This article provides insufficient context for those unfamiliar with the subject.(January 2013) |
Photon Doppler velocimetry (PDV) is a one-dimensional Fourier transform analysis of a heterodyne laser interferometry, used in the shock physics community to measure velocities in dynamic experiments with high temporal precision. PDV was developed at Lawrence Livermore National Laboratory by Oliver Strand. [1] In recent years PDV has achieved popularity in the shock physics community as an adjunct or replacement for velocity interferometer system for any reflector (VISAR), another time-resolved velocity interferometry system. Modern data acquisition technology and off-the-shelf optical telecommunications devices now enable the assembly of PDV systems within reasonable budgets.
The fundamental mechanism of PDV is the interference pattern created by two electromagnetic waves with a small difference in frequency. Since most PDV systems are constructed with available telecommunications equipment, a standard laser source for a PDV system is centered at 1550 nm (or 193.4 THz). If this source is then reflected off of a moving surface with some velocity (), the reflected light will be shifted in frequency () according to the relativistic Doppler shift equation.
If the shifted return light is then interfered with the original source, the resulting wave will have a beat frequency in the range of a few gigahertz. This beat frequency is slow enough that it can be monitored with a simple photodetector and high speed oscilloscope. By recording the beat frequency over time, a complete velocity history of the surface is obtained.
In theory, the analysis of a PDV data signal is quite simple, where the apparent velocity of the moving surface () is simply a function of the source wavelength () and the signal frequency ():
In practice, however, determining the instantaneous frequency () of the signal by inspection can be inaccurate and inefficient. Consequently, Fourier transform analysis is used to extract the most probable frequency components, which can then be used to calculate the velocity history.
By taking sequential FFTs over a time window that moves across the data signal, a 2D spectrogram can be created which indicates the frequency components most dominant in the data. The velocity history can then be extracted from the spectrogram.
PDV can measure a wide range of velocities (limited primarily by the time resolution of the signal recording equipment), and is relatively easy to set up and use.
Depending on the quality of the data signal and parameters of the FFT, the inherent error in PDV measurements can be high. However, there are ways to mitigate these problems and obtain velocity histories with very high accuracy. [2]
The 2009 PDV users conference was held at the University of Texas at Austin, Institute for Advanced Technology. [3] The 2010 PDV users conference was held at the Ohio State University. [4]
The Doppler effect is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The Doppler effect is named after the physicist Christian Doppler, who described the phenomenon in 1842. A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.
A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called heterodyning, which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is used to shift signals from one frequency range into another, and is also involved in the processes of modulation and demodulation. The two input frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a mixer.
Interferometry is a technique which uses the interference of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy, quantum mechanics, nuclear and particle physics, plasma physics, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, optometry, and making holograms.
Mode locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12 s) or femtoseconds (10−15 s). A laser operated in this way is sometimes referred to as a femtosecond laser, for example, in modern refractive surgery. The basis of the technique is to induce a fixed phase relationship between the longitudinal modes of the laser's resonant cavity. Constructive interference between these modes can cause the laser light to be produced as a train of pulses. The laser is then said to be "phase-locked" or "mode-locked".
In physics, coherence expresses the potential for two waves to interfere. Two monochromatic beams from a single source always interfere. Physical sources are not strictly monochromatic: they may be partly coherent. Beams from different sources are mutually incoherent.
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most common spectrum analyzers measure is electrical; however, spectral compositions of other signals, such as acoustic pressure waves and optical light waves, can be considered through the use of an appropriate transducer. Spectrum analyzers for other types of signals also exist, such as optical spectrum analyzers which use direct optical techniques such as a monochromator to make measurements.
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Tunable diode laser absorption spectroscopy is a technique for measuring the concentration of certain species such as methane, water vapor and many more, in a gaseous mixture using tunable diode lasers and laser absorption spectrometry. The advantage of TDLAS over other techniques for concentration measurement is its ability to achieve very low detection limits. Apart from concentration, it is also possible to determine the temperature, pressure, velocity and mass flux of the gas under observation. TDLAS is by far the most common laser based absorption technique for quantitative assessments of species in gas phase.
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Laser Doppler velocimetry, also known as laser Doppler anemometry, is the technique of using the Doppler shift in a laser beam to measure the velocity in transparent or semi-transparent fluid flows or the linear or vibratory motion of opaque, reflecting surfaces. The measurement with laser Doppler anemometry is absolute and linear with velocity and requires no pre-calibration.
The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferometer or Sagnac interferometer. A beam of light is split and the two beams are made to follow the same path but in opposite directions. On return to the point of entry the two light beams are allowed to exit the ring and undergo interference. The relative phases of the two exiting beams, and thus the position of the interference fringes, are shifted according to the angular velocity of the apparatus. In other words, when the interferometer is at rest with respect to a nonrotating frame, the light takes the same amount of time to traverse the ring in either direction. However, when the interferometer system is spun, one beam of light has a longer path to travel than the other in order to complete one circuit of the mechanical frame, and so takes longer, resulting in a phase difference between the two beams. Georges Sagnac set up this experiment in 1913 in an attempt to prove the existence of the aether that Einstein's theory of special relativity makes superfluous.
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Optical heterodyne detection is a method of extracting information encoded as modulation of the phase, frequency or both of electromagnetic radiation in the wavelength band of visible or infrared light. The light signal is compared with standard or reference light from a "local oscillator" (LO) that would have a fixed offset in frequency and phase from the signal if the latter carried null information. "Heterodyne" signifies more than one frequency, in contrast to the single frequency employed in homodyne detection.
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Optical coherence tomography (OCT) is a technique that displays images of the tissue by using the backscattered light.