Shock and Detonation Velocity Sensing at 100 GHz and 30 THz

Millimeter wave and mid-infrared interferometers have been developed on both sides of the THz domain to measure the fast propagation at a few km/s of an interface, created by a shock or a detonation, into a material of study. Our 92–96 GHz interferometer allows deep measurements beyond 100 millimeters through an explosive, but the corresponding wavelength (3.13 mm) limits the resolution to about 200 ns and the spatial resolution to a beam of about 16 mm in diameter. On the other hand, our Photonic Doppler Velocimeter operating in the 9–11 μm wavelength range (30 THz) was able to measure, up to now, a shock velocity through 10 mm of high-density polyethylene with a temporal resolution lower than 50 ns and a spatial resolution of about 1 mm in diameter. Both systems are used remotely using single-mode waveguides, which are 5 to 10 meters long, to avoid any damage on the systems. The signal processing is, however, different for the two systems. Since few interference fringes are obtained within 10 to 20 μs by the millimeter wave interferometer, a Lissajous phase analysis is typically used to get the displacement. The velocity is obtained after two derivative and filtering steps. This approach is more suitable to extract a single velocity profile. To extract two or more velocities, a more complex signal processing using convolutional neural networks is being developed. Thanks to his 30 THz frequency, the mid-infrared interferometer measurement can be processed by short-time Fourier transform analysis, which is less sensitive to the signal-to-noise ratio and allows multiple velocity profiles measurements. The performances of the mid-infrared interferometer will be compared to a millimeter wave interferometer in different experiments.