Fiber Bragg grating (FBG) is an important passive device widely used in various fields. Compared to holographic interference and phase mask methods, the point-by-point (PBP) technique using femtosecond (fs) laser offers a simple optical path and is not limited by masks, providing significant flexibility. This makes it a hot topic in both domestic and international research. However, fiber Bragg gratings with short grating regions still suffer from low reflectivity. To address this, several methods have been proposed, including adjusting laser pulse energy, altering the grating period, and modifying the refractive index modulation region. In this paper, we propose a biconcave-shaped FBG structure that demonstrates high reflectivity and a short grating region under the same femtosecond laser writing conditions. This structure can be flexibly adjusted according to the desired reflective characteristics, which is of great significance for the development of high-power fiber laser systems, fiber amplifiers, and other FBG-based applications.

Initially, the biconcave-shaped curvature is optimized to achieve optimal FBG reflective characteristics. The effects of modulation region length, period, grating modulation depth, and order on reflective performance are investigated, with a comparison to traditional FBGs. Preliminary experimental verification and simulation analysis of the sensing characteristics are also conducted.

As the curvature of the biconcave-shaped FBG decreases, its reflectivity initially increases and then decreases. The optimal reflectivity is achieved when the curvature value is 0.062π rad (Fig. 2). The simulation results indicate that as the grating length increases, the peak reflectivities of both types of gratings improve, accompanied by a narrowing of the 3 dB bandwidth (Fig. 3). An increase in grating period leads to a significant shift in the central wavelength of the reflected light from both gratings, showing a linear relationship (Fig. 4). As grating modulation depth increases, both types of FBGs exhibit noticeable shifts in Bragg wavelengths, and the 3 dB bandwidth increases significantly, along with an increase in peak reflectivity (Fig. 5). When both the grating period and order are doubled, the Bragg wavelength remains unchanged (Fig. 6). In all cases, the reflectivity of the biconcave-shaped FBG is twice that of the traditional FBG, while the 3 dB bandwidth remains nearly constant. The experimentally fabricated biconcave FBG has a reflectivity of 78.65%, double that of the traditional FBG. The 3 dB bandwidth is 1.43 nm, comparable to that of traditional FBGs (Fig. 7). This grating demonstrates temperature and strain sensing characteristics that closely match theoretical values.

To address the issue of low reflectivity in short grating area FBGs created using the PBP method with fs laser, a high-reflectivity FBG based on a biconcave-shaped structure is designed, and preliminary experimental validation and simulation analysis of its sensing characteristics are performed. Preliminary results show that when the curvature of the biconcave arc is 0.062π rad, the grating length is 400 μm, the period is 1.6 μm, the modulation depth is 0.0083, and the order is third, the reflectivity of the femtosecond laser-written FBG reaches 81.7%, nearly double that of the traditional FBG, with the 3 dB bandwidth remaining almost unchanged. The experimental results for the traditional biconcave FBGs, fabricated under the same conditions, are consistent with the theoretical simulations. The biconcave FBG proposed in this paper has the advantages of high reflectivity and flexible tunability, offering a theoretical basis for optimizing the performance of new micro-nano optical devices.