Design, Development and Validation of High Performance Fiber Bragg Grating Accelerometers

Abstract

Fiber Bragg grating (FBG) accelerometers have attracted the attention of researchers as an efficient and attractive alternative to conventional electrical accelerometers. They exploit the remarkable sensing capabilities of the FBG in combination with varieties of novel mechanical sensor heads. The FBG is an intrinsic optical sensor, which provides the measurand information in the wavelength-encoded format. The FBG accelerometers are light, compact, less noisy, highly sensitive, immune to electromagnetic interference, capable of sensing efficiently in harsh environments, and fit to carry out distributed sensing. These unique and promising characteristics have generated a lot of interest in exploring the use of FBG accelerometers in various fields of science and technology. Though various types of FBG accelerometers are proposed by different researchers to achieve specific characteristics, there is a need to realize high-performance FBG accelerometers, which have high in-axis sensitivity, low cross-axis sensitivity, self-temperature compensation capability, high linearity, reasonably good bandwidth, and wide dynamic range. To achieve the above objectives, three novel configurations i.e. Modular Double-L Cantilever, Monolithic T-Cantilever and Composite Triangular Cantilever based FBG accelerometers are evolved. Mathematical models and designs are analyzed through numerical simulations using MATLAB and finite element method (FEM) simulations using ANSYS. Precise fabrication sequences are adopted for realizing the mechanical sensors heads (MSH) and the FBGs. The FBGs are carefully integrated with the MSHs in optical differential sensing configuration to realize the novel FBG accelerometer prototypes. The accelerometers are characterized for their static, dynamic, and temperature characteristics. Close matching of the experimental results with the theoretical predictions proved the concepts and validated the designs. For the double-L cantilever based FBG accelerometer (DLC-FBGA), sensitivity of 406 pm/g with linearity of 99.8% over full-scale range of ± 6 g, cross-axis sensitivity of 0.5% of in-axis sensitivity, natural frequency of 86 Hz with a usable bandwidth of 5-50 Hz, and self-temperature compensation with an error of 0.02 pm/oC are achieved. For the monolithic T-cantilever based FBG accelerometer (MTC-FBGA), sensitivity of 821 pm/g linearity of 99.7%, range of ± 3g, cross-axis sensitivity of 0.3%, natural frequency of 64 Hz with a usable bandwidth of 5-40 Hz and self-temperature compensation with an error of 0.07 pm/oC are achieved. For the composite triangular cantilever based FBG accelerometer (CTC-FBGA), very high sensitivity of 1721.6 pm/g with a linearity of 99.3%, cross-axis sensitivity of 0.7%, natural frequency of 23 Hz, bandwidth up to 10 Hz, self-temperature compensation with an error of 0.03 pm/oC and a dynamic range of 80 dB are achieved. The highly sensitive self-temperature compensated high-performance accelerometers can faithfully sense and measure low amplitude and low-frequency vibrations as well as accelerations related to inertial navigation, seismic vibration, and structural health monitoring of medium to large scale civil, aerospace, and defense structures. The mathematical models developed for the FBG accelerometers also provide enough flexibility to further optimize the design parameters to achieve specific desired performance characteristics. Using the recently researched high sensitivity etched FBGs and nano-material coated FBGs, the sensitivity of the proposed configurations can be enhanced many folds.