investigation on the existence of additional delayed-time in chaos synchronization of two uni-directionally coupled vertical-cavity surface-emitting lasers (VCSELs) for the first time. Under a generalized synchronization scenario, we demonstrate that there exists an additional delayed-time in addition to the time-of-flight between the two coupled VCSELs. The cross-correlation function analysis has been used as a method to determine the coupling delay and synchronization quality between two uni-directionally coupled chaotic VCSELs. We show that the injection strength significantly influences the additional delayed-time, and the injection strength analysis eventually substantiates the existence of additional delayed-time between the coupled lasers in experiments. The experimental results are in accordance with the numerical results. Additionally, we numerically study the effect of laser's internal parameter mismatches on additional delayed-time and synchronization quality between the lasers.

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

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.

Flat Broadband Chaos in Vertical-Cavity Surface-Emitting Lasers Subject to Chaotic Optical Injection

Four schemes for the generation of chaos in vertical-cavity surface-emitting lasers (VCSELs) by optical feedback and/or optical injection have been evaluated experimentally. The experimental results show that the preferred scheme is to effect chaotic optical injection into a stable operating VCSEL. This is an effective method to generate a flat broad-bandwidth chaotic signal with the bandwidth of the chaotic signal being enhanced by more than four times compared with that obtained using optical feedback. In addition, the chaotic signal generated by chaotic optical injection has a flat power spectrum. If a continuous wave (CW) signal is injected into a chaotic VCSEL, the bandwidth of chaos can also be enhanced, but less than that using the preferred scheme. Moreover, the spectrum in this case is not very flat. For CW optical injection into a CW VCSEL, the power spectrum of the chaos has many peaks and so is not suitable for applications in chaotic optical communication and random number generation. Injecting chaotic light into a chaotic VCSEL provides little enhancement of the chaos bandwidth.

Message Encoding/Decoding Using Unpredictability-Enhanced Chaotic VCSELs

Successful message encoding/decoding in an unpredictability-enhanced chaotic vertical-cavity surface-emitting lasers (VCSELs) system is achieved numerically via the chaos-shift-keying technique. Both VCSELs are subject to variable-polarization optical feedback (VPOF), and polarization-preserved optical injection is adopted. Better decoding performance is achieved by choosing proper polarizer angle, and the security is, to some extent, enhanced owing to the VPOF.

Bidirectional Dual-Channel Communication Based on Polarization-Division-Multiplexed Chaos Synchronization in Mutually Coupled VCSELs

A bidirectional dual-channel chaos communication scheme based on the synchronization in the linear polarization modes of vertical-cavity surface-emitting lasers (VCSELs) is introduced numerically. In this scheme, two mutually coupled VCSELs are identically driven by a third chaotic VCSEL. It is demonstrated that high quality chaos synchronization in both the XP and YP modes can be achieved simultaneously. With the polarization-division-multiplexing technology and chaos modulation technology, the proposed scheme affords bidirectional dual-channel message transmission on one public link.

Numerical characterization of time delay signature in chaotic vertical-cavity surface-emitting lasers with optical feedback

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 (γs). 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).

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

Time-delay (TD) signatures of chaotic outputs generated by vertical-cavity surface-emitting lasers (VCSELs) are investigated quantitatively by using two proposed estimators, the peak signal to mean ratio (PSMR) calculated based on the auto-correlation function, and the valley signal to mean ratio (VSMR) calculated based on an information-theory-based estimator, the permutation entropy (PE) function. The VCSELs subject to variable-polarization optical feedback (VPOF) are considered, and the effects of polarizer angle, feedback strength, feedback delay and injection current are discussed. When the feedback strength is small, the TD signatures for the fully developed chaotic outputs of VCSELs can be well concealed for all polarizer angles. While for large feedback strength, the TD signatures cannot be concealed for any polarizer angle. However, for moderate feedback strength, the TD signatures of VCSELs with VPOF at intermediate polarizer angles can be better concealed than those of VCSELs with conventional polarization-selected optical feedback. Thus, the VCSELs with VPOF are extremely interesting for the security-enhanced chaotic optical communication systems.

Time-Delay Signature of Chaotic Vertical-Cavity Surface-Emitting Lasers with Polarization-Rotated Optical Feedback

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.

Synchronization of chaotic VCSELs by external chaotic signal parameter modulation

Synchronization of chaotic vertical-cavity surface-emitting lasers (VCSELs) is achieved by external chaotic signal modulation successfully. Simulation indicates that we can get chaos synchronization if the intensity of external chaotic signal is large enough. First of all, we use direct current modulation to achieve the chaos of VCSELs, and determine the laser's chaotic state by analyzing time series of the output and the corresponding power spectrum. And then we achieve synchronization of the two chaotic systems by external chaotic signal parameter modulation. We also find that the larger the modulation intensity is, the easier it is to achieve synchronization for chaotic VCSELs. This approach can also be applied to systems with a number of modulated lasers.

Nonlinear dynamics of the polarization of multitransverse mode vertical-cavity surface-emitting lasers under current modulation

In this paper we report on a theoretical investigation of the nonlinear dynamics of the polarization of multitransverse mode vertical-cavity surface-emitting lasers (VCSELs) under current modulation. Special attention is given to the comparison with a previously studied case of single-transverse mode VCSEL emitting in two orthogonal polarizations. The consideration of spatial effects in VCSEL modifies the polarization dynamics that accompanies the period doubling route to chaos for large modulation amplitudes. Depending on the modulation parameters, the excitation of a higher order transverse mode may either induce chaotic pulsing in an otherwise regularly pulsating VCSEL, or induce a time-periodic pulsing dynamics in an otherwise chaotic VCSEL. Bifurcation diagrams obtained for different modulation frequencies, several values of the dichroism, and different transverse mode characteristics allow us to identify the different scenarios of polarization dynamics in a directly modulated VCSEL. Temporal analysis of carrier number radial profile reveals considerable changes for the multitransverse mode case only constituting the physical origin of the reported changes in the temporal and polarization dynamics.

Polarization-preserved and polarization-rotated synchronization of chaotic vertical-cavity surface-emitting lasers

The synchronization process in a vertical-cavity surface-emitting laser (VCSEL) based master-slave configuration, where the master laser has been destabilised by optical feedback from an external mirror, has been investigated. The dynamics of both VCSEL polarization modes have been modeled and used to study the chaos synchronization mechanism in both polarization preserved, and polarization rotated unidirectionally coupled master and slave configurations. Two types of synchronization have been identified in the polarization rotated case, one of which may afford an opportunity to effect multiplexed message encoding using orthogonal laser modes.

Experimental Demonstration of VCSEL-Based Chaotic Optical Communications

Message encoding and decoding using chaotic vertical-cavity surface-emitting lasers (VCSELs) has been experimentally demonstrated. A 200-MHz message has been successfully encoded in an external-cavity VCSEL transmitter and decoded in a stand-alone VCSEL receiver. Message recovery has been achieved with 9-dB signal-to-noise ratio.

Modeling of optical synchronization of chaotic external-cavity VCSELs

The performance of a master-slave configuration for effecting the synchronization of chaotic vertical-cavity surface-emitting lasers (VCSELs) is studied using numerical simulations. The dynamical evolution of optically coupled VCSELs is examined using a traveling wave model which is valid in the strong optical feedback regime. It is shown that the proposed configuration is capable of effecting synchronization in a robust manner. The opportunity for exploiting synchronized chaos in secure optical communication systems is indicated.