Spatially resolved 3D measurements of long-period gratings written by fs-laser inscription in large mode area fibers

High-power fiber lasers have reached kW power levels. The most important non-linear process limiting power scaling of industrial fiber lasers is stimulated Raman scattering. Long period gratings (LPGs) couple forward propagating core light to forward propagating cladding light and are well suited as a filter for the unwanted Raman scattering. In this paper we show for the first time of our knowledge the inscription of LPGs in large-mode-area (LMA) fibers with ultra-short laser pulses. We investigate the influence of different inscription parameters with a 3D, spatially resolved measurement of the induced index change. We present results from gratings with an attenuation of 8.5 dB at the desired wavelength with a small out-of-band loss of 1 dB.