Fiber Bragg gratings in active large mode area fiber written with femtosecond pulses

Fiber gratings allow in-line spectral control of the guided light and can be used as narrow-band reflectors or for the selective coupling of light into other fiber modes. However, fibers used in "state of the art"- fiber lasers possess material properties and geometries that make it difficult or impossible to inscribe gratings with standard cw-UV-Laser processing. These difficulties can be overcome by techniques that rely on the refractive index change caused by nonlinear absorption of focused femtosecond pulses.