Bandwidth Of Chaos Research Articles

Concealment of Chaos Time-Delay Signature in Three-Cascaded Vertical-Cavity Surface-Emitting Lasers

The time-delay signature and the chaos bandwidth in three-cascaded vertical-cavity surface-emitting lasers have been investigated experimentally. A peak value of autocorrelation coefficient at the feedback round trip time and the ratio between this peak value and its background are used to quantitatively identify the time-delay (TD) signature of chaos. A new concept—peak to side-peak ratio is introduced for better quantification of the TD signature concealment. The peak to side-peak ratio is defined as the ratio between the peak value of autocorrelation coefficient at the TD and the peak value at a delay time other than the TD and zero delay time. Three injection cases, namely, small bandwidth, intermediate bandwidth, and wide bandwidth of the injecting chaos signals, have been used to study the effect of the bandwidth of the injecting chaos on the TD concealment. The experimental results show that the time-delay signature can be totally concealed in the slave laser subject to the intermediate bandwidth of chaotic optical injection over a wide frequency detuning range.

Photonic generation of polarization-resolved wideband chaos with time-delay concealment in three-cascaded vertical-cavity surface-emitting lasers.

Optical chaos generated by chaotic lasers has been widely used in several important applications, such as chaos-based communications and high-speed random-number generators. However, these applications are susceptible to degradation by the presence of time-delay (TD) signature identified from the chaotic output. Here we propose to achieve the concealment of TD signature, along with the enhancement of chaos bandwidth, in three-cascaded vertical-cavity surface-emitting lasers (VCSELs). The cascaded system is composed of an external-cavity master VCSEL, a solitary intermediate VCSEL, and a solitary slave VCSEL. Through mapping the evolutions of TD signature and chaos bandwidth in the parameter space of the injection strength and frequency detuning, photonic generation of polarization-resolved wideband chaos with TD concealment is numerically demonstrated for wide regions of the injection parameters.

Enhanced Flat Broadband Optical Chaos Using Low-Cost VCSEL and Fiber Ring Resonator

Low-cost vertical-cavity surface-emitting laser (VCSEL) and semiconductor optical amplifier (SOA) have been explored experimentally in a setup with a fiber ring resonator to generate flat broadband chaos. The effect of the frequency detuning between the fiber grating frequency and the free-running VCSEL's frequency, and the bias current of the SOA on the bandwidth of chaos has been studied. The results show that the bandwidth of the chaos is strongly dependent on the frequency detuning and the bias current of the SOA. Using the conventional definition of chaos bandwidth, the bandwidth of chaos with the fiber ring resonator can be about three times greater than that with a conventional mirror. In order to consider the flatness of the chaos spectra, the flat bandwidth of chaos is redefined as the frequency range between dc and the frequency where the power fluctuation is within ±3 dB. With this definition, the flat bandwidth of chaos with the fiber ring resonator is >10 times higher than that with conventional mirror feedback.

Fast phase response and chaos bandwidth enhancement in semiconductor lasers subject to optical feedback and injection.

We numerically show the quantitative relation between the chaos bandwidth enhancement and fast phase dynamics in semiconductor lasers with optical feedback and optical injection. The injection increases the coupling between the intensity and the phase leading to a competition between the relaxation oscillation (RO) frequency and the intrinsic response frequency of the phase. For large feedback strengths, it is found that the chaos bandwidth is determined by the intrinsic phase response frequency. For smaller feedback strengths, the system is not chaotic and its bandwidth is determined by the RO frequency.

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.

Heterodyne Random Bit Generation Using an Optically Injected Semiconductor Laser in Chaos

Heterodyne generation of parallel random bit streams from chaotic emission of an optically injected semiconductor laser is investigated. The continuous-wave optical injection invokes chaotic dynamics in the laser. The broadband chaotic emission is detected through optical heterodyning and electrical heterodyning into different channels. The channels digitize the signals into parallel independent random bit streams. Because of efficient utilization of different portions of the chaos bandwidth, heterodyne detections enable parallel generation of random bit streams, offer high total output bit rates, and require no high-bandwidth analogue-to-digital converters. In the experiment, two optical heterodyne channels and four electrical heterodyne channels are implemented. Each channel is required to digitize only 2.5 GHz of a much broader chaos bandwidth. The sampling rate is 10 GHz with five least significant bits selected from every 8-bit sample. The total output bit rate reaches 100 Gb/s and 200 Gb/s for optical and electrical heterodyning, respectively. The standard test suite of the National Institute of Standards and Technology verifies the randomness of both individual and interleaved output bit streams.

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.

Loss of Time Delay Signature in Broadband Cascade-Coupled Semiconductor Lasers

Numerical and experimental investigations of time delay concealment and chaos bandwidth enhancement in three semiconductor lasers with a cascade scheme are presented. The cascade system is composed of an external-cavity master laser, a solitary intermediate laser, and a solitary slave laser. The results show that the first two lasers are used for concealing time delay signature, and the last two lasers are used for bandwidth enhancement. For large parameter regions of injection strength and frequency detuning, time delay concealment and bandwidth enhancement of chaotic signals are simultaneously achieved by adopting the simple cascade configuration.

Effect of transmission fiber on chaos communication system based on erbium-doped fiber ring laser

We report a numerical study on the deterioration of eye opening penalty (EOP) and bit error rate (BER) of the received message from a communication system based on chaos generated in erbium-doped fiber ring laser (EDFRL) under the influence of amplifier noise, transmission fiber dispersion, and nonlinearity. Three different transmission fibers with practical parameters are considered. We find that in order to realize high-bit-rate chaotic communication, the residual fiber dispersion should be kept as small as possible. Furthermore, if the transmission power is optimized to balance the effect of amplifier noise and fiber nonlinearity, a message with bit rate on the order of several gigabits/second can be transmitted over hundreds of kilometers with acceptable BER deterioration. A comparison between open- and closed-loop structures is also conducted. Moreover, we find that the maximum bit rate is limited by the bandwidth of the chaos determined by the nonlinearity of the fiber ring laser and the laser configuration. It is possible to optimize these parameters to provide the maximum chaos bandwidth and hence the maximum message bit rate.