Up to now, high-power fiber lasers have been widely used in industrial processing, national defense, scientific research, and other fields. However, stimulated Raman scattering (SRS) is one of the main factors limiting the power scaling of such fiber lasers. In recent years, chirped and tilted fiber Bragg gratings (CTFBGs) have been demonstrated to suppress the SRS in high-power lasers by filtering the Raman light. CTFBGs are traditionally fabricated by the ultraviolet laser phase mask method, which requires the fibers to be hydrogen-loaded and thermal-annealed before and after grating inscription, respectively. This process is time-consuming and costly, especially when CTFBGs are fabricated with large-core fibers since the time for hydrogen loading and thermal annealing increases as the fiber core expands. The development of the femtosecond (fs) laser inscription method provides an alternative to the fabrication of CTFBGs. As it has no requirement on fiber photosensitivity, hydrogen loading and thermal annealing are no longer necessary, which greatly shortens the fabrication period of CTFBGs. However, although chirped FBGs and tilted FBGs have been fabricated by fs-lasers, fs-laser-inscribed CTFBGs have not been reported yet, and this paper is expected to fill this research gap.
The inscription system for CTFBGs is shown in Fig. 1. The fs-laser emitted from the laser source is reflected by a reflecting mirror and is then focused on a fiber after passing through a chirped phase mask and a cylindrical lens successively. The fiber is fixed on a piezoelectric platform by a pair of fiber holders, and the reflecting mirror and the cylindrical lens are fixed on a one-dimension translation stage. Because the waist width of focal spot is smaller than the diameter of the fiber core, a tilted grating structure can be obtained by the oblique scanning of the fiber with the piezoelectric platform. Moreover, a larger chirp and a longer grating length can be achieved by moving the translation stage along the fiber axis.
Four CTFBGs with different tilt angles are fabricated, with their micrographs shown in Figs. 2(b) and 2(c). The tilted grating structure is clear and completely covers the fiber core. Fig. 3 presents the measured spectra of the CTFBGs. The transmission spectra suggest that as the tilt angle increases, the center wavelength moves towards a short wavelength. In addition, the depth decreases while the width increases. However, the Bragg wavelength of the CTFBGs does not change as the tilt angle increases according to the reflection spectra of the CTFBGs.
This paper takes the lead in inscribing CTFBGs with different tilt angles in large-mode-area double-cladding fibers with core/cladding diameter of 20/400 μm by fs-lasers, with a maximum filtering depth and bandwidth of ca. 15 dB and ca. 8.9 nm, respectively. This paper is of great significance to the research and development of CTFBGs.
