Abstract-Diffraction gratings have been widely used in wavelength-controlled non-mechanical laser beam steering for high data-rate indoor optical wireless communications (OWC). Existing free-space diffraction gratings suffer from inherent difficulties of limited diffraction efficiency, bulky configuration, high cost and significant coupling loss with optical fiber links. In this work, a new optical approach for highly efficient, compact and fiber compatible laser beam steering using an in-fiber diffraction grating is proposed and experimentally demonstrated for the first time to our best knowledge. In-fiber diffraction is made possible based on a 45° tilted fiber grating (TFG), where wavelength dependent lateral scattering is obtained due to the strongly tilted grating structure. Improved diffraction efficiency of 93.5% has been achieved. In addition, the 45° TFG works perfectly for both light emission and reception, enabling full-duplex optical wireless transmission. Utility of the 45° TFG in all-fiber laser beam steering for multi-user full duplex optical wireless communications has been verified in experiments. 1.4 m free-space full-duplex wireless transmission has been demonstrated with data rate up to 12 Gb/s per beam using 2.4 GHz bandwidth OFDM signals.
Abstract:A damage identification system of carbon fiber reinforced plastics (CFRP) structures is investigated using fiber Bragg grating (FBG) sensors and back propagation (BP) neural network. FBG sensors are applied to construct the sensing network to detect the structural dynamic response signals generated by active actuation. The damage identification model is built based on the BP neural network. The dynamic signal characteristics extracted by the Fourier transform are the inputs, and the damage states are the outputs of the model. Besides, damages are simulated by placing lumped masses with different weights instead of inducing real damages, which is confirmed to be feasible by finite element analysis (FEA). At last, the damage identification system is verified on a CFRP plate with 300 mm × 300 mm experimental area, with the accurate identification of varied damage states. The system provides a practical way for CFRP structural damage identification.
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