A novel, generic approach for fiber-optical sensing in rapidly rotating structures is presented. This approach does not require optical ingress via the central rotation axis. In this work, strain sensing at rotation speeds of up to 5000 rpm is demonstrated, and higher speeds should be possible. We demonstrate measurement of the rotation-induced strain in a rotating body at 500 rpm intervals up to 5000 rpm, and results agree very well with predictions.
A new optical fiber head for Photonic Doppler Velocimetry (PDV) made from a combination of fiber types [multimode (MM) and single-mode (SM)] and lenses is described. The input laser beam is delivered by a SM fiber and imaged onto the target by simple optics, including an imaging lens centered inside a larger lens, whose role is to collect and image the back reflected light into the MM collection fiber. The large core of the MM fiber enhances the collection efficiency and also reduces its dependence on the target angle. Transmission through the MM fiber reduces the heterodyne fringe visibility considerably, but the Fourier analysis still enables very accurate resolution of the fringe frequency (and hence the velocity). The new PDV head with 20 GHz bandwidth was tested in a dynamical shock wave experiment to measure velocities of ∼3 km/s (∼3.9 GHz), and the results agreed very well with measurements by a standard SM PDV.
We report a highly localized, rapid-response pressure measurement of a shock wave front in a solid by utilizing a miniature fiber-optic-based probe. The probe used was a 100 μm-long fiber Bragg grating (FBG) inscribed on a standard silica communication fiber, 125 μm in diameter. The optical fiber was embedded within a ceramic zirconia ferrule and was shocked axially by a polycarbonate impactor fired from a gas gun. In a second ferrule, included in the same experiment, a 1 mm long FBG was embedded for comparison. Both FBGs were positioned at the front face of their respective ferrules, in order to sense the region where the shock wave is pristine, with no release waves, and where the stress conditions were expected to be constant for a few hundreds of nanoseconds. A simulation has been performed using LS-DYNA software describing the temporal dependence of the axial stress operating on the zirconia target and the embedded fiber gratings. The reflected spectra of both fiber grating probes were interrogated by an array of wavelength division demultiplexers and 200 MHz InGaAs detectors. Both probes exhibited a wavelength shift that corresponded to the pressure profile of the shock wave that traveled through the fiber, agreeing quite well with the predictions of the simulation. The wavelength blueshift was about 3.5 nm under a calculated shock pressure in the silica of 320 MPa, induced by a shock pressure of 700 MPa in the host zirconia target. Overall, the 100 μm probe demonstrated superior measurement capabilities to the 1 mm probe, both in time response and localization, as well as better agreement with the simulation. Multiple probes can be applied to provide high resolution mapping of shock phenomena in space and time, thus assisting in establishing the dynamic properties of materials under impact loading.
FBGs respond to external pressures in ways that reflect both the strain-optic effect and the geometrical variations, both induced by the applied pressure. While the response to static isotropic pressure is quite straight forward and intuitive, the response to anisotropic shock waves is much more complex and depends also on the relative orientation between the fiber and the shock propagation direction. We describe and explain experimental results for both cases.
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