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A Three-Phase PDV Signal-Processing Method that Eliminates the Influence of Acceleration on Determining Velocity
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Photonic Doppler Velocimetry (PDV) is a fiber-based diagnostic for the extreme conditions created by high-speed impact, explosive detonation, electrical pulsed power, and intense laser ablation. PDV is a conceptually simple application of the optical Doppler effect, but measurements above 1 km/s only became practical at the beginning of the twenty-first century. This review discusses the evolution of PDV, its operational details, practical analysis, and outstanding challenges.
We have represented a system description used to study of the kinematic parameters of processes of destruction of chondrites under the action of intense shock waves. Interferometric method of measuring speed PDV was the basis for a compact, easy-to-use device intended for the analysis products of target destruction. Using the Fourier transform method of analysis, the dynamic rates of expansion for chondrite targets in interaction with high-power laser radiation and the dependence of the shock wave velocity on the thickness of the target were obtained.
A method for determination of the contact-area functions for diamond indenters of nanohardness testers using a metrological atomic-force microscope with three-coordinate laser interferometer is proposed. Face shapes of a number of indenters of Berkovich pyramid type are measured. The precision of the indenter surface coordinates measurement is 1 nm. It is demonstrated that the indenter tip shape changes in the course of its use; in particular, for the first 100 nm the deviation from the ideal pyramid can exceed 30 nm. Thus, one of the methods for verification of the contact-area function for an indenter is its periodic calibration using a metrological atomic-force microscope.
While photonic Doppler velocimetry (PDV) is becoming a common diagnostic in dynamic compression research, its limiting accuracy and precision are not well understood. Velocity resolution is known to be inversely proportional to the time resolution, but resolution estimates differ by one to two orders of magnitude. Furthermore, resolution varies with the number of recorded signals and how these signals are analyzed. Numerical simulations reveal factors that affect accuracy and precision in PDV, and the results may be extended to a broad class of measurements. After systematic effects are eliminated, the limiting velocity uncertainty in a PDV measurement is governed by the sampling rate, the signal noise fraction, and the analysis time duration.
A robust analysis method is presented for multiple-phase heterodyne velocimetry measurements. By combining information from three phase-shifted signals, it is possible to eliminate coherent intensity variations and incoherent light from the measurement. The three data signals are reduced to a pair of quadrature signals, allowing unambiguous calculation of target displacement. The analysis relies on a minimum number of adjustable parameters, and these parameters can be precisely determined from simple interferometer characterization.
Laser Doppler velocimetry, also known as photonicDopplervelocimetry (PDV), is a diagnostic commonly used in shock compression experiments. To quantitatively compare experimental results to computational results, the uncertainty in the velocimetrymeasurements must be well characterized. That is, we must know which features in the experimental data are statistically significant and which are not. In this contribution, we will present recent work on understanding the resolution, precision, and accuracy of PDV measurements. We will discuss how the uncertainty is a function of signal strength, noise, and signal digitization, and how the uncertainty is influenced by the analysis method. We will show how knowledge of uncertainty can be used to rigorously compare experiment with computational predictions.
At LLNL, we have been using heterodyne techniques for the past year and a half to measure velocities up to several kilometers-per-second on different types of experiments. We assembled this diagnostic, which we call the Heterodyne Velocimeter (HetV), using commercially available products developed for the communications industry. We use a 1550 nm fiber laser and single mode fibers to deliver light to and from the target. The return Doppler-shifted light is mixed with the original laser light to generate a beat frequency proportional to the velocity. At a velocity of 1000 m/s, the beat signal has a frequency of 1.29 GHz. We record the beat signals directly onto fast digitizers. The maximum velocity is limited by the bandwidth of the electronics and the sampling rate of the digitizers. The record length is limited by the amount of memory contained in the digitizers. This paper describes our approach to measuring velocities with this technique and presents recent data obtained with the HetV.