Extreme events and crises observed in an all-solid-state laser with modulation of losses.

We report observations of extreme events (or dissipative optical rogue waves) in a laser with a modulated parameter (cavity losses). Experimental data supporting the hypothesis that these events are related with multi-stability and external crises is presented. It is also shown that the time separation between a pulse and an extreme event can be predicted more accurately than that between pulses of average intensity, in agreement with the theoretical description and opening the road to the prediction and control of extreme optical events.

Extreme events in the Ti:sapphire laser.

We report experimental and theoretical evidence of the existence of extreme value events in the form of scarce and randomly emerging giant pulses in the femtosecond (self-pulsing or Kerr-lens mode-locked) Ti:sapphire laser. This laser displays complex dynamical behavior, including deterministic chaos, in two different regimes. The extreme value pulses are observed in the chaotic state of only one of these two regimes. The observations agree with the predictions of a well-tested theoretical model that does not include noise or self-Q-switching into its framework. This implies that, in this laser, the extreme effects have a nontrivial dynamical origin. The Ti:sapphire laser is hence revealed as a new and convenient system for the study of these effects.

Deterministic optical rogue waves.

Experimental observations of rare giant pulses or rogue waves were done in the output intensity of an optically injected semiconductor laser. The long-tailed probability distribution function of the pulse amplitude displays clear non-Gaussian features that confirm the rogue wave character of the intensity pulsations. Simulations of a simple rate equation model show good qualitative agreement with the experiments and provide a framework for understanding the observed extreme amplitude events as the result of a deterministic nonlinear process.

Extreme events and single-pulse spatial patterns observed in a self-pulsing all-solid-state laser.

The passively Q-switched, self-pulsing all-solid-state laser is a device of widespread use in many applications. Depending on the condition of saturation of the absorber, which is easy to adjust, different dynamical regimes are observed: continuous-wave emission, stable oscillations, period doubling bifurcations, chaos, and, within some chaotic regimes, extreme events (EEs) in the form of pulses of extraordinary intensity. These pulses are sometimes called "dissipative optical rogue waves." The mechanism of their formation in this laser is unknown. Previous observations suggest they are caused by the interaction of a few transverse modes. Here we report a direct observation of the pulse-to-pulse evolution of the transverse pattern. In the periodical regimes, sequences of intensities are correlated with sequences of patterns. In the chaotic ones, a few different patterns alternate, and the EEs are related with even fewer ones. In addition, the series of patterns and the pulse intensities before and after an EE are markedly repetitive. These observations demonstrate that EEs follow a deterministic evolution, and that they can appear even in a system with few interacting modes. This information plays a crucial role for the development of a mathematical description of EEs in this laser. This would allow managing the formation of EE through control of chaos, which is of both academic and practical interest (laser rangefinder).

Efficacy of polyethylene glycol-propylene glycol-based lubricant eye drops in reducing squamous metaplasia in patients with dry eye disease.

PURPOSE: Squamous metaplasia in dry eye disease (DED) manifesting as the loss of conjunctival goblet cells results in reduced mucin secretion and tear film instability. The objective of this study was to evaluate the efficacy of a polyethylene glycol-propylene glycol/hydroxypropyl-guar (PEG-PG/HP-guar) artificial tear formulation in reducing the squamous metaplasia in patients with DED using conjunctival impression cytology (CIC). METHODS: In this Phase IV, single-arm, open-label study, DED patients (aged ≥18 years) with a corneal staining sum score ≥3 and tear film break-up time (TFBUT) <7 s self-administered the PEG-PG/HP-guar artificial tears, 3 times a day for a period of 90 days. The primary end point was the change from baseline in goblet cell density (Nelson's CIC grading score) over the treatment period. Other end points were change in the corneal and conjunctival staining scores, and TFBUT. Statistical evaluation was performed using a paired t-test. RESULTS: In total, 49 patients (n=98 eyes) completed the study. Compared with baseline, there was a significant reduction in the mean CIC scores (ie, improvement in goblet cell density) at Days 30, 60, and 90 (1.6±0.5 vs 1.2±0.5, 0.9±0.5, and 0.8±0.5; P<0.0001). At Day 90, 22% of eyes demonstrated squamous metaplasia Grade 0 (ie, normal epithelium). Similar improvements were observed in the corneal staining scores (5.7 vs 3.1, 1.1, and 0.5; P<0.0001), conjunctival staining scores (5.5 vs 3.6, 1.6, and 0.9; P<0.0001), and TFBUT (4.8 vs 5.8, 6.3, and 6.8 s; P<0.0001) at Days 30, 60, and 90, respectively. CONCLUSION: In this study, treatment with PEG-PG/HP-guar artificial tears for 90 days decreased CIC score, reduced corneal and conjunctival staining, and increased TFBUT in patients with DED. These results suggest that PEG-PG/HP-guar artificial tears can improve the ocular surface health and reverse the changes induced by squamous metaplasia in DED.

Multiplicative noise in the longitudinal mode dynamics of a bulk semiconductor laser.

We analyze theoretically and experimentally the influence of current noise on the longitudinal mode hopping dynamics of a bulk semiconductor laser. It is shown that the mean residence times on each mode have different sensitivity to external noise added to the bias current. In particular, an increase of the noise level enhances the residence time on the longitudinal mode that dominates at low current, evidencing the multiplicative nature of the stochastic process. A two-mode rate equation model for a semiconductor laser is able to reproduce the experimental findings. Under a suitable separation of the involved time scales, the model can be reduced to a one-dimensional bistable potential system with a multiplicative stochastic term related to the current noise strength. The reduced model clarifies the influence of the different noise sources on the hopping dynamics.

Stochastic resonance in bulk semiconductor lasers.

The stochastic time scale of the mode hopping in a bulk semiconductor laser can be varied maintaining the symmetry of the residence times by a proper tuning of the laser substrate temperature and pumping current. While the addition of external noise to the pumping current affects the symmetry of the mode-hopping process, a sinusoidal modulation does not, providing that the modulation amplitude is below a critical value. In this case, we observe stochastic resonance in the modal intensities of the laser. We show the occurrence of the phenomenon in the spectral domain, and we characterize it by a statistical analysis based on the residence times probability distributions. The evidence of bona fide resonance is also provided, varying the modulation frequency and analyzing a proper statistical indicator. Changing the temperature of the laser substrate we show that resonance occurs at different modulation periods always equal to the double of the average residence time measured without modulation.

Experimental evidence of van der Pol-Fitzhugh-Nagumo dynamics in semiconductor optical amplifiers.

Thermo-optical pulsing in semiconductor amplifiers is experimentally shown to correspond to a very common excitable scenario (the van der Pol-Fitzhugh-Nagumo system). Self-sustained oscillations appear in the sequence predicted by this simple dynamical model as we change either the injection level or the bias current. Periodic modulation of these parameters leads to the characteristic phase-locking structure. Furthermore, coherence resonance is observed when external noise is added to the system.

Coupled optical excitable cells.

In this work we investigate experimentally the dynamics of two coupled optical excitable cells, namely, two semiconductor lasers with optical feedback. We analyze the dynamics observed in terms of the statistical properties of the time series and in terms of the phase space reconstruction from the data. We build a model based on a simple set of deterministic equations (on a two torus) plus noise in order to capture the essential features of the dynamics observed. We discuss the validity of our theoretical results in terms of families of excitable systems and coupling terms.

Effects of Systane(®) Balance on noninvasive tear film break-up time in patients with lipid-deficient dry eye.

PURPOSE: To evaluate the ability of Systane(®) Balance (SYSB) administered four times per day for 4 weeks to increase noninvasive tear film break-up time (NITFBUT) over baseline compared with a saline (SAL) control in patients with lipid-deficient dry eye (DE). PATIENTS AND METHODS: Patients aged ≥18 years with DE and evidence of meibomian gland dysfunction (ie, abnormal gland expression and missing meibomian glands) were included in this randomized, parallel-group, controlled, investigator-masked comparison study. Patients were randomized to SYSB or SAL four times daily for 4 weeks. The primary efficacy variable was mean change in NITFBUT from baseline at week 4. Ocular surface staining, goblet cell density, and meibomian gland expression were also assessed. Safety assessments included adverse events (AEs), best-corrected visual acuity, and ocular signs. RESULTS: A total of 49 patients received study treatments (SYSB, n=25; SAL, n=24). Most patients were women (67.4%) and Caucasian (63.3%); mean ± standard deviation (SD) age was 44±19 years. DE characteristics at baseline were similar between groups. After 4 weeks of treatment, the mean ± SD NITFBUT increase from baseline was significantly greater with SYSB (2.83±0.74 seconds) compared with SAL (0.66±0.55 seconds; P<0.001, t-test). Improvements in conjunctival and corneal staining, percentage of patients with increased goblet cell density, and meibomian gland expression were also observed with 4 weeks of SYSB over SAL. No AEs were reported for either treatment group; best-corrected visual acuity and ocular signs remained stable or improved compared with baseline. CONCLUSION: SYSB restored tear film stability, improved ocular surface healing, and improved meibomian gland functionality after 4 weeks of use in patients with lipid-deficient DE. No AEs were reported with either SYSB or SAL.

Cavity solitons as pixels in semiconductor microcavities.

Cavity solitons are localized intensity peaks that can form in a homogeneous background of radiation. They are generated by shining laser pulses into optical cavities that contain a nonlinear medium driven by a coherent field (holding beam). The ability to switch cavity solitons on and off and to control their location and motion by applying laser pulses makes them interesting as potential 'pixels' for reconfigurable arrays or all-optical processing units. Theoretical work on cavity solitons has stimulated a variety of experiments in macroscopic cavities and in systems with optical feedback. But for practical devices, it is desirable to generate cavity solitons in semiconductor structures, which would allow fast response and miniaturization. The existence of cavity solitons in semiconductor microcavities has been predicted theoretically, and precursors of cavity solitons have been observed, but clear experimental realization has been hindered by boundary-dependence of the resulting optical patterns-cavity solitons should be self-confined. Here we demonstrate the generation of cavity solitons in vertical cavity semiconductor microresonators that are electrically pumped above transparency but slightly below lasing threshold. We show that the generated optical spots can be written, erased and manipulated as objects independent of each other and of the boundary. Numerical simulations allow for a clearer interpretation of experimental results.