Velocity Extraction from Photon-Doppler-Velocimetry Spectrograms

Los Alamos National Laboratory

Scientists at Los Alamos National Laboratory use an optical interferometric technique to observe moving surfaces with high time resolution. Light from a signal laser travels through an optical fiber before shining on the surface under test. The small portion of the beam that reflects from the surface and re-enters the fiber is mixed with a reference laser signal and the combined signal as detected by a fast optical detector is digitized at rates up to 50 GHz. The resulting data stream must then be analyzed to identify signals that encode the motion of the surface. The velocity of the surface along the direction of the laser beam produces a proportional frequency shift in the reflected light; these frequency shifts can be determined from a spectrogram of signal intensity as a function of both frequency and time calculated from the ( V(t) ) data using standard fast Fourier transform techniques. 

The reflected light is often weak and may vary in intensity over the short time of the experiment, which makes separating the signal from background noise difficult. Some experiments may cause the surface to eject particles, which can efficiently scatter light back to the fiber. In such cases many different velocities may be present in the spectrogram. Los Alamos seeks a more automated way to process photon Doppler velocimetry (PDV) files containing ( V(t) ) data to produce surface velocity and uncertainty as functions of time, where such surfaces exists, and to quantify velocity distributions for ejecta.

Advisor(s): Peter N. Saeta.

Team: Isabella (Isabel) Yibei Duan ’22, Nicholas A Koskelo ’20, and Rikki M Walters ’20.