Junji Ohtsubo

Chaos synchronization and chaotic signal masking in semiconductor lasers with optical feedback

Theoretical and experimental investigations of chaos synchronization and its application to chaotic data transmissions in semiconductor lasers with optical feedback are presented. Two schemes of chaos synchronization-complete and generalized synchronization-are discussed in the delay differential systems. The conditions for chaos synchronization in the systems and the robustness for the parameter mismatches are studied. The possibility of secure communications based on the chaos masking technique in semiconductor lasers with optical feedback is also discussed, and message transmission of a 1.5-GHz sinusoidal signal is demonstrated. The method of bandwidth enhancement of chaotic carriers is proposed for broad-band chaos communications.

Experimental synchronization of chaotic oscillations in external-cavity semiconductor lasers.

Synchronization of fast chaotic oscillations of the order of gigahertz is experimentally observed in two external-cavity semiconductor lasers.

Velocity measurement of a diffuse object by using time-varying speckles

The relation between the velocity of a moving diffuse object and the mean number of integrated speckles in the certain time interval at the far-field plane of the object is obtained. By using this relation based on the first order statistics of integrated speckles, a new method of measuring the velocity of the diffuse object is proposed and experimentally verified. The first order probability density function of time-integrated speckles is also studied from which the validity of the present method is verified.

Bandwidth-enhanced chaos synchronization in strongly injection-locked semiconductor lasers with optical feedback

Bandwidth-enhanced chaos synchronization in strongly injection-locked semiconductor lasers with optical feedback is numerically studied based on laser rate equations. The bandwidth of the chaotic carrier frequency in a semiconductor laser with optical feedback is expanded roughly three times by strong optical injection compared with the bandwidth when there is no optical injection. Using a bandwidth-enhanced semiconductor laser as a chaotic transmitter and receiver, we synchronized transmitter and the receiver lasers in a complete chaos synchronization scheme.

Chaos control and noise suppression in external-cavity semiconductor lasers

Feedback-induced chaos and intensity noise enhancement in a laser diode with external optical feedback are studied by computer simulations. The enhancement of relative intensity noise (RIN) that is often observed in experiments is considered as a result of the feedback-induced deterministic chaos and the intensity noise suppression is treated from the viewpoint of chaos control. Especially, the conventional noise suppressing technique known as a high-frequency injection modulation is turned into a problem of stabilizing chaos through parameter modulations. We developed an analytical method which allows to optimize the modulation frequency from the linear stability analysis of the dynamical model that describes the laser diode with external feedback. The robustness of the modulation with respect to the modulation frequency and depth is verified and the results suggest the feasibility of applying our method to actual noise suppression. The RIN in the low-frequency region (up to 100 MHz) is shown to be reduced to the solitary laser level when the feedback-induced chaos is effectively controlled with the optimized modulation frequency.

Observation of the synchronization of chaos in mutually injected vertical-cavity surface-emitting semiconductor lasers.

Synchronization of chaotic oscillations was observed in mutually injected vertical-cavity surface-emitting lasers (VCSELs) in a low-frequency fluctuation regime. In the experiments, only one of the two polarization modes (x mode) showed synchronized oscillations, and the other polarization components (y mode) were synchronized as a result of the effect of anticorrelated oscillations that is a characteristic feature of VCSELs.

1.5-GHz message transmission based on synchronization of chaos in semiconductor lasers.

Sinusoidal message transmission up to 1.5 GHz is performed based on synchronization of chaos in experimental nonlinear systems of semiconductor lasers with optical feedback. In the chaotic systems the message is almost entirely suppressed in the receiver output, even if the message has nonnegligible power in the transmitter.

Statistical properties of laser speckle produced in the diffraction field

The statistical properties (i.e., the probability density function and the average contrast) of laser speckle produced by a weak diffuse object in the diffraction field have been theoretically and experimentally studied with the assumption of the Gaussian statistics for the formation of speckles. The general formulas for the probability density function and the average contrast, which are valid for an entire range of object surface and for the whole diffraction field, are introduced, and their special cases, which have been studied in the past, are derived and discussed. These formulas for the probability density function and the averagae contrast are actually evaluated in the diffraction field of a weak diffuse object illuminated by the Gaussian laser beam. The experimental study has been conducted to verify the above theoretical study, and its results are in good agreement with the theoretical ones. The circularity and noncircularity of the speckle statistics in the diffraction field are discussed on the basis of the theoretical study.

Characteristics of a semiconductor laser with external feedback

The theoretical analysis of the compound cavity of a semiconductor laser with external optical feedback is conducted. For large optical feedback, the output power from the laser and its oscillation frequency differ from those for small optical feedback. From the rate equations of the compound cavity, the conditions of the laser oscillation are derived in the presence of large optical feedback The dependencies of the output power and the laser oscillation frequency on the external-cavity length are investigated. Some new results involving laser oscillation depending on the external-cavity length are presented. The experimental results are compared with theoretical predictions. The dependence of the laser oscillation on the external-cavity length is qualitatively explained in the present model. >

Statistical properties of speckle intensity variations in the diffraction field under illumination of coherent light

Abstract The statistical properties of speckle intensity variations produced by coherent light in the diffraction field of diffusely transmitting objects are investigated experimentally with intimate relation to their surface roughness. The probability distribution and the average contrast of intensity variations in speckle patterns are studied as a function of the surface roughness of the objects and the receiving plane in the diffraction field.