David W. Sukow

Fast Random Bit Generation Using a Chaotic Laser: Approaching the Information Theoretic Limit

We design and implement a chaotic-based system, enabling ultra-fast random bit sequence generation. The potential of this system to realize bit rates of 160 Gb/s for 8-bit digitization and 480 Gb/s for 16-bit digitization is demonstrated. In addition, we provide detailed insight into the interplay of dynamical properties, acquisition conditions, and post-processing, using simple and robust procedures. We employ the chaotic output of a semiconductor laser subjected to polarization-rotated feedback. We show that not only dynamics affect the randomness of the bits, but also the digitization conditions and postprocessing must be considered for successful random bit generation. Applying these general guidelines, extensible to other chaos-based systems, we can define the optimal conditions for random bit generation. We experimentally demonstrate the relevance of these criteria by extending the bit rate of our random bit generator by about two orders of magnitude. Finally, we discuss the information theoretic limits, showing that following our approach we reach the maximum possible generation rate.

Entraining power-dropout events in an external-cavity semiconductor laser using weak modulation of the injection current

We measure experimentally the effects of injection current modulation on the statistical distribution of time intervals between power-dropout events occuring in an external-cavity semiconductor laser operating in the low-frequency fluctuation regime. These statistical distributions are sensitive indicators of the presence of pump current modulation. Under most circumstances, we find that weak low-frequency (in the vicinity of 19 MHz) modulation of the current causes the dropouts to occur preferentially at intervals that are integral multiples of the modulation period. The dropout events can be entrained by the periodic perturbations when the modulation amplitude is large (peak-to-peak amplitude /spl ges/8% of the dc injection current). We conjecture that modulation induces a dropout when the modulation frequency is equal to the difference in frequency between a mode of the extended cavity laser and its adjacent antimode. We also find that the statistical distribution of the dropout events is unaffected by the periodic perturbations when the modulation frequency is equal to the free spectral range of the external cavity. Numerical simulations of the extended-cavity laser display qualitatively similar behavior. The relationship of these phenomena to stochastic resonance is discussed and a possible use of the modulated laser dynamics for chaos communication is described.

Square-wave self-modulation in diode lasers with polarization-rotated optical feedback.

The square-wave response of edge-emitting diode lasers subject to a delayed polarization-rotated optical feedback is studied in detail. Specifically, the polarization state of the feedback is rotated such that the natural laser mode is coupled into the orthogonal, unsupported mode. Square-wave self-modulated polarization intensities oscillating in antiphase are observed experimentally. We find numerically that these oscillations naturally appear for a broad range of values of parameters, provided that the feedback is sufficiently strong and the differential losses in the normally unsupported polarization mode are small. We then investigate the laser equations analytically and find that the square-wave oscillations are the result of a bifurcation phenomenon.

Experimental synchronization of chaos in diode lasers with polarization-rotated feedback and injection

We demonstrate experimental chaos synchronization between two chaotic semiconductor lasers subjected to polarization-rotated optical feedback and unidirectional injection. This system allows high-quality synchronization to be obtained between dissimilar lasers in a wide range of chaotic operating regimes. Another feature of this system is its operation at high characteristic frequencies, taking advantage of all-optical implementation. Time series and RF spectra showing synchronization are confirmed by high correlation coefficients in excess of 0.85.

Experimental demonstration of suppression of low-frequency fluctuations and stabilization of an external-cavity laser diode

We demonstrate experimentally all-optical stabilization of a single-mode laser diode subject to external optical feedback operating in the low-frequency fluctuations (LFF) regime, by the technique of applying a second delayed optical feedback. We interpret our results as suppression of LFF through destruction of the antimodes responsible for the LFF crises and stabilization of the laser through creation of new maximum gain modes, in agreement with recent theoretical predictions.

Bifurcations in a semiconductor laser subject to delayed incoherent feedback

We examine experimentally and numerically the bifurcation sequence and route into chaos of a semiconductor laser subject to delayed incoherent feedback. We show that the sequence of bifurcations follows a three frequency scenario in which the steady-state of the laser destabilizes by a Hopf bifurcation at the relaxation frequency of the laser. Specifically, we show experimentally and numerically that the Hopf bifurcation can be supercritical or subcritical depending on the length of the delay and the pumping of the laser above threshold. This is followed by a torus bifurcation at the external cavity frequency and further by a tertiary bifurcation at a significantly lower frequency.

Mixed external cavity mode dynamics in a semiconductor laser

We observe experimentally and numerically novel mixed-mode dynamic states of a diode laser subject to two delayed optical feedbacks. These states have been proposed and analyzed within the framework of the Lang-Kobayashi single-feedback model. Such states are combinations of two distinct external cavity modes and can be identified through a characteristic sequence of a Hopf bifurcation followed by a secondary quasi-periodic bifurcation. We present experimental and numerical results that demonstrate such sequences.

Identity synchronization in diode lasers with unidirectional feedback and injection of rotated optical fields

Identity synchronization is observed experimentally and numerically in the chaotic dynamics of a system of two unidirectionally coupled semiconductor lasers. The transmitter and receiver lasers are subjected to polarization-rotated optical feedback and injection, respectively. Numerical and analytical results show that identity synchronization requires parameter matching through a relationship between the injection and feedback strengths, and linewidth enhancement factors of the lasers. Inverse synchronization is also observed experimentally.

Polarization dynamics in semiconductor lasers with incoherent optical feedback

The chaotic dynamics of a semiconductor laser subject to a delayed polarization-rotated optical feedback is investigated theoretically and experimentally. An extension of the usual one-polarization model is derived to account for two orthogonal polarizations of the optical field. The two-polarization model is motivated by observations of lag synchronization in our experiments using polarization-rotated optical feedback and uni-directional injection. Experimental data confirm the predictions of the two-field model. We also show that the two-polarization model can be reduced to the one-polarization model.

Experimental criteria for high-speed random bit generation using a chaotic semiconductor laser

Here, we investigate in detail the factors that affect the randomness and possible bit rates of a random bit generator based on a chaotic semiconductor laser: dynamical properties of the system, digitization conditions and postprocessing of the original signal. Implementing the proper balance, our random bit generator is demonstrated to have the potential for bit rates up to 480 Gbit/s.