Femtosecond Fiber Bragg Gratings Research Articles

Temperature and strain gradient monitoring of the buoyancy material curing reaction with a submillimeter spatial resolution fs-FBG array

Millimeter-scale temperature and strain gradient measurements are crucial for understanding the mechanical properties of buoyancy materials. In this paper, we successfully fabricated ultrashort and high-density fiber Bragg grating (FBG) array based on femtosecond laser point-by-point writing technology. By optimizing the energy and number of femtosecond laser pulse, and introducing secondary tilt apodization technology, submillimeter-spatial-resolution FBG arrays were prepared. To the best of our knowledge, this is the first application of femtosecond FBG array for high spatial resolution temperature and strain gradient in situ monitoring of buoyancy materials. FBGs exhibit excellent anti-chirp performance and achieve submillimeter spatial resolution curing monitoring under temperature and strain monitoring gradients of 2.6 °C/mm and 231 με/mm, respectively.

Strain and Temperature Sensing Based on Different Temperature Coefficients fs-FBG Arrays for Intelligent Buoyancy Materials.

The temperature and strain fields monitoring during the preparation process of buoyancy materials, as well as the health status after molding, are important for mastering the mechanical properties of buoyancy materials and ensuring the safety of operators and equipment. This paper proposes a short and high-density femtosecond fiber Bragg grating (fs-FBG) array based on different temperature coefficients fibers. By optimizing the parameters of femtosecond laser point-by-point writing technology, high-performance fs-FBG arrays with millimeter level gating length and millimeter level spatial resolution were prepared on two types of fibers. These were successfully embedded in buoyancy materials to achieve in-situ online monitoring of the curing process and after molding. The experimental results show that the fs-FBG array sensor has good anti-chirp performance and achieves online monitoring of millimeter-level spatial resolution. Intelligent buoyancy materials can provide real-time feedback on the health status of equipment in harsh underwater environments. The system can achieve temperature monitoring with an accuracy of 0.56 °C and deformation monitoring with sub-millimeter accuracy; the error is in the order of micrometers, which is of great significance in the field of deep-sea exploration.

Research on spacecraft in orbit perception based on artificial neural networks and digital twin technology using grating arrays

In order to improve the safety of spacecraft, the research on artificial neural network and digital twin technology based on, to our best knowledge, a novel fiber Bragg grating (FBG) sensor array is proposed for intelligent sensing monitoring of spacecraft on-orbit collisions. Femtosecond FBG arrays were fabricated on the novel oxide-doped fiber by point-by-point writing technique. The femtosecond FBG is analyzed using the time-dependent perturbation theory of quantum mechanics. The FBG array can achieve high-temperature measurement of 1100 °C and large strain measurement of 15000 µε. The sensing arrays were deployed on the surface of the spacecraft. Constructed the multi-layer perceptron neural network structure and convolutional neural network structure. 1200 samples were trained. Conducted model accuracy testing. The accuracy rate is above 98%, and accuracy verification has been implemented. The digital twin model was designed based on various data such as strain and temperature of the spacecraft structure under impact monitored by FBG sensors. A precise mapping has been formed between the physical entities of spacecraft and digital twins. Empower spacecraft with functions such as self-monitoring, judgment, and response. To ensure the stable and safe operation of spacecraft.

Fabrication of Low-Loss Femtosecond Fiber Bragg Gratings Based on Beam Shaping by Small Aperture

Fabrication of Low-Loss Femtosecond Fiber Bragg Gratings Based on Beam Shaping by Small Aperture

Application of Quasi-Distributed High Temperature Sensor Based on Femtosecond Fiber Bragg Grating

Application of Quasi-Distributed High Temperature Sensor Based on Femtosecond Fiber Bragg Grating

Design and integration of femtosecond Fiber Bragg gratings temperature probes inside actively cooled ITER-like plasma-facing components

Measuring the temperature in plasma-facing components (PFCs) provides information both on plasma parameters in the divertor region and on the thermal stress experienced by PFCs. Fiber Bragg gratings (FBGs) are interesting candidates for this application because they are immune to electromagnetic interferences and their ability to be multiplexed allows an extended spatial coverage. Four fibers, each of them including eleven regenerated Bragg gratings, have been embedded in tungsten-coated graphite components and operated up to their signal-collapsing limit at 800 °C. Extending the measurement range towards higher temperatures increases the sensitivity to plasma parameters and allows withstanding higher energy experiments. To overcome thermal limitations, the system is up-graded using femtosecond laser inscribed fibers. In addition to their outstanding thermal stability, femtosecond FBGs benefit from higher signal-to-noise ratios than regenerated FBGs. The paper addresses femtosecond FBGs design and issues relative to their integration inside the actively cooled ITER-like PFCs of the WEST tokamak. The period and length of the gratings are designed to increase the number of measurement spots to fourteen gratings per fiber, regularly distributed over 17 cm, while ensuring robust detection even with strong thermal gradients (no overlapping or deformation of Bragg peaks). The system operates up to 1200 °C with gradients reaching 200 °C/mm perpendicularly and 40 °C/mm in parallel to the fiber. FBGs are inserted in actively cooled ITER-like PFCs through a 2.5 mm deep lateral groove localized at 5 mm beneath the top of bulk tungsten monoblocks. A PFC mock-up machined with a groove has been tested under HHF facility to assess the effect of the groove on monoblocks thermal behavior. The test demonstrates that machined monoblocks behave as expected from simulation and can withstand 20 MW/m2 heat flux (i.e. 1200 °C in the fiber) with 20 % overheating as compared to intact monoblocks.

Wavelength-switchable erbium-doped fiber laser based on femtosecond FBG inscribed on fiber core and cladding through the coating

Wavelength-switchable and multiple-wavelength outputs from fiber laser systems have many potential applications but, at present, methods for their generation are quite complex. Here we propose a simple approach, demonstrating a dual-wavelength switchable erbium-doped fiber laser that incorporates a single fiber Bragg grating (FBG), fabricated by a femtosecond laser in a plane-by-plane inscription method through a fiber coating, for a switchable 1564.66 and 1562.87 nm dual-wavelength laser operation. An 800 nm femtosecond pulsed laser acts as the inscription laser, focused by a 63× oil lens, fabricating a first-order FBG with an interval of 541 nm directly, on both the fiber core and cladding, through the polyimide coating. A 4 m-long erbium-doped fiber was selected as the gain medium. A lasing threshold of 66 mW was used in the experiment, and a switchable laser with stable single- and dual-wavelength outputs was achieved by adjusting a polarization controller. We were able to generate two different single-wavelength lasers with power fluctuations lower than 0.09 dB when measured over 40 min. In addition, dual-wavelength laser output could be obtained with power shifts of less than 0.9 dB at 26 °C.

Femtosecond FBG Written through the Coating for Sensing Applications.

Type I fiber Bragg gratings (FBG) written through the coating of various off-the-shelf silica fibers with a femtosecond laser and the phase-mask technique are reported. Inscription through most of the common coating compositions (acrylate, silicone and polyimide) is reported as well as writing through the polyimide coating of various fiber cladding diameters, down to 50 µm. The long term annealing behavior of type I gratings written in a pure silica core fiber is also reported as well as a comparison of the mechanical resistance of type I and II FBG. The high mechanical resistance of the resulting type I FBG is shown to be useful for the fabrication of various distributed FBG arrays written using a single period phase-mask. The strain sensing response of such distributed arrays is also presented.

Observation of ~20 ns group delay in a low-loss apodized fiber Bragg grating.

We report a record group delay of 19.5 ns (an equivalent group index of 292) measured in a strongly apodized, 2 cm long, femtosecond fiber Bragg grating (FBG). This significant (~4-fold) improvement over the previous record results from the presence of a Fabry-Perot arising from the apodization. The measured group-index spectrum is well explained by a model that accounts for the apodized profiles of the index modulation, propagation loss, and birefringence of the grating. The peak power loss inferred from this model is only ~0.12 m⁻¹, which is one of the lowest values reported for an FBG.