Novel fibre laser sources based on point-by-point femtosecond laser inscribed fibre Bragg gratings

Like any other laser, a fibre laser consists of an active medium (in this case a piece of doped optical fibre) between two feedback elements. While Fresnel reflection from the fibre end facet can assume the role of a broadband partially reflecting output coupler, fibre Bragg gratings (FBGs) are an obvious choice for the implementation of a frequency selective high reflector in an all-fibre design were bulk optics can be avoided. However, the standard fabrication method for FBGs, the phase-mask technique, relies on the use of photosensitive fibre and is hence not well suited to rare-earth-doped fibres since they are typically based on aluminosilicate glass that exhibits a very low photosensitivity. Thus, either hydrogen loading has to be used which leads to a degradation of the laser performance, or the FBG has to be written into a photosensitive fibre which is subsequently sliced onto the laser fibre, again impairing the overall performance. Moreover, phasemask written gratings typically anneal out at high temperatures which renders them useless for high power laser applications. Recently, the femtosecond point-by-point (PbP) method has been introduced for the fabrication of FBGs in standard telecom fibres [1]. Due to its flexibility, this technique has proven to be extremely versatile. A femtosecond laser pulse modifies the refractive index in a small area within the core of a fibre and by synchronising the translation of the fibre with the 1 kHz repetition rate of the femtosecond laser, highly reflecting narrowband gratings with excellent thermal stability can be fabricated at arbitrary wavelengths. Based on that, we have demonstrated that the PbP method can be utilised to realise high power double-clad Ytterbium fibre lasers with unprecedented narrow linewidth output radiation [2] and that power scaling to the hundreds of Watt level is feasible [3]. In this paper we will give a short review of our activities in this area and will then present our most recent results. We will demonstrate that PbP FBGs can be inscribed into the active core of virtually any kind of laser fibre including Thulmium and Holmium doped double-clad fibres for applications in the 2 μm range, but also into polarisation maintaining (PANDA-style) fibres, the latter resulting in an extremely robust all-fibre oscillator with a switchable linear Polarisation. In addition, we will present results of extensive morphological and high-temperature annealing studies which we have performed on PbP FBGs. These investigations reveal the underlying structural changes which happen inside the glass-matrix. They also show that the scattering properties off the refractive index modulations constituting the Bragg grating as well as the short wavelength attenuation are analogous to those associated with type II gratings written with UV lasers.