Information and communication technologies with chaotic lasers (Applications)
1. Fast physical random number generation
Random number generators in digital information systems exploit physical entropy sources, such as electronic and photonic noise, to add unpredictability to deterministically generated pseudo-random sequences. However, there is a large gap between the generation rates achieved with existing physical sources and the high data rates of many computation and communication systems, which is a fundamental weakness in these systems. We show that good-quality random bit sequences can be generated at very fast bit rates using physical chaos in semiconductor lasers. Streams of bits which pass standard statistical tests for randomness have been generated at rates of up to 1.7 gigabit per second by sampling the fluctuating optical output of two chaotic lasers. This rate is an order of magnitude faster than that of previously reported devices for physical random bit generators with verified randomness. This means that the performance of random number generators can be greatly improved by using chaotic laser devices as physical entropy sources.
Nature Photonics, vol.2, no.12, pp.728-732 (2008).
IEEE Journal of Quantum Electronics, vol.45, no.11, pp.1367-1379 (2009).
Optics Express, vol.17, no.11, pp.9053-9061 (2009).
2. New information security and cryptography
We demonstrate a method for information theoretic secure key generation based on practical physical limits of continuous data acquisition from a rapidly fluctuating random signal generated by a chaotic semiconductor laser and transmitted over an optical fiber. Users independently and randomly grab sections of the chaotic waveform, separate into tag and key parts, and exchange tag parts to identify the common key parts which can be used to make secure keys. The frequency of trigger pulses in the waveform is used to control the security of the scheme which depends on the small probability of a third party coincidentally acquiring samples acquired by two other users.
Applied Physics Letters, vol.83, no.15, pp.3213-3215 (2003).
3. Optical secure communication with synchronized chaotic lasers
We experimentally demonstrate optical secure communication by using chaos synchronization in two microchip lasers. The output of the microchip laser in the transmitter is externally modulated with an acousto-optic modulator. One encodes a digital message in the chaotic carrier by turning the modulation on and off. Because the accuracy of synchronization for the slave laser in the receiver tends to be degraded in the presence of external modulation in the injection laser signal, one can distinguish two binary states. The digital message can be recovered as an envelope of the chaotic oscillation when the difference between the two laser outputs of the transmitter and the receiver is calculated.
Progress in Optics, edited by E. Wolf, vol.48, chap.5, pp.203-341, Elsevier, The Netherlands (2005).
IEEE Journal of Quantum Electronics, vol. 39, no. 8, pp. 963-970 (2003).
Optics Letters, vol.26, no.12, pp. 866-868 (2001).
4. Generation of ultra-fast chaos
Chaotic signals with a flat power spectrum over 20 GHz have been generated using two commercially available semiconductor lasers coupled in a unidirectional master?slave scheme. The master laser has an external optical feedback that induces optical chaos in the laser output. A part of the chaotic light output from the master laser is injected into the slave laser. We experimentally demonstrated the generation of broad-band signals up to 22 GHz using lasers whose relaxation oscillation frequency in the free-running state is only around 6.4 GHz. We also show that the experimental results can be well reproduced by numerical simulations using two coupled rate equations. The numerical investigation shows that the high-frequency broad-band signal generation is owing to two key effects: high-frequency oscillations as a result of beating between the master and slave laser lights, and spectrum flattening due to the injection of the chaotic signal. The flatness, stability, and tunability of the power spectra demonstrated in our experiments suggests that the proposed system can be potentially useful for generation of high-frequency broad-band random signals.
IEEE Journal of Quantum Electronics, vol.39, no.11, pp.1462-1467 (2003).
Optics Express, vol.17, no.22, pp.19536-19543 (2009).
5. Multiplexing communication using multiple synchronization of chaos
We experimentally demonstrate the dual synchronization of chaos in two pairs of Nd:YVO4 microchip lasers in a one-way coupling configuration over one transmission channel. Dual synchronization is achieved when the optical frequency is matched between the corresponding pairs of lasers by using injection locking. We investigate the influence of optical injection from the two master lasers to one slave laser, and found that the dual synchronization is observed when the injection locking is achieved between either of the master lasers and the slave laser. Under the condition of the injection locking between both of the master lasers and the slave laser, the injection locking is alternately achieved and the accuracy of dual synchronization is degraded. We also confirm these results by numerical calculation.
Physical Review E, vol.67, no.2, pp.026220-1-8 (2003).
6. Blind source separation of mixed chaotic signals by independent component analysis
We experimentally demonstrate blind source separation of chaos generated in microchip solid-state lasers and electronic circuits by using independent component analysis. Two chaotic source signals are linearly mixed with randomly selected mixing ratio and independent component analysis is applied for the mixed signals to extract the source signals. We investigate blind source separation of many chaotic laser signals and succeed 100- signal separation of chaotic temporal waveforms. Longer temporal waveforms are required with increase of the number of mixed signals.
Optics Express, vol.16, no.2, pp.725-730 (2008).
Electronics Letters, vol.44, no.3, pp.248-250 (2008).
Chaos, synchronization, and consistency in lasers (Basic science)
1. Consistency of nonlinear systems driven by a common signal
The consistency of a nonlinear system’s response to a repeated complex waveform drive signal is an important consideration in classical and quantum systems as diverse as lasers, neuronal networks, and manufacturing plants.We show from a consideration of different characteristic waveforms that there is typically an optimal drive amplitude for the most consistent response; internal noise sources dominate for small amplitude driving while deterministic system nonlinearity reduces consistency for large amplitudes. We test this general concept and its measurement experimentally and numerically on the specific example of a laser system.
Physical Review Letters, vol.93, pp.244102-1-4 (2004).
Physical Review E, vol.78, no.3, pp.036203-1-036203-5 (2008).
2. Generalized synchronization of chaos in identical laser systems
We demonstrate generalized synchronization of chaos in a two-mode laser system. The total intensity of the laser output (the sum of the individual mode intensities) is used as the drive signal. This lumped variable transmitted to the identical response system does not generate identical synchronization. Generalized synchronization is observed instead of identical synchronization because of the hidden internal degrees of freedom.
Physical Review Letters, vol.91, no.17, pp.174101-1-4 (2003).
Physical Review Letters, vol.93, pp.084101-1-4 (2004).
3. Common-chaotic-signal induced synchronization in semiconductor lasers
We experimentally and numerically observe synchronization of two semiconductor lasers commonly driven by a chaotic semiconductor laser subject to optical feedback. Under condition that the relaxation oscillation frequency is matched between the two response lasers, but mismatched between the drive and the two response lasers, we show that it is possible to observe strongly correlated synchronization between the two response lasers even when the correlation between the drive and response lasers is low. We also show that the cross correlation between the two responses is larger than that between drive and responses over a wide parameter region.
Optics Express, vol.15, no.7, pp.3974-3980 (2007).
Optics Express, vol.17, no.12, pp.10025-10034 (2009).
4. Synchronization of chaos in lasers
We investigate the detailed characteristics of chaos synchronization in semiconductor lasers subject to polarization- rotated optical feedback. The emission of the dominant TE mode of a drive laser is rotated 90 deg and fed back to the laser with time delay. The polarization-rotated TE mode is also injected with time delay into the TM mode of a second laser. Two types of synchronization with different time-lags are found, as in the case for synchronization in semiconductor lasers with nonrotated optical feedback. However, a significant difference to the nonrotated optical feedback case is that neither of the two types of synchronization requires matching of optical carrier frequency between the two lasers.
IEEE Journal of Quantum Electronics, vol.42, no.3, pp.342-350 (2006).
Physical Review E, vol. 62, no. 2, pp. 1960-1971 (2000).
5. Dual synchronization of chaos in electronic circuits
We demonstrate the dual synchronization of chaos in two pairs of one-way coupled Colpitts electronic oscillators by both experiment and numerical simulation. We use the cross coupling method, where the difference in voltage between the sum of two master oscillators and one slave oscillator is injected into the other slave oscillator as an electrical current, for dual synchronization of chaos. We have investigated the regions for achieving dual synchronization of chaos when one of the internal parameters is mismatched between the master and slave oscillators. We numerically obtain a similar curve for the accuracy of synchronization to that obtained from our experiments. A communication scheme using dual synchronization of chaos is also proposed and demonstrated.
Physical Review E, vol.68, no.5, pp.056207-1-11 (2003).
Novel optical devices
1. Fractal pattern of chaotic light scattering in regular polyhedral mirror ball structures
We experimentally observe fractal patterns of chaotic light scattering in regular polyhedral mirror ball structures that consist of spherical reflectors located at the vertices of polyhedra as optical scattering devices. We measure the fractal dimension of the basin boundaries of the light scattering patterns in the regular polyhedral mirror ball structures.
Physical Review E, vol.76, pp.046213-1-6 (2007).