Daniel Hennequin

Characterization and modelling of the hollow beam produced by a real conical lens

The properties of the hollow beam produced by a conical lens are studied in detail. In particular, the impact of a rounded vertex is examined. It is shown that it could lead to drastic changes in the transverse distribution of the hollow beam, determined by the ratio between the transverse size of the incident beam and the size of the blunt area. An adequate choice for this ratio allows us to either minimize the losses or optimize the distribution symmetry.

Homoclinic chaos in a laser containing a saturable absorber

Checks of homoclinic chaos made with nonlinear analysis techniques have been performed on the signals coming from a CO2 laser containing CH3I as a saturable absorber. The one-dimensional return maps of the regimes appearing inside the alternating periodic chaotic sequence are typical of homoclinic chaos. Numerical simulations give results in good agreement with the experimental observations. In the case of a fast absorber, a homoclinic tangency to a cycle created in a suberitical Hopf bifurcation is seen to be responsible for the homo-clinic behavior observed in the model.

Instabilities and chaos in an infrared laser with saturable absorber: experiments and vibrorotational model

The instabilities and chaos in a CO2 laser containing SF6 and 15NH3 absorbers have been studied as a function of the parameters. By making use of a phase-portrait analysis, the instabilities have been classified through laser-control orbits in the phase space around the laser-with-saturable-absorber (LSA) fixed points. A chaotic regime, their a sequence of period-doubling bifurcations, has been observed for an instability of limit cycles reached through one fixed point. The transition between different instability operations presents an intermediate regime, around defined as the hesitation regime and have characterized through the fluctuations in the return times. which we have phenomena have been reproduced within a model, including the rotational–vibrational structure of The observed and absorber media. The numerical analysis has shown that the LSA time evolution, as described the amplifier orbits in the LSA phase space, depends on the relative attractions of the saddle point and the through homoclinic saddle focus fixed points.

Basic transverse dynamics of a photorefractive oscillator

The basic phenomena of transverse dynamics of a two-wave mixing photorefractive oscillator (PRO) in the weakly multimode case are considered. The evolution of the transverse patterns as a function of two control parameters, namely, the cavity length and the PRO gain, is investigated. The behavior is described in terms of the modes of the empty cavity. It is found that the mode interactions occur differently, depending on whether the modes belong to the same frequency-degenerate family or not, although in both cases the observed behavior is periodic.

Spatio-temporal dynamics of a weakly multimode CO2 laser

Abstract The behavior of a slightly asymmetric weakly multimode CO 2 laser is studied experimentally and compared with that of perfectly cylindrically symmetrical lasers. It is found that in most cases, the system symmetry breaking destroys the vortices, although it appears that for a very small loss of symmetry, the behavior of the laser remains symmetrical.

Frequency conversion in external cavity semiconductor lasers exposed to optical injection

We have investigated experimentally and theoretically the phenomenon of frequency conversion in a semiconductor laser subjected simultaneously to delayed feedback and optical injection. We report observations of frequency conversion in the nearly degenerate detuning range, even with the production of a cascade of higher harmonics, through the process of multiwave mixing. We demonstrate that the external feedback exerts a strong influence on the conversion process, by narrowing the linewidth of the converted signal, mainly, by affecting the conversion efficiency. Maxima in the efficiency occur whenever the frequency of the injected signal fulfils the phase conditions imposed by the external cavity; moreover, the choice of the feedback parameters controls the detuning range within which frequency conversion is allowed.

Instabilities in a bimode CO2 laser with a saturable absorber

Abstract We report on the observation of mode hopping in a bimode CO 2 laser with a saturable absorber. When the control parameters are changed, the output intensity of this laser shows a succession of periodic regimes that may become completely erratic. Each of these regimes may be explained considering the role played alternatively by two unstable fixed points corresponding to the steady state intensity of each mode. In a particular range of the parameter space, a description of multipeaked regimes is given using the Farey arithmetic, and it ressembles that observed in the Belousov-Zhabotinskii reaction. Transverse effects are observed that result from the nonlinear coupling of transverse cavity modes with both the active and passive media.

Dark optical lattice of ring traps for cold atoms

We propose an optical lattice for cold atoms made of a one-dimensional stack of dark ring traps. It is obtained through the interference pattern of a standard Gaussian beam with a counterpropagating hollow beam obtained using a setup with two conical lenses. The traps of the resulting lattice are characterized by a high confinement and a filling rate much larger than unity, even if loaded with cold atoms from a magneto-optical trap. We have implemented this system experimentally, and demonstrated its feasibility. Applications in statistical physics, quantum computing, and Bose-Einstein condensate dynamics are conceivable.

Multi-stability for an extended-cavity diode laser with intracavity atomic absorber

Either single- or multi-mode emission from a diode laser coupled to an external cavity containing a cell filled with caesium vapour has been observed. The oscillation frequency of each operating mode of the laser is locked to a hyperfine transition or crossover resonance of the saturated absorption spectrum of the caesium line. A model based on the rate equations for external-feedback diode lasers, modified in order to include frequency-dependent losses and dispersion of the laser field through the atomic absorber, has been developed. We show that the atomic absorption resonances determine the lasing frequencies. A multi-stable behaviour is obtained.

Improved correlation dimension estimates through change of variable

Abstract To evaluate the correlation dimension of chaotic regimes of a CO 2 laser with modulated losses, attractor reconstruction using the method of delays is performed using the logarithm of the intensity rather than the intensity itself. Improved convergence with respect to the embedding dimension and better reliability are observed.