Fiber Bragg gratings (FBGs) fabricated by femtosecond laser have the advantages of light weight, high capacity wavelength division multiplexing, high mechanical strength, and excellent thermal stability. As a significant sensitive component of sensors, FBGs have been deployed widely in aerospace, nuclear power, metallurgy, bridges, and tunnels. One common method of their construction is directly writing FBGs point by point; however, this suffers from high loss. Various techniques have been proposed to reduce this loss, such as improving the writing path to line by line, shaping femtosecond laser beams, and selecting optical fibers with smaller core diameters. Despite reductions in loss, there remain deficiencies in their production, such as low fabrication efficiency and generality. In this study, an inscription method of FBG based on a small-aperture beam shaping technique has been proposed, which can be helpful for the efficient fabrication of low-loss FBGs.
First, the energy distribution of a focused Gaussian beam limited by aperture is analyzed, and the aperture condition of the filamentary focal field is obtained. A femtosecond laser writing device based on small aperture shaping is then built. When the aperture is gradually reduced from 10.0 mm to 0.5 mm, a series of second-order FBGs are written on standard quartz single-mode fibers with the coating removed. The lengths of the FBGs are 3 mm, the reflectivities are approximately 90%, and the center wavelengths are near 1550 nm. The microscopic images of the FBG are obtained via a charge coupled device (CCD) camera along and perpendicular to the laser incidence direction. The corresponding transmission spectra are obtained by an FBG interrogator.
As the aperture decreases, the length of the grating fringes perpendicular to the laser incidence direction increases significantly faster than along the incidence direction; the shapes of the grating fringes thus gradually change from elliptical to filamentous. When the aperture is 10.0 mm, the insertion loss and short-wavelength loss (at 1510 nm) are 0.9 dB and 4.01 dB, respectively; when the aperture is reduced to 1.0 mm, these two types of losses decrease to 0.11 dB and 0.35 dB, respectively (Fig. 5). This is because the filamentous grating fringe reduces the curvature of the typical circular fringe, leading to less Mie scattering of incident light. At the same time, because the area of the coupling between the filamentous grating fringes and the fundamental mode of the fiber is greater, the same coupling amplitude requires smaller refractive index modulation. Therefore, the small aperture has a significant suppression effect on losses. The short-wavelength loss of FBG manifests in the form of oscillation over a wide spectral range, mainly due to the excitation of cladding modes by refractive index perturbations in the grating region, which are coupled with the fundamental mode of the fiber core. The oscillation amplitude mainly depends on the energy of the low azimuth cladding mode. When the aperture is reduced to 1.0 mm, the filamentous fringes have a smooth refractive index modulation, resulting in the lower excitation of high-azimuth cladding modes but the higher excitation of some low-azimuth cladding modes (Fig. 7). The low-azimuth cladding modes carry more energy, resulting in higher coupling efficiency with the fundamental mode of the core. The oscillation is reduced by writing an FBG on a coated fiber or an FBG with a relatively low reflectivity (such as 40%; Fig. 9).
In this study, a low-loss femtosecond fiber grating fabrication technology based on small aperture shaping is proposed. The filamentous shaping effect of the small aperture on the grating fringe is theoretically analyzed and experimentally demonstrated. A series of FBGs with a central wavelength of 1550 nm and a reflectivity of 90% are fabricated by using different apertures. As the aperture is reduced from 10.0 mm to 1.0 mm, the grating fringe shape gradually transitions from circular to filamentous, while the insertion loss is reduced from 0.90 dB to 0.11 dB and the short-wavelength loss is reduced from 4.01 dB to 0.35 dB. Compared to circular grating fringes, the filamentous grating fringes reduce the Mie scattering of incident light and enhance the coupling of fundamental modes of the core, effectively reducing loss. The filamentous fringes also enhance the excitation of low-azimuth cladding modes, leading to greater oscillations at the short-wavelength side. These oscillations can be effectively suppressed by writing an FBG on a coated fiber or an FBG with a relatively low reflection.
