Growth and stability of UV and VIS femtosecond written fiber Bragg gratings in different rare earth doped fibers

Abstract

Using femtosecond (fs) radiation and multi-photon absorption processes for fiber Bragg grating (FBG) inscription offers the advantage of writing FBGs independent of the chemical fiber composition. Especially for fiber laser applications the fabrication of FBGs integrated in rare earth doped fibers is a favorable option for monolithic fiber lasers. In this paper we report on the growth and stability of femtosecond generated fiber Bragg gratings in different rare earth doped fibers. For this purpose we use two different fs laser wavelengths at 266 nm and 400 nm and a modified Talbot-interferometer setup for the generation of first order Bragg gratings. We study the growth characteristics of FBGs in terms of reflectivity, Bragg wavelength and spectral grating width during the writing with UV and VIS fs radiation. For these experiments fibers drawn in-house at the IPHT are used, which possess varying contents of Ytterbium and/or Cerium with a comparable Phosphor and Aluminum co-doping and a standard geometry (125 µm cladding, 8-10 µm core diameter). We observe different kinds of grating growth processes depending on the inscription wavelength and the specific doping level of the fibers. It is possible to produce high reflective Type I gratings by UV fs exposure and high reflective Type II gratings with higher temperature stability by VIS fs exposure. The transformation from Type I to Type II gratings with a 400 nm inscription wavelength is studied in dependence on the exposure conditions. Our experimental results underline the role of doping for fs material photosensitivity and for FBG inscription process.