Shaping the spectral response of ultrashort pulse phase mask written fiber Bragg gratings

We present an approach to control the spectral profile of ultrashort pulse phase mask written fiber Bragg gratings (FBG) by adding shaping apertures to the inscription scheme. They can have the form of a simple slit aperture, offering control over the grating length and enabling extremely short FBG. Furthermore, the aperture opening relative to the fiber core cross-section can be varied along the fiber propagation axis. This controls the coupling strength along the grating, allowing for e.g. side lobe suppression of constant period FBG or controlling ripples on chirped FBG. Shaping apertures enable a next degree of flexibility for the reliable phase mask inscription technique.