K. Alan Shore

Chaos-based communications at high bit rates using commercial fibre-optic links

Chaotic signals have been proposed as broadband information carriers with the potential of providing a high level of robustness and privacy in data transmission. Laboratory demonstrations of chaos-based optical communications have already shown the potential of this technology, but a field experiment using commercial optical networks has not been undertaken so far. Here we demonstrate high-speed long-distance communication based on chaos synchronization over a commercial fibre-optic channel. An optical carrier wave generated by a chaotic laser is used to encode a message for transmission over 120 km of optical fibre in the metropolitan area network of Athens, Greece. The message is decoded using an appropriate second laser which, by synchronizing with the chaotic carrier, allows for the separation of the carrier and the message. Transmission rates in the gigabit per second range are achieved, with corresponding bit-error rates below 10-7. The system uses matched pairs of semiconductor lasers as chaotic emitters and receivers, and off-the-shelf fibre-optic telecommunication components. Our results show that information can be transmitted at high bit rates using deterministic chaos in a manner that is robust to perturbations and channel disturbances unavoidable under real-world conditions.

Synchronization of chaos in unidirectionally coupled vertical-cavity surface-emitting semiconductor lasers.

Synchronization of chaos is achieved experimentally in unidirectionally coupled external-cavity vertical-cavity surface-emitting semiconductor lasers operating in an open-loop regime. Synchronization is observed when the polarization of the transmitter is perpendicular to the polarization (x polarization) of the free-running receiver. The ratio of transmitter output to y-polarized receiver output power shows normal (positive-slope) synchronization. However, inverse (negative-slope) synchronization is found to arise between the transmitter output and the x-polarized receiver output power.

Suppression of polarization switching in vertical-cavity surface-emitting lasers by use of optical feedback

The polarization properties of vertical-cavity surface-emitting lasers (VCSELs) subject to optical feedback are studied experimentally. It is thereby demonstrated that polarization-selective optical feedback can be utilized to entirely eliminate VCSEL polarization switching over the entire device operating range.

Enhanced chaos synchronization in unidirectionally coupled vertical-cavity surface-emitting semiconductor lasers with polarization-preserved injection

We experimentally study chaos synchronization in unidirectionally coupled vertical-cavity surface-emitting semiconductor lasers (VCSELs) with polarization-preserved and polarization-selected optical injection. The measurements show, in agreement with theoretical predictions, that the maximum cross coefficient of 0.884 obtained with polarization-preserved optical injection is significantly higher than the maximum cross coefficient of 0.724 obtained with polarization-selected optical injection.

Effect of chaos pass filtering on message decoding quality using chaotic external-cavity laser diodes

An experimental demonstration of the effect of chaos pass filtering on high-frequency message transmission in the complete synchronization regime is reported. The opportunity for chaotic message decoding at frequencies up to 6 GHz is shown.

Dual-channel chaotic optical communications using external-cavity semiconductor lasers

We experimentally demonstrate that two chaotic optical communication channels can be configured over a single transmission path by using two external-cavity laser diodes as transmitter lasers and a single stand-alone receiver laser. The two chaotic communication channels are shown to operate independently. Two messages, at different modulation frequencies, generated via direct-current modulation of the transmitter lasers, can be masked by the chaos and recovered at the receiver laser by the use of a decoder laser. The decoder laser is used to select the channel to be utilized. We show that there is good message recovery with little cross talk between the two channels. The channels operate at different laser wavelengths and thus constitute a wavelength-division-multiplexing scheme.

Multi-interface roughness effects on electron mobility in a Ga0.5In0.5P/GaAs multisubband coupled quantum well structure

We analyse the effect of interface roughness scattering on low temperature electron mobility μn mediated by intersubband interactions in a multisubband coupled Ga0.5In0.5P/GaAs quantum well structure. We consider a barrier δ-doped double quantum well system in which the subband electron mobility is limited by the interface roughness scattering μIRn and ionized impurity scattering μimpn. We analyse the effect of the intersubband interaction and coupling of subband wavefunctions through the barrier on the intrasubband and intersubband transport scattering rates. We show that the intersubband interaction controls the roughness potential of different interfaces through the dielectric screening matrix. In the case of lowest subband occupancy, the mobility is mainly governed by the interface roughness of the central barrier. Whereas when two subbands are occupied, the interface roughness of the outer barrier predominates due to intersubband effects. The influence of the intersubband interaction also exhibits interesting results on the well width up to which the interface roughness dominates in a double quantum well structure.

Experimental study of polarization switching of vertical-cavity surface-emitting lasers as a dynamical bifurcation

We study the role of the bias current sweep rate in measurements of polarization switching (PS) of vertical-cavity surface-emitting lasers (VCSELs). We show that the polarization-resolved L-I (light-intensity) curve depends on the current sweep rate. As the current sweep rate increases, the PS occurs at higher bias currents for upward scans and at lower bias currents for downward scans. We also show that the delay of the dynamical bifurcation follows a power law relationship with the frequency of the ramp, in good agreement with recent theoretical predictions.

Comparison of closed-loop and open-loop feedback schemes of message decoding using chaotic laser diodes

An experimental study of a chaotic optical communication scheme using an external cavity laser diode to encode a message is reported. The message is decoded in closed- and open-loop receiver schemes. Both schemes show effective decoding of a 1-GHz message. The synchronization and effectiveness of encoding are compared in both schemes.

Experimental Study of Time-Delay Signatures in Vertical-Cavity Surface-Emitting Lasers Subject to Double-Cavity Polarization-Rotated Optical Feedback

Suppression of time-delay (TD) signature in a vertical-cavity surface-emitting laser subject to double-cavity polarization-rotated optical feedback has been verified experimentally. The TD signature, represented by the autocorrelation coefficient at the delay time in the external cavity, is measured in the two orthogonal polarizations of the laser. The results show that TD signatures with double-cavity feedback can be 22% to 35% weaker than those with single cavity feedback for the same experimental parameters. For appropriate feedback polarization, TD signatures become invisible. This is in good agreement with the theoretical prediction. Our results also show that higher feedback strength results in greater suppression in TD signatures in a wide range of polarization angle.