Two beam interferometric inscription of UV femtosecond fiber Bragg gratings

Abstract

The inscription of fiber Bragg gratings with femtosecond laser radiation gives access to a wide range of new materials beyond conventional UV-photosensitive and special hydrogen loaded materials. Typically, infrared wavelengths have been used until now for femtosecond Bragg grating inscription. The use of UV femtosecond laser radiation gives more direct access to two-photon absorption in materials with high band-gaps and could achieve gratings with high spatial resolution. The refractive index modulation process for UV femtosecond pulses is then obtained as a combination of classical color center absorption and high intensity structural modification. Besides the choice of the inscription laser wavelength, also the inscription method is of great practical importance for the grating properties. The most commonly used phase mask technique suffers from mask degradation effects in case of high intensity laser pulses and gives only limited flexibility in variation of the Bragg reflection wavelengths. We have therefore investigated the use of an interferometric technique in combination with UV femtosecond laser pulses. The relevant parameters of spatial and temporal coherence have been considered and the wavelength tuning and reflection properties have been tested for several different types of fibers.