Mathieu Devautour

Management of the high-order mode content in large (40 μm) core photonic bandgap Bragg fiber laser

Very large-mode-area Yb(3+)-doped single-mode photonic bandgap (PBG) Bragg fiber oscillators are considered. The transverse hole-burning effect is numerically modeled, which helps properly design the PBG cladding and the Yb(3+)-doped region for the high-order mode content to be carefully controlled. A ratio of the Yb(3+)-doped region diameter to the overall core diameter of 40% allows for single-mode emission, even for small spool diameters of 15 cm. Such a fiber was manufactured and subsequently used as the core element of a cw oscillator. Very good beam quality parameter M(2)=1.12 and slope efficiency of 80% were measured. Insensitivity to bending, exemplified by the absence of temporal drift of the beam, was demonstrated for curvature diameter as small as 15 cm.

Nonchemical-vapor-deposition process for fabrication of highly efficient Yb-doped large core fibers

A new fabrication process of active optical silica glass based on direct sand vitrification is proposed. This method, an alternative to chemical vapor deposition (CVD), allows the fabrication of homogeneous and highly Yb(3+)-doped rods that are ten times larger in diameter than those produced by CVD. For large-mode-area fibers fabricated by the stack-and-draw method, this is a tremendous technical breakthrough that could offer great flexibility in fiber design. As a proof of concept, we focused here on the fabrication and characterization of active core material surrounded by pure silica. Consequently, we draw a simple multimode step-index fiber. The index ripple in the core that matches our objectives is approximately 2.2x10(-4). For this first demonstration, the core material is codoped with Yb(2)O(3) (3600 parts in 10(6)(ppm) by weight) and Al(2)O(3), yielding a 180 dB m(-1) absorption at a wavelength of 975 nm, whereas the background loss is around 0.8 dB m(-1). The continuous-wave laser obtained with this fiber exhibits 74% slope efficiency.

Microstructured fibres and applications

In this paper, we explore some new concepts of optical fibre designs exhibiting large active core surrounded by resonant cladding for high power delivery.

Alternative designs for high power single mode active optical fibres

In this paper, we propose, through different theoretical examples of composite fibre designs, some alternative ways to push up the limits of active optical fibres. Highly ytterbium-doped large cores are required for high power delivery and low non linear limitations. Such cores usually support several guided modes. We demonstrate in this paper that by surrounding the core with a tailored cladding, single mode transverse emission under amplification or in the lasing regime is possible. A 3D modelling of transverse mode competition in active fibres is first presented and applied to several cladding designs. This numerical model, which is wavelength sensitive, is of great interest for laser cavity design in continuous wave regime for example as both composite fibre and cavity designs are considered. Accurate modelling of modal competition requires to compute overlap between electric field and active region for each electromagnetic mode (Γmn) as a function of the transverse plane coordinates Γmn(x, y). The fibre design, the laser cavity, the pump power have to be considered in order to obtain an accurate description of 3D spatial population inversion and its consequences on modal competition under active regime. The numerical model is first applied to a fibre exhibiting an all solid cladding made of 2D array of high refractive index rods and then to a multi-layered fibre. The latter is used to illustrate our purpose in figure 1 which shows the comparison between lasers made of fibres with or without tailored cladding. Both fibres exhibit an Yb-doped (5625 wt-ppm) core with a diameter of 30 µm and a refractive index higher than that of silica (5×10−3). The pump power is 60 W. When the core is surrounded by pure silica, 17 modes can propagate and LP01 carries less than 20% of the total output power which is distributed among almost all modes (Fig. 1b). The laser obtained is highly multimode and the spatial quality of the beam is expected to be very low. On the contrary, when the structured cladding surrounds the core (Fig. 1a), almost all the laser power is emitted on the LP01 mode (Fig. 1c). With a 4% – 99%, 2 m long cavity, less than 8% of total power is emitted on the LP41, all other modes do not exist.

Optical Fiber Design And Fabrication: Discussion On Recent Developments

Level of emitted power and beam quality of singlemode fiber lasers have been drastically increased at the expense of loss due to bend sensitivity, simplicity of manufacturing and packaging. Furthermore, the extension of the spectral coverage was primarily explored by exploiting non‐linear effects, neglecting numerous possible transitions of rare earths. Through different research areas, we demonstrate the possibilities offered by new fiber designs and alternative methods of manufacturing. Photonic Band Gap fibers reconcile diffraction limited beam and large mode area with low bending loss. 80% slope efficiency is demonstrated together with a robust propagation allowing the fiber to be tightly bent until wounding radii as small as 6 cm. Highly ytterbium doped multimode core surrounded by high refractive index rods fiber exhibits a transverse singlemode behavior under continuous wave laser regime. A robust LP01 mode is observed and filtering effect is clearly observed. A non CVD process based on silica sand...