A semiconductor laser with delayed optical feedback is an experimental implementation of a nominally infinite dimensional dynamical system. As such, time series analysis of the output power from this laser system is an excellent test of complexity analysis tools, as applied to experimental data. Additionally, the systematic characterization of the range and variation in complexity that can be obtained in the output power from the system, which is available to be used in applications like secure communication, is of interest. Output power time series from a semiconductor laser system, as a function of the optical feedback level and the laser injection current, have been analyzed for complexity using permutation entropy. High resolution maps of permutation entropy as a function of optical feedback level and injection current have been achieved for the first time. This confirms prior research that identifies a coherence collapse region which is found to be uninterrupted with respect to any embedded islands with different dynamics. The results also show new observations of low optical feedback dynamics which occur in a region below that for coherence collapse. The map of the complexity shows a strong dependence on the delay time used in the permutation entropy calculation. Short delay times, which sample information at the complete measurement bandwidth, produce maps with drastically different systematic variation in complexity throughout the coherence collapse region, compared to maps generated with a delay time that matches the optical feedback delay. Evaluating the complexity with a permutation entropy delay equal to the external cavity delay produces results consistent with the notion of weak/strong chaos, as well as categorizing the dynamics as being of high complexity where the external cavity delay time is harder to identify. These are both desirable features for secure communication applications. The results also show permutation entropy as a function of delay time can be used to detect key frequencies driving the dynamics, including any that may exist due to, or arise from, technicalities of device fabrication and/or noise. A more complete insight into complexity as measured by permutation entropy is gained by considering multiple delay times.

Particles as small as 0.3 μm in diameter have been successfully removed from a glass surface using a single ultraviolet pulse from a frequency doubled copper vapor laser (255.3 nm). Quantitative analysis of the particle density before and after laser irradiation shows that laser cleaning occurs after a fluence threshold is reached. The cleaning efficiency after threshold follows a nonlinear trend with respect to fluence. A model is presented which reveals that the cleaning efficiency is a function of the irradiance distribution of the beam used. Results of modeling thermal expansion of the substrate and particles, and particle adhesion do not confirm a thermal expansion mechanism for laser cleaning in this study, in contrast with other recent reports.

No abstract

An experimental comparison of phase conjugate feedback to conventional optical feedback in a semiconductor laser is presented, which contrasts the optical frequency spectra and power noise spectra with varying levels of feedback. These spectra are correlated with the single frequency regimes III and V and the chaotically unstable regime IV. Conventional feedback is derived from a mirror and variable neutral density filter, and the phase conjugate feedback is derived from a self-pumped rhodium doped barium titanate photorefractive crystal. Both systems show two stable, single frequency operation regions analogous to regime III and V operation separated by a single region of unstable operation analogous to regime IV. It is found that phase conjugate feedback leads to distinctive behaviors including: differences in the relative intensity noise spectra; dynamically varying output frequency spectra close to the transition from regime IV to V; systematic power transfer from one laser diode longitudinal mode to a nearest neighbor through regime IV; and a very much larger range of feedback levels leading to unstable output. The level of optical feedback leading to a transition from regime III to IV is the same for conventional optical feedback and phase conjugate feedback when the correction for the different coupling efficiencies is made. The transition from regime IV to V occurs for much higher levels of feedback when PCF is used leading to the larger range of feedback levels giving chaotic behavior with PCF. The results are discussed in the context of existing theoretical models of laser diodes with phase conjugate feedback.

We have investigated the scaling of peak vacuum ultraviolet output power from homogeneous Xe dielectric barrier discharges excited by short voltage pulses. Increasing the Xe fill pressure above 1 bar provides an increased output pulse energy, a shortened pulse duration and increases in the peak output power of two to three orders of magnitude. High peak power pulses of up to 6 W cm−2 are generated with a high efficiency for pulse rates up to 50 kHz. We show that the temporal pulse characteristics are in good agreement with results from detailed computer modelling of the discharge kinetics.

A 4-section semiconductor laser with integrated optical feedback has been shown experimentally to be capable of operating in either the short- or long-cavity regime, by controlling the device relaxation oscillation frequency relative to the external cavity frequency. Systematic increase of the laser injection current, and the resulting increase in relaxation oscillation frequency, allowed the transition between the two regimes of operation to be observed. The system displayed a gradual transition from a dynamic dominated by regular pulse packages in the short-cavity regime to one dominated by broadband chaotic output when operating in the long-cavity regime. This suggests that the "short cavity" regular pulse packages continue to co-exist with the "long cavity" broadband chaotic dynamic in the system studied. It is the relative power associated with each of these dynamics that changes. This may occur more generally in similar systems.

2009) Automated correlation dimension analysis of optically injected solid state lasers. Optics Express, Vol. 17, Issue 9, pp. 7592-7608. Access to the published version:http://dx.Abstract: Nonlinear lasers are excellent systems from which to obtain high signal-to-noise experimental data of nonlinear dynamical variables to be used to develop and demonstrate robust nonlinear dynamics analysis techniques. Here we investigate the dynamical complexity of such a system: an optically injected Nd:YVO 4 solid state laser. We show that a map of the correlation dimension as a function of the injection strength and frequency detuning, extracted from the laser output power time-series data, is an excellent mirror of the dynamics map generated from a theoretical model of the system. An automated computational protocol has been designed and implemented to achieve this. The correlation dimension map is also contrasted with prior research that mapped the peak intensity of the output power as an experimentally accessible measurand reflecting the dynamical state of the system [Valling et al., Phys. Rev. A 72, 033810 (2005)].

No abstract