Shock wave diagnostics with an ultra-short optical fiber probe

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.