Mode selective fiber Bragg gratings

Focussing ultrashort laser pulses allows for inscribing fiber Bragg gratings (FBGs) directly into rare earth doped fiber cores - without prior photosensitivity treatment. High reflective FBGs can be written into active Large Mode Area (LMA) Fibers with 20 micron core diameter using a phase mask scanning technique. Here, we demonstrate fiber Bragg gratings (FBGs), which cover only a fraction of the core. With this additional degree of freedom it is possible to taylor the reflectivity of individual modes. We show for example how those FBGs can be used in few mode LMA fibers to suppress reflections into higher order modes.