Inscription of fiber Bragg gratings in non-photosensitive and rare-earth doped fibers applying ultrafast lasers
Up to now, Fiber Bragg Gratings(FBG) were mainly written by absorption of an UV interference pattern inducing a periodical permanent refractive index modification in the fiber core. However, this method requires the use of photosensitive fibers and is difficult to apply for the realisation of FBGs in rare-earth-doped fibers, which is necessary for the realization of stable and robust fiber lasers. Therefore, an alternative technique allowing the flexible inscription of FBGs in fibers almost independently of their chemical composition was developed based on the non-linear absorption of femtosecond pulses to induce a refractive index change into the glass material. In this work, we investigate the inscription of FBGs with femtosecond pulses into different non-photosensitive fibers (e.g. standard telecommunication fibers, rare-earth doped fibers and polarization maintaining fibers). Because of the short coherence length of the pulses, a standard interferometric method is difficult to implement with the required accuracy. Instead, a phase mask was used to produce the interference pattern by overlapping a pair of diffracted beams in the phase mask vicinity. In order to increase the grating reflectivity and to allow a better control of the grating bandwidth, the grating length was increased by translating both phase mask and fiber under the writing beam. Using this method, highly reflective 40 mm long FBGs were demonstrated. Thermal investigations demonstrated that the stability of the femtosecond written FBGs is better than for typical type I UV gratings. Furthermore, we demonstrated the first inscription of FBGs in rare-earth doped fibers and polarization maintaining fibers using femtosecond pulses. These FBGs have been successfully implemented as resonator mirrors in different fiber lasers types. This opens the possibility for the realization of more robust and powerful fiber lasers in which the FBGs are directly written into the rare-earth doped fiber.