Femtosecond inscription of chirped fiber Bragg gratings and fiber Bragg grating arrays using a single uniform phase-mask

Chirped fiber Bragg gratings (CFBGs) and arrays of fiber Bragg gratings (FBGs) are inscribed with femtosecond pulses and a single uniform phase-mask (PM). The inscription setup includes an 800 nm NIR femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. The wavelength tunability is achieved by a defocusing lens, and movement of the PM. A first FBG is inscribed, followed by nine cascading FBGs, where each one is inscribed only after the movement of the PM and a movement of the fiber, thus allowing inscription on a fresh fiber section without overlapping. The transmission and reflection spectra of these ten cascaded FBGs are measured showing a spectral shift of ∼0.9 nm between each one, and a total shift of ∼8 nm. The transmission dip of each FBG is roughly −10 dB. The CFBG is inscribed with the same single uniform PM and with the same setup, but with different movement increments of the PM and fiber. The transmission and reflection spectra of the resulting CFBG are measured with a strength of ∼20 dB and a continuous bandwidth of ∼4.7 nm at FWHM. This yields a chirp rate of ∼1 nm/cm, while the full length of the CFBG is ∼45 mm.