Phase Mask Integrated Aperture Shaping for Efficient Writing of Multiple Notch Filters in the Form of Fiber Bragg Gratings

Fiber Bragg gratings (FBG), consisting of a periodical modulation of the refractive index in the fiber core, offer fiber integrated narrowband filtering. In astronomy they can be utilized e.g. as hydroxyl (OH) emission line filters for observation in the mid-IR wavelength range [1]. Phase mask inscription with IR-femtosecond lasers is an ideal tool for realization of those filters, as it provides a highly reproducible inscription process creating low loss and high performance FBG, suitable also for various fiber types enabling e.g. filters in the 2 µm wavelength range. As the OH emission lines vary in their bandwidth and strength, ideally the notch filters have to be adapted as well to provide optimal filtering with minimum losses. For a targeted reflectivity, the spectral bandwidth of an FBG is controlled by the length of the structure. Also, for spectral side lobe suppression, the FBG have to be apodized. Specially designed shaping apertures [2] have shown potential to overcome the somewhat restricted envelope shaping for the phase mask inscription technique. Here, we present an integration of the shaping aperture into the phase mask, offering a more stable writing process and an increasing reproducibility.