Femtosecond Inscription of a Spectral Array of Seven Fiber Bragg Gratings at the Same Spot Using a Single Uniform Phase-Mask

Femtosecond induced refractive index changes in transparent materials have been widely explored in the past twenty-five years. Many devices and applications such as fiber Bragg gratings (FBGs), waveguides, couplers, and filters have been presented using fs inscriptions [1]. During this period, fs inscriptions of FBGs have been widely explored, and grating inscriptions in different types of fibers have been demonstrated. Today, there are mainly two different approaches for inscribing FBGs with a fs laser. First, there are the point-by-point (PbP) [2], the line-by-line (LbL) [3], and the (PlbPl) [4] techniques in which grating planes are formed one by one. These techniques offer high flexibly in grating inscription, particularly regarding the choice of the Bragg wavelength and the grating profile. However, the inscription system is very demanding with regard to both optical and mechanical accuracy aspects. Second, there is the phase-mask (PM) technique, where the whole grating length is inscribed at once with a PM and a focusing cylindrical lens [5]. Grating inscription using a PM is a simpler and more robust technique, yet less flexible because the Bragg wavelength is pre-determined by the PM period. Fortunately, tuning the Bragg wavelength has been demonstrated by introducing defocusing and aberrations to the fs inscription beam [6]–[8].