Kazuyuki Yoshimura

Fast random bit generation with bandwidth-enhanced chaos in semiconductor lasers.

We experimentally demonstrate random bit generation using multi-bit samples of bandwidth-enhanced chaos in semiconductor lasers. Chaotic fluctuation of laser output is generated in a semiconductor laser with optical feedback and the chaotic output is injected into a second semiconductor laser to obtain a chaotic intensity signal with bandwidth enhanced up to 16 GHz. The chaotic signal is converted to an 8-bit digital signal by sampling with a digital oscilloscope at 12.5 Giga samples per second (GS/s). Random bits are generated by bitwise exclusive-OR operation on corresponding bits in samples of the chaotic signal and its time-delayed signal. Statistical tests verify the randomness of bit sequences obtained using 1 to 6 bits per sample, corresponding to fast random bit generation rates from 12.5 to 75 Gigabit per second (Gb/s) ( = 6 bit x 12.5 GS/s).

Fast Random Number Generation With Bandwidth-Enhanced Chaotic Semiconductor Lasers at 8

We propose a scheme for fast random number generation with bandwidth-enhanced chaotic semiconductor lasers. Chaotic laser intensity output and its time-delayed signal are sampled at 50 GigaSample per second and converted into eight-bit values. The order of the eight-bit samples of the time-delayed signal is reversed, and bitwise exclusive-or operation is executed between the bit-order-reversed samples and the original eight-bit samples. With this method, it is not necessary to eliminate any of the bits in eight-bit samples in order to obtain good-quality random bit sequences. The equivalent generation rate of 8×50 Gb/s is achieved in an experiment using bandwidth-enhanced chaotic semiconductor lasers.

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We investigate the characteristics of fast random bit generation using chaotic semiconductor lasers. The optical amplitudes of two lasers with chaotic oscillations induced by optical feedback are each sampled at a fixed rate to extract binary bit sequences which are then combined by an exclusive-OR operation to obtain a single random bit sequence. Bit sequences generated at rate of 1 Giga bit per second are verified to pass statistical tests of randomness. We describe the dependence of randomness on laser parameters, in particular the injection current, the external cavity length and the feedback strength. The results provide clear empirical guidelines for tuning the chaotic laser parameters to achieve random bit sequences. This study shows that chaotic laser devices can be fast and reliable sources of physical entropy for computing and communication applications.

50 Gb/s

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.

Characteristics of Fast Physical Random Bit Generation Using Chaotic Semiconductor Lasers

We report a novel chaos semiconductor laser chip in which a distributed feedback (DFB) laser, two semiconductor optical amplifiers (SOAs) and a photodiode (PD) are monolithically integrated with a passive ring waveguide. The ring-type structure with the two separate SOAs achieves stronger delayed optical feedback compared to previous chaos laser chips which use linear waveguide and facet-reflection. The integrated PD allows efficient detection of the optical signal with low optical loss. A rich variety of dynamical behaviors and optical signals can be selectively generated via injection currents to the two separate SOAs. In particular, the strong optical feedback makes possible the generation of strong broadband optical chaos, with very flat spectrum of ±6.5 dB up to 10 GHz. The stability and quality of the chaotic mode is demonstrated using strict statistical tests of randomness applied to long binary sequences extracted by sampling the optical intensity signal.

Common-chaotic-signal induced synchronization in semiconductor lasers

We generate random bit sequences from chaotic temporal waveforms by using photonic integrated circuits (PICs) with different external cavity lengths. We investigate the condition for generating random bits at different sampling rates of single-bit generation method with the PICs. We succeed in generating certified random bit sequences by using the PIC with 3, 4, 5, or 10-mm-long external cavity, whereas random bits cannot pass all the statistical tests of randomness when the PIC with 1 or 2 mm-long external cavity is used.

Chaos laser chips with delayed optical feedback using a passive ring waveguide.

It has been proposed that a secure key distribution scheme using correlated random bit sequences can be implemented using common random-signal induced synchronization of semiconductor laser systems. In this scheme it is necessary to use laser systems consisting of multiple cascaded lasers to be secure against a powerful eavesdropper. In this paper, we report the results of an experimental study that demonstrate that the common random-signal induced synchronization is possible in cascaded semiconductor laser systems. We also show that the correlated random bit sequences generated in the synchronized cascaded laser systems can be used to create an information-theoretically secure key between two legitimate users.

Fast physical random bit generation with photonic integrated circuits with different external cavity lengths for chaos generation

We experimentally show that a random optical pulse train can be generated by modulating a bistable semiconductor ring laser. When the ring laser is switched from the monostable to the bistable regime, it randomly selects one of two different stable unidirectional lasing modes, clockwise or counterclockwise modes. Non-deterministic random pulse sequences are generated by driving the switch parameter, the injection current, with a periodic pulse signal. The origin of the nondeterministic randomness is the amplified spontaneous emission noise coupled to the counter-propagating lasing modes. The statistical randomness properties are optimized by adjusting the relative strength of amplified spontaneous emission noise sources for the two lasing modes. It is also shown that it is possible to generate optical pulse sequences which pass a standard suite of statistical randomness tests.

Information-theoretic secure key distribution based on common random-signal induced synchronization in unidirectionally-coupled cascades of semiconductor lasers

We investigate the dynamics of two semiconductor lasers with separate optical feedback when they are driven by a common signal injected from a chaotic laser under the condition of non-identical drive and response. We experimentally and numerically show conditions under which the outputs of the two lasers can be highly correlated with each other even though the correlation with the drive signal is low. In particular, the effects of the phase of the feedback light on the correlation characteristics are described. The maximum correlation between the two response lasers is obtained when the phase of the feedback light is matched between the two response lasers, while the minimum correlation is observed when the difference in the optical phase is pi. On the other hand, the correlation between the drive and response is not sensitive to the phase of the feedback light, unlike the previously studied case of identical drive and response. We numerically examine the difference between the maximum and minimum cross correlations over a wide range of parameters, and show that it is largest when there is a balance between the injection strength and the feedback strength.

Random optical pulse generation with bistable semiconductor ring lasers

We present an experimental method for directly observing the amplification of microscopic intrinsic noise in a high-dimensional chaotic laser system, a laser with delayed feedback. In the experiment, the chaotic laser system is repeatedly switched from a stable lasing state to a chaotic state, and the time evolution of an ensemble of chaotic states starting from the same initial state is measured. It is experimentally demonstrated that intrinsic noises amplified by the chaotic dynamics are transformed into macroscopic fluctuating signals, and the probability density of the output light intensity actually converges to a natural invariant probability density in a strongly chaotic regime. Moreover, with the experimental method, we discuss the application of the chaotic laser systems to physical random bit generators. It is experimentally shown that the convergence to the invariant density plays an important role in nondeterministic random bit generation, which could be desirable for future ultimate secure communication systems.