Doppler Frequency-Multiplied Terahertz-Wave Doppler Interferometric Velocimeter With High Temporal Resolution

Due to the ability of terahertz (THz) waves to penetrate nonpolar opaque optical materials, Doppler interferometric velocimeters using THz waves can measure the velocity of a target inside or behind opaque optical materials, such as shock or detonation waves inside high explosives. To increase the temporal resolution of transient velocity measurements, a shorter time window is necessary during time–frequency analysis. However, the submillimeter wavelength of THz waves means that a shorter time window (e.g., nanoseconds) leads to extremely large velocity uncertainty. To address this challenge, a Doppler frequency-multiplied terahertz-wave Doppler velocimeter (DFM-TDV) was proposed. By frequency multiplication of the Doppler frequency shift, the velocity uncertainty can be suppressed under the same time window, allowing for a narrower time window while maintaining the same level of velocity uncertainty. The design and performance of the DFM-TDV were discussed, and its capability was tested through synthetic Doppler signal experiments and detonation-driven flyer experiments. The velocity uncertainty and temporal resolution of the measured velocity were improved by factors of 6.7 and 4, respectively, with a multiplication factor of 16.