Fiber-optic pressure and mass velocity sensor

Measuring shock wave propagation in solid media, materials under extreme strain conditions including bullet and blast proof materials


Three sensors mounted in orthogonal axes can be used to reconstruct the three vector components of mass velocity.

Laser-generated shock waves are used in several applications including nuclear fusion experiments, synthesis of protective nano-laminate materials, and in basic studies on materials under extreme conditions. However, direct measurements of shock waves propagating in solid and opaque media are difficult due to the high pressure generated by the shock and the inability to optically image the shock front respectively. In designing a pressure sensor for these applications, other constraints must be addressed such as rigidity of the sensor to withstand the shock wave, the necessarily small size of the sensor, and required high response time to avoid measurement errors.

Recently, Navy researchers have developed a device for measuring shock wave pressure in a solid media using fiber optics. Three sensor types: fiber Fabry-Perot sensor; fiber Bragg grating sensor; and a Michelson interferometer fiber tip sensor have been employed in the invention to measure this change in pressure. The fiber Fabry-Perot and fiber Bragg grating sensors provide a point-like measurement of the local disturbance. They respond to local volumetric compression of the cavity and therefore respond to the change in optical path length induced by the shock wave. The interferometer responds to displacements anywhere along the sensing fiber. The invention has a high operating bandwidth (>10 MHz), is sufficiently rigid to withstand the force of the shock wave, has tunable sensitivity for the application, and is immune to electromagnetic interference.

Applications of this sensor include characterizing shock wave propagation effects in solid materials for understanding energy dissipation and channeling. The sensor responds to shock velocity preferentially in the axial direction along the fiber and is, therefore, a directional sensor. Three sensors mounted in orthogonal axes can be used to reconstruct the three vector components of mass velocity. By combining this three-axis velocity sensor with a fiber optic pressure sensor, the energy in the shock wave can also be characterized (this measurement requires three components of velocity and pressure) for the case when the shock wavefront is nonplanar. For the case of a planar shock wavefront, a single velocity measurement in the direction of shock propagation and pressure measurement is adequate to calculate shock energy.

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