The preparation process and structural health monitoring after forming of composite materials have always been one of the research directions of many researchers. Fiber Bragg gratings (FBGs) are used for composite material monitoring due to their small size, ease of reuse, and good compatibility with materials [[1], [2], [3]]. Especially in harsh marine environments, FBGs have become the preferred sensor.

G Wang et al. [4] used FBG sensors to achieve real-time status monitoring of ships. J.H. Zhou et al. [5] demonstrated that the accuracy and anti-interference performance of FBGs were superior to strain gauges. Compared to structural deformation monitoring, the monitoring of composite preparation process is also important. For example, for solid buoyancy materials commonly used in deep submersibles, they have poor thermal conductivity. During the preparation process, there are large temperature and strain gradients within the material, leading to bending deformation and cracking in practical applications [[6], [7], [8]]. Monitoring the strain and temperature gradients is important for optimizing the curing process and understanding mechanical performance, which requires small-size and high-density sensor arrays.

A few FBGs prepared by traditional ultraviolet excimer laser phase mask technology have been embedded into buoyancy materials to monitor the solidification process. However, the grating length is generally between 5 mm and 1 cm, which is prone to chirping when subjected to nonuniform strain, the grating length needs to be further reduced [[9], [10], [11]]. Therefore, it is necessary to optimize FBG sensors and develop ultrashort FBGs array to achieve high-precision monitoring of temperature and strain gradients during the curing process of buoyancy materials. In recent years, femtosecond (fs) laser point-by-point (PbP) writing technology has rapidly developed due to its high flexibility, which can realize the fabrication of FBGs with arbitrary grating lengths and create favorable conditions for ultrashort and high-density FBG arrays, thereby improving the antichirp performance and spatial resolution of sensors. F. Mumtaz et al. [12] used fs laser PbP writing technology to prepare FBG arrays in multimode coreless fibers, with grating length >6 mm. X.Y. Liu et al. [13] used fs-laser PbP writing technology to shorten the grating length and spatial resolution to 2 mm. Sanzhar Korganbayev et al. [14] prepared a fs-FBG array with a grating length of 1.15 mm and an interval of 0.05 mm, the 3 dB bandwidth of the FBG was 0.7 nm, and the reflectivity was 10%∼20%. As the grating length of the femtosecond fiber Bragg grating (fs-FBG) continues to decrease to the submillimeter level, the reflectivity sharply decreases, and the 3 dB bandwidth increases, reaching several nm or even tens of nm. In wavelength division multiplexing (WDM) systems, the bandwidth of tens of nanometers limits the number of multiplexed FBGs, making it difficult to achieve high-density FBG arrays. Y.P. Wang et al. achieved excellent progress on ultrashort FBGs by writing multiple FBGs with the same central wavelength in the fiber core and radial position around the fiber, which increased the reflectivity of the FBG from 28% to 93% and the 3 dB bandwidth from 0.96 nm to 1.44 nm [15]; additionally, cascading 2 FBGs with tens of μm grating lengths achieved spatial resolution of the submillimeter [16]. However, a bandwidth of nearly 10 nm is not convenient in WDM systems, which limits the density of the fs-FBG array. For buoyancy material specimens, it is necessary to study ultrashort and high-density FBG arrays to achieve large-scale temperature and strain gradient monitoring.

In this paper, to realize ultrashort and high-density FBG arrays, we optimized the fs-laser writing process and fabricated a fs-FBG array with a submillimeter gating length. The FBG array was embedded in the buoyancy materials, and the temperature and strain gradients were successfully monitored during the curing process of the buoyancy materials, providing a technical means for structural health monitoring of solid buoyancy materials after molding.