Current laser interferometry pressure pulse detection techniques use the shock-induced change in index of refraction to track detonation and/or shock fronts in a single fiber optic by reflecting laser light off the boundary created between the unshocked material and the shocked material. This technique cannot be used in regimes where there isn't a strong enough shock front to reflect the laser light above the noise floor. Other embedded fiber techniques use a chirped fiber Bragg grating to track a shock position versus time by correlating a known spectrum of light to a calibrated position. These two systems use a single fiber and measure only time-of-arrival for a strong shock.
An Embedded Fiber Optic Sensor (EFOS) system was developed that can measure apparent particle velocity time histories in low- to high-shock regimes, and for non-shocks. The apparent particle velocity traces give time-of-arrival data and can be transformed into pressure time histories, a capability unique to this system.
Many explosives and/or combustion events have short run-up distances requiring sub-millimeter measuring techniques. The EFOS system utilizes Corning SMF-28 9/125-μm diameter fibers (but not limited to glass fibers) placed at known distances from a target surface, and connected to infrared detectors coupled with Photonic Doppler Velocimetry (PDV). The PDV system uses the Doppler shifted beat frequency of reflected infrared laser light as compared to a reference leg of the laser source with a heterodyned signal. The probes detect apparent particle velocity traces similar to those seen in traditional laser interferometer particle velocity measurements that help interrogate the transient phenomena of explosives or shock waves, but at the micro scale and potentially smaller.