Valérie Doya

Experimental study of pump power absorption along rare-earth-doped double clad optical fibres

We present an experimental study of the influence of the inner cladding geometry on pump power absorption in double clad fibres. The experimental results are analyzed by taking into consideration the spatial intensity distribution of the modes in the inner cladding. Six different geometries and core dopant are investigated. By comparing the pump power absorption evolution along each fibre, we show that using a chaotic inner cladding geometry, a constant absorption coefficient is achieved independently of the launching light condition.

Selective amplification of scars in a chaotic optical fiber.

In this Letter we propose an original mechanism to select scar modes through coherent gain amplification in a multimode D-shaped fiber. More precisely, we demonstrate the selective amplification of scar modes by positioning a gain region in the vicinity of the self-focal point of the shortest periodic orbit in the transverse motion.

Condensation and thermalization of classsical optical waves in a waveguide

We consider the long-term evolution of a random nonlinear wave that propagates in a multimode optical waveguide. The optical wave exhibits a thermalization process characterized by an irreversible evolution toward an equilibrium state. The tails of the equilibrium distribution satisfy the property of energy equipartition among the modes of the waveguide. As a consequence of this thermalization, the optical field undergoes a process of classical wave condensation, which is characterized by a macroscopic occupation of the fundamental mode of the waveguide. Considering the nonlinear Schr¨ odinger equation with a confining potential, we formulate a wave turbulence description of the random wave into the basis of the eigenmodes of the waveguide. The condensate amplitudeiscalculatedanalyticallyasafunctionofthewaveenergy,anditisfoundinquantitativeagreementwith the numerical simulations. The analysis reveals that the waveguide configuration introduces an effective physical frequency cutoff, which regularizes the ultraviolet catastrophe inherent to the ensemble of classical nonlinear waves. The numerical simulations have been performed in the framework of a readily accessible nonlinear fiber optics experiment.

Numerical Modeling of Pump Absorption in Coiled and Twisted Double-Clad Fibers

We report the analysis of multimode pump absorption in double-clad rare-earth-doped fibers under realistic bending conditions. The finite-element beam-propagation method is used for the analysis. The fiber bending is approximated by the transformation of the refractive index profile of the fiber. Double-clad fibers of hexagonal, circular and stadium-like cross-sections are studied as examples. In addition, the double-clad waveguide structure of a two-fiber bundle is investigated. Simulations show that the bending effects cannot be neglected in double-clad fiber multimode pump propagation analysis and optimization. The reported rigorous numerical model opens new way to design double-clad fibers and to optimize the pump absorption efficiency. We show that high pump-absorption efficiency better than the ideal (ergodic) limit, can be achieved by simultaneous coiling and twisting of the double-clad fiber.

Unexpected light behaviour in periodic segmented waveguides.

In this article, it is shown that multimode periodic segmented waveguides (PSW) are versatile optical systems in which properties of wave chaos can be highlighted. Numerical wave analysis reveals that structures of quantum phase space of PSW are similar to Poincaré sections which display a mixed phase space where stability islands are surrounded by a chaotic sea. Then, unexpected light behavior can occur such as, input gaussian beams do not diverge during the propagation in a highly multimode waveguide.

Optical scar in a chaotic fiber

Abstract.We propose to use a multimode optical fiber with a D-shaped cross section as a privileged system to image wavefunctions of a chaotic system. Scar modes are in particular the subject of our investigations. We study their imprints on the statistics of intensity and we show how the introduction of a localized gain region in the fiber is used to perform a selective amplification of scar modes.

Complex behaviour of a ray in a Gaussian index profile periodically segmented waveguide

In this paper, we present a numerical analysis concerning ray propagation in a multimode periodic segmented waveguide with a Gaussian index segment profile. We show that this simple waveguide configuration exhibits a complex ray dynamics that can be regular or chaotic depending on the initial conditions.

Pump absorption in coiled and twisted double-clad hexagonal fiber: effect of launching conditions and core location

Ever extending applications of fiber lasers require energy efficient, high-power, small footprint and reliable fiber lasers and laser wavelength versatility. To meet these demands, next generation of active fibers for high-power fiber lasers is coming out that will eventually offer tailored spectroscopic properties, high robustness and reduced cooling requirements and improved efficiency through tailored pump absorption. We report on numerical modelling of the efficiency of the pump absorption in double clad active fibers with hexagonal shape of the inner cladding cross section and rare-earth-doped core. We analyze both the effect of different radii of the spool on which the fiber is coiled and different fiber twisting rates. Two different launching conditions were investigated: the Gaussian input pump beam and a speckle pattern that mimics the output of the pump laser diode pigtail. We have found that by asymmetric position of the rare-earth-doped core we can significantly improve the pump absorption.

Modal-field spectra analysis of pump absorption efficiency in double-clad rare-earth doped fibers (Conference Presentation)

High-power fiber lasers became important devices in many industrial and health care fields. The key for high-power operation of fiber lasers is the double-clad fiber technology transforming lower-brightness pumps into high-brightness laser beams. Efficient pump absorption in the active core of the double-clad fiber is crucial for reliable and economic operation of high power fiber lasers. In our recent work we extensively studied the dependence of the pump absorption efficiency on bending and twisting of the fiber. For the first time we theoretically predicted and later experimentally demonstrated significant enhancement of pump absorption efficiency by simultaneous bending and twisting of the double-clad fiber. In this contribution we provide extension of our previous theoretical studies using beam propagation model incorporating laser rate equations. The effect of bending and twisting on signal amplification in the double-clad fiber is analyzed for different input signal powers, and moreover, pump field modal spectra are evaluated. The results show that in correspondence with pump absorption efficiency the gain of the amplifier is enhanced under the conditions of simultaneously bent and twisted fiber. The key to understand the effect of bending and twisting on pump absorption efficiency consists in modal spectra of pump field propagating in the first clad of the double clad fiber. Three cases of straight, bent only, and simultaneously bent and twisted fiber are compared. The comparison shows that bending causes increase of the spectral range of propagating modes, but does not bring about mode-mixing. Substantial mode-mixing is established only in simultaneously bent and twisted fiber.

Regular modes in a mixed-dynamics-based optical fiber

A multimode optical fiber with a truncated transverse cross section acts as a powerful versatile support to investigate the wave features of complex ray dynamics. In this paper, we concentrate on the case of a geometry inducing mixed dynamics. We highlight that regular modes associated with stable periodic orbits present an enhanced spatial intensity localization. We report the statistics of the inverse participation ratio whose features are analogous to those of Anderson localized modes. Our study is supported by both numerical and experimental results on the spatial localization and spectral regularity of the regular modes.