Mid-infrared sub-wavelength grating mirror design: tolerance and influence of technological constraints

High polarization selective Si/SiO2 mid-infrared sub-wavelength grating mirrors with large bandwidth adapted to VCSEL integration are compared. These mirrors have been automatically designed for operation at \lambda = 2.3 $\mu$m by an optimization algorithm which maximizes a specially defined quality factor. Several technological constraints in relation with the grating manufacturing process have been imposed within the optimization algorithm and their impact on the optical properties of the mirror have been evaluated. Furthermore, through the tolerance computation of the different dimensions of the structure, the robustness with respect to fabrication errors has been tested. Finally, it appears that the increase of the optical performances of the mirror imposes a less tolerant design with severer technological constraints resulting in a more stringent control of the manufacturing process.Comment: The final publication is available at http://iopscience.iop.org/2040-8986/13/12/125502

On the statistical interpretation of optical rogue waves

Numerical simulations are used to discuss various aspects of "optical rogue wave" statistics observed in noise-driven fiber supercontinuum generation associated with highly incoherent spectra. In particular, we consider how long wavelength spectral filtering influences the characteristics of the statistical distribution of peak power, and we contrast the statistics of the spectrally filtered SC with the statistics of both the peak power of the most red-shifted soliton in the SC and the maximum peak power across the full temporal field with no spectral selection. For the latter case, we show that the unfiltered statistical distribution can still exhibit a long-tail, but the extreme-events in this case correspond to collisions between solitons of different frequencies. These results confirm the importance of collision dynamics in supercontinuum generation. We also show that the collision-induced events satisfy an extended hydrodynamic definition of "rogue wave" characteristics.Comment: Paper accepted for publication in the European Physical Journal ST, Special Topics. Discussion and Debate: Rogue Waves - towards a unifying concept? To appear 201

Great ape gestures : intentional communication with a rich set of innate signals

Great apes give gestures deliberately and voluntarily, in order to influence particular target audiences, whose direction of attention they take into account when choosing which type of gesture to use. These facts make the study of ape gesture directly relevant to understanding the evolutionary precursors of human language; here we present an assessment of ape gesture from that perspective, focusing on the work of the “St Andrews Group” of researchers. Intended meanings of ape gestures are relatively few and simple. As with human words, ape gestures often have several distinct meanings, which are effectively disambiguated by behavioural context. Compared to the signalling of most other animals, great ape gestural repertoires are large. Because of this, and the relatively small number of intended meanings they achieve, ape gestures are redundant, with extensive overlaps in meaning. The great majority of gestures are innate, in the sense that the species’ biological inheritance includes the potential to develop each gestural form and use it for a specific range of purposes. Moreover, the phylogenetic origin of many gestures is relatively old, since gestures are extensively shared between different genera in the great ape family. Acquisition of an adult repertoire is a process of first exploring the innate species potential for many gestures and then gradual restriction to a final (active) repertoire that is much smaller. No evidence of syntactic structure has yet been detected.Publisher PDFPeer reviewe

Real-time measurements of dissipative solitons in a mode-locked fiber laser

Dissipative solitons are remarkable localized states of a physical system that arise from the dynamical balance between nonlinearity, dispersion and environmental energy exchange. They are the most universal form of soliton that can exist in nature, and are seen in far-from-equilibrium systems in many fields including chemistry, biology, and physics. There has been particular interest in studying their properties in mode-locked lasers producing ultrashort light pulses, but experiments have been limited by the lack of convenient measurement techniques able to track the soliton evolution in real-time. Here, we use dispersive Fourier transform and time lens measurements to simultaneously measure real-time spectral and temporal evolution of dissipative solitons in a fiber laser as the turn-on dynamics pass through a transient unstable regime with complex break-up and collision dynamics before stabilizing to a regular mode-locked pulse train. Our measurements enable reconstruction of the soliton amplitude and phase and calculation of the corresponding complex-valued eigenvalue spectrum to provide further physical insight. These findings are significant in showing how real-time measurements can provide new perspectives into the ultrafast transient dynamics of complex systems.Comment: See also M. Narhi, P. Ryczkowski, C. Billet, G. Genty, J. M. Dudley, Ultrafast Simultaneous Real Time Spectral and Temporal Measurements of Fibre Laser Modelocking Dynamics, 2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference, paper EE-3.5 (2017

Robust design of Si/Si3N4 high contrast grating mirror for mid-infrared VCSEL application

A Si/Si3N4 high contrast grating mirror has been designed for a VCSEL integration in mid-infrared ({\lambda} = 2.65 $\mu$m). The use of an optimization algorithm which maximizes a VCSEL mirror quality factor allowed the adjustment of the grating parameters while keeping large and shallow grating pattern. The robustness with respect to fabrication error has been enhanced thanks to a precise study of the grating dimension tolerances. The final mirror exhibits large high reflectivity bandwidth with a polarization selectivity and several percent of tolerance on the grating dimensions.Comment: The final publication is available at http://www.springerlink.com, Optical and Quantum Electronics (2012) Online Firs

Transitions dipolaires induites par collisions sur un faisceau de formaldéhyde. Effet d'un champ électrique

Using a H2CO molecular beam, a method is first described which permits the observation of rotational transitions ΔJ = ± n (n ≽ 1), induced by collisions with the molecules of a target gas. In the case H2CO → H 2CO, selection rules are defined and rotational resonance effects are observed on the differential cross sections for some ΔJ = — 1 transitions. It appears that a direct electric field E considerably modifies the H2CO cross sections σJ. Using the pair H 2CO → NH3, the study of σJ variations with E shows that the ΔJ = 0 transitions become forbidden, when E is intense. The experimental results are compared with the theoretical transition probabilities variations, due to E and calculated in the Born approximation. An experimental method is deduced which permits, by application of an electric field, to select AJ = 0 transitions on a molecular beam.Nous présentons d'abord une méthode expérimentale qui permet d'observer les transitions rotationnelles ΔJ = ± n (n ≽ 1) induites par collisions entre les molécules d'un faisceau de formaldéhyde et un gaz tampon. En prenant H2CO comme gaz tampon, nous précisons les règles de sélection et, sur les sections de collisions correspondant à plusieurs transitions ΔJ = — 1, nous mettons en évidence des effets de résonance rotationnelle. Nous constatons qu'un champ électrique continu modifie considérablement les sections de collisions du formaldéhyde. Sur le système dipolaire H2 CO-NH3, l'étude des variations caractéristiques de celles-ci en fonction du champ appliqué montre que les transitions ΔJ. = 0 de H2CO deviennent interdites en champ fort. Nous comparons les résultats expérimentaux aux variations de probabilité de transition dues au champ électrique et calculées dans l'approximation de Born. On déduit une méthode expérimentale permettant de trier par effet de champ les transitions ΔJ = 0 sur le faisceau moléculaire