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.

Chaotic message encoding and decoding in unidirectionally coupled vertical-cavity surface-emitting lasers (VCSELs) with polarization-preserved and polarization-selected optical injection has been studied experimentally. A GHz message has been successfully encoded in the chaotic transmitter and decoded from the receiver with polarization-preserved optical injection. In contrast decoding using polarization-selected optical injection was achieved at only 330 MHz. It has also been demonstrated that GHz message extraction can be achieved using both normal and inverse chaos synchronization thus providing an opportunity for exploiting polarization properties of VCSELs for duplexed chaotic message transmission.

A chaotic semiconductor laser subject to double optical feedback is characterised in terms of the statistical analysis, power spectrum analysis and forecast error analysis of the chaotic dynamics. Varying the relative lengths of the two external cavities, the dynamics are observed and compared to that of a single cavity. We show that in the case of integer multiple external-cavity lengths the round-trip time delays can be deduced from the forecast error analysis. In the case of fractional multiple cavity lengths, only the mean value of the cavity resonance frequencies can be deduced from the power spectra and hence the cavity time delays cannot be found. Cavity time delays that differ more than 0.4 ns can be obtained from the forecast error analysis. In the case of similar cavity lengths, the cavity time delays cannot be extracted. The results show that double optical feedback may offer particular advantages for enhancing security in chaos encryption.

The nonlinear dynamics of two semiconductor laser systems: (i) with optical feedback, and (ii) with optical feedback and direct current modulation are evaluated from multi-GHz-bandwidth output power time-series. Animations of compilations of the RF spectrum (from the FFT of the time-series) as a function of optical feedback level, injection current and modulation signal strength is demonstrated as a new tool to give insight into the dynamics. The results are contrasted with prior art and new observations include fine structure in the RF spectrum at low levels of optical feedback and non-stationary switching between periodic and chaotic dynamics for some sets of laser system parameters. Correlation dimension analysis successfully identifies periodic dynamics but most of the dynamical states are too complex to be extracted using standard algorithms.