Spatially coherent top-hat beam output from a large mode area microstructured single-mode fiber

Summary form only given. Fibre technology has improved the compactness, stability and versatility of laser systems. Most of applications need a spatially coherent beam and therefore single-mode fibres are mandatory. Nevertheless the intensity profile of theses fibres usually has a Gaussian-like shape. For some applications, this profile must be transformed to exhibit a `top-hat' intensity profile. Different techniques already exist to get such a profile [1], but with strong drawbacks: losses, alignment difficulties or spatial incoherence. To overcome these difficulties, an elegant and efficient solution is to achieve a single-mode fibre which directly delivers a spatially coherent `tophat' beam. In fact, microstructured optical fibre technology provides a powerful means to develop a large-mode-area fibre with top-hat fundamental mode profile. Accordingly, we have designed, realized and characterized low-losses large mode area microstructured fibres delivering a top-hat beam [2]. As can be seen on Fig. 1, a seven missing air hole architecture has been chosen with a silica core surrounded by a thin step index ring made of germanium doped silica. This is a key element since it is responsible of the flattened transverse energy distribution of the mode. We have fabricated the fiber by means of the usual stack and draw technique and the Germanium oxide ring has been deposited by using an Outside Vapour Deposition OVD method. The fundamental mode field diameter of the fibre is about 21 μm, giving an effective area of 360 μm2. To ensure the spatial coherence of the output beam, we measure the modal content of two 5m-long-fibres by S2 measurement [3]. The first one was designed to be multi-mode (by increasing the pitch and air filling fraction of the cladding) and it is used to validate the measurement. The second one is the single-mode fibre with a tophat beam output. The result of the S2 measurement is shown in the Fig. 2. By scanning the wavelength of the light coupled in the fibre, we measure with a CCD-camera the spectral dependence of the output beam. By this way we get a spectral interferogram on each pixel. The treatment of the Fourier Transforms of these interferograms gives the profiles and the weights of the modes [3]. In the Fig. 2(a) the curves represent the sums of these Fourier Transforms. The blue-dotted one corresponds to the multi-mode fibre; the peak at 9 ps is the result of the interferences between the two modes of the fibre. No such a peak appears in the red curve corresponding to the second fibre whatever the injection conditions. It clearly demonstrates that the second fibre is single-mode. Conclusion: We present a low-loss fibre exhibiting a `top-hat' beam output with a great stability. We also demonstrate by an S2 measurement that the fibre is single-mode. To the best of our knowledge it is the first realisation of a single-mode fibre which delivers a top-hat beam. To meet industrial requirements in beamshaping this fibre presents a great interest, mainly to conserve coherent beams and great depth of focus.