Embedded fiber-optic strain sensing networks for airworthy assessment of operational Unmanned Aerial Vehicles (UAVs) are presented. Sensing is based on in-flight fiber Bragg grating technology, as well as on on-ground Rayleigh backscattering distributed strain sensing. While the in-flight instrumentation monitors loads, looking for excessive values, the Rayleigh-based technique is used for high spatial resolution strain distribution along the UAV wings, under prescribed loading. Consistency of measurements over time indicates structural integrity. Simultaneous strain measurements using both distributed Rayleigh and fiber Bragg gratings, on the same fiber, promises to combine high spatial resolution, though practically static measurements with dynamic, though discrete ones.
The effect of the gain dependency of the Brillouin linewidth on double-slope-assisted Brillouin optical domain, dynamic sensing techniques is studied. These double-slope methods are immune to pump-power related peak Brillouin gain variations, however, they are not immune to gain-dependent variations of the shape of the Brillouin gain spectrum. For a 15ns pump pulse, this gain-dependent shape of the normalized Brillouin gain is shown to lead to a 5.5 [%/dB] strain error when the double-slope sum-difference approach is used, compared with a 7 [%/dB] error when the double-slope ratio approach is used. Ways to compensate for these issues are discussed.
Using a tailored-frequency probe, this paper presents a distributed slope-assisted BOTDA (SA-BOTDA) measurement of an optical fiber, bonded to a bent cantilever, where the fiber's static Brillouin Frequency Shift continuously varies along its length by an amount of the order of or larger than the Brillouin bandwidth. While standard SA-BOTDA methods, employing a fixed probe frequency, may suffer from distortions in measuring vibrations around this Brillouininhomogeneous static state, a tailored probe preserves the full dynamic range offered by the slope of Brillouin gain spectrum. The computerized measurement system is capable of real-time continuous monitoring of the strain, including averaging.
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