Fiber Bragg gratings (FBGs) are key components for optical telecommunication systems and sensoring applications, allowing the realization of integrated devices by avoiding complex alignment schemes and decreasing coupling losses. FBGs also allow in-line spectral control of guided light and can be used as narrow-band reflectors for compact fiber laser applications. They are constituted of a periodic refractive index change in the fiber core. 
    The inscription of the FBGs was realized by using a phase-mask scanning technique. The phase-mask of period 2.15 μm was written in a fused-silica substrate with electron-beam lithography and an ion- etching technique. The diffraction efficiency in the +1st and −1st diffraction orders was 35%, and the transmission of the incoming beam (zeroth order) was less than 5%. The interference pattern obtained by overlapping the +1st and −1st diffraction orders results in a fiber grating period of 1.075 μm. This corresponds to the second diffraction order for the design wavelength of 1.55 μm within the fiber. Al- though the intensity of the +1st and −1st diffraction orders have been optimized during the phase-mask design, a non-negligible part of the diffracted light is still contained in the other diffraction orders, which can lead to multiple interference patterns. By using the order walk-off effect, we obtained pure two-beam interference for a distance of 1.5 mm between the phase mask and the fiber. 
    The experimental setup for the FBG inscription is shown in Figure. The fiber used for our experiments is an aluminosilicate Er-doped fiber (Liekki Er80-8/150) free from Ge or any other photosensible dopants. The femtosecond laser beam is focused into the core of the single-mode Er-doped fiber by a cylindrical lens f = 40 mm to get enough peak intensity for the refractive index change. The calculated focused beam width of 8.5 μm is comparable with the 8 μm of the core diameter. To get sufficient grating reflectivity, we translated both the phase mask and the fiber with a constant velocity of 4 mm/min by using a high-precision translation stage. Hence a grating having a length of 40 mm was inscribed with a pulse energy of 600 μJ. The grating reflectivity R 98.7% was deduced from the trans- mission loss. 
    Using the doped fiber containing the FBG, we set up a fiber laser as can be seen in Figure. Lasing operation was achieved at 1554.43 nm in accordance with the measurement of the transmission spectrum. Note that the laser output bandwidth is much narrower than the FBG reflection/transmission peak. The laser also has a remarkably high signal-to-noise ratio of 60 dB because of the small reflection bandwidth of the Bragg grating, allowing a wavelength-selective feedback into the cavity. Thus spontaneous emission could not be amplified by another pass through the laser cavity. The evolution of the laser output power versus pump power exhibits a slope efficiency of 21.1%. The maximum output power obtained was 38 mW for a launched power of 290 mW. As no saturation was observed, higher pump powers should yield significantly higher output powers. In this study, we have demonstrated the first, to our knowledge, FBG inscription into a nonphotosensitive Er-doped fiber by using femtosecond pulses and a phase-mask scanning technique. This opens the possibility for highly integrated fiber laser systems.

    Erbium fiber laser based on intracore femtosecond-written fiber Bragg grating
    E. Wikszak, J. Thomas, J. Burghoff, B. Ortaç, J. Limpert, S. Nolte, U. Fuchs and A. TünnermannOptics Letters, vol. 31, pp. 2390-2392 (2006)

    We report the inscription of a FBG into a polarization maintaining (PM) Yb-doped fiber using IR femtosecond pulses and a phase-mask scanning technique for the first time to our knowledge. This FBG was used to realize a stable, linearly polarized, compact fiber laser with a slope efficiency of 27% and an unsaturated output power of 100 mW. The grating was written in a panda-type PM Yb-doped fiber (fiber core diameter 4.8 μm, cladding diameter 125 μm) with near-IR (800 nm – 50 fs) ultra-short pulses.

    Linearly polarized ytterbium fiber based on intracore femtosecond-written fiber Bragg gratings

    E. Wikszak, J. Thomas, S. Klingebiel, B. Ortaç, J. Limpert, S. Nolte and A. TünnermannOptics Letters, vol. 32, pp. 2756-2758 (2007)