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WO1999035519A2 - Diffusion de rayleigh modulee et filtree - Google Patents

Diffusion de rayleigh modulee et filtree Download PDF

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Publication number
WO1999035519A2
WO1999035519A2 PCT/US1999/000691 US9900691W WO9935519A2 WO 1999035519 A2 WO1999035519 A2 WO 1999035519A2 US 9900691 W US9900691 W US 9900691W WO 9935519 A2 WO9935519 A2 WO 9935519A2
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Prior art keywords
laser
frequency
velocity
pattern
scattered
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PCT/US1999/000691
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English (en)
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WO1999035519A3 (fr
Inventor
Philip L. Varghese
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The Board Of Regents, The University Of Texas System
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Application filed by The Board Of Regents, The University Of Texas System filed Critical The Board Of Regents, The University Of Texas System
Priority to AU24556/99A priority Critical patent/AU2455699A/en
Publication of WO1999035519A2 publication Critical patent/WO1999035519A2/fr
Publication of WO1999035519A3 publication Critical patent/WO1999035519A3/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
    • G01P5/00—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft
    • G01P5/26—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft by measuring the direct influence of the streaming fluid on the properties of a detecting optical wave
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
    • G01S17/50—Systems of measurement based on relative movement of target
    • G01S17/58—Velocity or trajectory determination systems; Sense-of-movement determination systems

Definitions

  • This invention pertains to a system that employs modulated filtered Rayleigh light scattering that is useful, for example, in measuring the velocity of a moving stream of gas.
  • Optical diagnostic techniques are becoming an increasingly important component of aerospace research. Employing light as the probe has three distinct advantages. First, optical techniques do not disturb a flow, as would a physically intrusive probe. Second, in the extreme conditions that exist in the most advanced areas of aerospace research, normal physical instrumentation may not be capable of withstanding the pressures or temperatures involved. Finally, there are some types of diagnostics that can only be reasonably accomplished in-situ by an optical technique. These include detection of atomic or molecular species.
  • the ability to make precise velocity measurements of various moving flows of gas, or otherwise is an area of technology in which new and improved techniques are sought.
  • improved velocity techniques are sought for conducting basic research in fluid dynamics and for applied research such as in wind tunnel applications for aircraft design.
  • the previously proposed methods have been limited in accuracy and precision, and required multiple lasers to acquire a single velocity vector of a sample.
  • the method is non-invasive and thus does not perturb the flow being studied. It has very high dynamic range permitting both very low and very high speeds to be measured as required.
  • the method is a novel modification of an existing laser-based velocity measurement technique called Filtered Rayleigh Scattering (FRS).
  • FRS Filtered Rayleigh Scattering
  • My invention consists of modulating the laser frequency and using a phase-sensitive detection at the second harmonic of the modulation frequency. This provides improved detectivity of the typically weak Rayleigh scattered signal in the presence of stray light interference. Additionally, the modulation provides the basis for stabilizing the laser frequency and thus increases the accuracy of the measurement. Unlike most prior art related to Raman spectroscopy which seeks to diminish Rayleigh scattered signals, this invention takes advantage of the Rayleigh scattered light to directly provide input for velocity measurement.
  • a common diode laser such as the one used in this invention, is relatively inexpensive, currently measures less than a millimeter in size while encapsulated in a housing the size of a pencil eraser, but is capable for instance of emitting 50mW of continuous power.
  • Larger, yet still relatively compact, monolithic diode arrays and power-amplified diode lasers have been fabricated that are capable of output powers greater than 100W.
  • Another feature of diode lasers, making them unique, is that they can be rapidly tuned in frequency at megahertz repetition rates by modulating the laser input current. The majority of these lasers emit in the near-infrared to the red wavelengths of the electromagnetic spectrum.
  • Modulation spectroscopy techniques employ an instrument known as a lock-in amplifier that filters out nearly all signals except for those at a specific frequency and phase. Therefore, it can extract weak signals from large background noise level when it is set at the fundamental or second harmonic of the frequency used to modulate the laser wavelength as it traverses an absorption line.
  • Spectroscopists take advantage of the ability of a diode laser to be tuned rapidly in wavelength at this modulation frequency to allow its signal to be extracted by a lock-in amplifier. Since the diode lasers used in the practice of this invention are frequency tunable, directly, this invention may be practiced without the need for optical mechanical modulation devices that have been employed previously to modulate the frequency of non-diode lasers, such as Ti: Sapphire lasers.
  • One can also simultaneously scan the laser slowly across a broader bandwidth to examine such things as the multiple line structures of atomic and molecular absorption in the near infrared spectrum.
  • alkali metal vapors have strong electronic transitions within the emission bandwidths of some of the more common and powerful diode lasers. These electronic transitions are often split into a number of closely- spaced absorption lines that are known collectively as hyperfme structure. The lines are also very narrow in bandwidth. Alkali vapor cells, therefore, are excellent filters for use in very high-resolution frequency discrimination. Additionally, an alkali vapor cell has very high optical depth at only modest vapor pressures and lengths, i.e. it absorbs all (or nearly all) light at the absorption wavelength.
  • This invention employs the cost, durability, and size advantages of a diode laser to provide an accurate velocity measurement.
  • the system uses a modulated diode laser to scatter off the gas to be probed. Then that light is passed through an alkali vapor cell of known composition and well-established absorption characteristics before being detected.
  • the narrow absorption lines in the alkali cell act as a high-resolution frequency detector and, using modulation spectroscopy techniques, the relatively weak diode laser signal may be extracted from background interference and noise and amplified for analysis.
  • the Doppler frequency shift in the scattered light may be measured, and from that, the velocity may be calculated.
  • the present invention may be practiced without the use of seeding.
  • the Doppler shift is the cornerstone of optical velocimetry techniques. If the directional vectors and the wavelength of the light source are known, then it is a simple matter to convert a Doppler frequency shift to velocity. The difficulty is then a matter of accurately determining the frequency shift in the scattered light. This may be accomplished by using some form of filter that absorbs at a known frequency, as in a FRS experiment. At the edge of an absorption "line,” there is a very narrow bandwidth over which the absorption increases rapidly and therefore the amount of light that is allowed to pass through falls rapidly. This band provides a conversion of frequency to transmission that can then be registered by a photodetector and analyzed.
  • the frequency that the lock-in amplifier uses to extract signal is the frequency at which the spectroscopic probe wavelength is modulated, dithering rapidly across a narrow band of wavelength. Where the dither traverses back and forth over a steeper part of the absorption, the detector sees a larger fluctuation, which translates in the lock-in signal to be greater amplitude. Therefore, detection at the dithering frequency results in the lock-in recording a signal based upon the slope of the absorption feature and not its absolute intensity. This produces a lock-in amplified signal that approximates a first derivative of the original signal.
  • detection at the second harmonic, or twice the frequency, of the modulation signal results in a lock-in output result approximating the slope of the slope, or the second derivative of the original signal.
  • detection at the Nth harmonic of the modulation frequency results in an approximation to an Nth derivative signal.
  • the quality of the approximation is based on a number of factors.
  • the first is the amplitude of the modulation in the spectroscopic probe wavelength. As the modulation dithers through a greater band, the resulting signal will be stronger, because more of the original signal is scanned with each pass. The larger fluctuation in signal translates to greater output signal amplitude. It has been proposed that the optimal dither frequency bandwidth, or modulation depth, should be about 2.2 times the half width at half maximum (HWHM) of the signal for maximum signal. Additionally, the greater the modulation depth, the greater the distortion in the resultant signal from approximating a true derivative signal, because fine structure smaller than the dithering bandwidth become blurred. Therefore, a balance may be found between the competing factors.
  • Another variable is the speed of the lock-in amplifier.
  • Current lock-in amplifiers have a maximum frequency to which they can lock as well as an inherent time constant. This time constant is often adjustable to serve as an integration time for the detector to improve noise suppression in the output signal. If one is dithering about a set frequency, then the temporal resolution of the detection scheme is limited by the lock-in time constant, which is generally significantly slower than any other time constant in the system. If one scans through a larger bandwidth of frequencies to look at an absorption structure while modulating the signal, then to avoid distortion, the features of the structure must be scanned through at a speed much slower than either the modulation frequency or the time constant of the amplifier.
  • FIG. 2 shows the absorption profile of a cell of rubidium vapor around 780nm. It is being scanned at a repetition rate of 5Hz . Due to the properties of the laser, as current is increased, output frequency is decreased, so in reading the plot from left to right, frequency is decreasing, while the wavelength is increasing.
  • the 2F signal of that absorption profile was recorded and is shown in FIG 3.
  • the second harmonic peaks are at the points of the absorbtions maximum positive curvature (at the absorption signal minima), and its troughs are at the absorption profiles points of maximum negative curvature (at the edges of the line profiles and the signal maxima between the third and fourth minima).
  • this invention is an apparatus useful for measuring velocity of a fluid, which comprises: a frequency-tunable diode laser; a reference system that provides a reference absorption pattern from the laser; optics that collect light scattered from the fluid when irradiated with the laser; a scattering detection system that provides a scattered absorption pattern; and a computation system that determines a frequency shift between the reference pattern and the scattered pattern and that calculates velocity of the fluid based on the shift.
  • this invention is a process useful for measuring velocity of a fluid, comprising: irradiating the flow with a modulated laser beam from a diode laser; collecting light scattered from the fluid; obtaining a reference absorption pattern of the laser beam and a scattered absorption pattern of the scattered light; determining the frequency shift based on the reference absorption pattern and the scattered absorption pattern; determining fluid velocity based on the frequency shift.
  • this invention is a process useful for manufacturing an instrument for measuring the velocity of a fluid, which comprises: providing a frequency- tunable diode laser; providing a feedback system that provides a reference absorption pattern from the laser; providing optics that collect light scattered from the fluid when irradiated with the laser; providing a scattering detection system that provides a scattered absorption pattern; and providing a computation system that determines a frequency shift between the reference pattern and the scattered pattern and that calculates velocity of the fluid based on the shift.
  • this invention is an apparatus useful for measuring velocity of a fluid, which comprises: a frequency-tunable laser, wherein a beam from the laser may be directed at the fluid; a laser controller connected to the laser; a reference frequency discriminator such as a heated rubidium cell that provides information to a detector which feeds back information to the laser via a first lock-in amplifier, a summing amplifier, and a laser current controller which is connected to the laser; optics that collect Rayleigh scattering from the fluid when irradiated with the laser; and a second frequency discrimination detector such as a heated rubidium cell that receives the Rayleigh scattering from the optics and advances output data to a second detector which further conveys data to a high speed analog/digital converter and computer via a lock-in amplifier; wherein a high frequency oscillator provides input to a summing amplifier that feeds the laser current controller and the first and second lock-in amplifiers.
  • a reference frequency discriminator such as a heated rubidium cell that provides information to a detector which feed
  • this invention is a process useful for measuring velocity of a fluid, comprising: irradiating the flow with a laser beam; collecting Rayleigh scattering from the fluid; measuring the Doppler shift of the laser beam; reducing light interference by modulating filtered Rayleigh scattering; and determining fluid velocity based on the frequency of the Doppler shifted laser beam.
  • this invention is a process for manufacturing an apparatus useful for measuring velocity of a fluid, which comprises: providing a frequency- tunable laser, wherein a beam from the laser may be directed at the fluid; connecting a laser controller to the laser; providing a reference heated rubidium cell that provides information to a detector which is connected to and feeds back information to the laser via a first lock-in amplifier, a summing amplifier, and a laser current controller which is connected to the laser; providing optics that collect Rayleigh scattering from the fluid when irradiated with the laser; providing a second heated rubidium cell that receives the Rayleigh scattering from the optics and advances output data to a second detector connected to the second cell which further conveys data to a high speed analog/digital converter and computer via a lock-in amplifier; providing a high frequency oscillator which provides input to a summing amplifier that is connected to and feeds data to the laser current controller and the first and second lock-in amplifiers.
  • One novel aspect of this invention is the modulation of the laser frequency which simultaneously permits sensitive detection of the signal in the presence of bright interferences, and improved laser frequency stabilization by locking the laser to the atomic or molecular transition being used as a filter. Modulation frequencies of several MHz are easily used.
  • the tuning capability also provides an instrument of this invention with a very high dynamic range.
  • the laser can be tuned to a desired offset from the atomic or molecular resonance, so that the Doppler shifted Rayleigh scattering coincides with peak sensitivity of the response curve.
  • the same system can be used to measure velocities from zero to several km/s with similar precision. This invention thus represents a significant improvement over conventional Filtered Rayleigh Scattering with a fixed-frequency laser source.
  • LDV laser doppler velocimetry
  • This invention improves on other laser-based velocity measurement techniques, such as laser doppler velocimetry (LDV).
  • LDV requires two laser beams to be crossed in space and provides a single point measurement of one velocity component.
  • This inventions permits simultaneous multi-point measurements. Further, by viewing a single laser beam at two different angles two components of velocity can be measured simultaneously with a single laser beam.
  • LDV measurements are compromised by having several scattering particles in the probe volume. This is not a problem in the practice of this invention. In common with other optical techniques, and in contrast to conventional probe sensors, this invention is non- invasive and does not perturb the flow being studied.
  • This invention is useful for sensitive measurements of fluid velocity from low speeds to very high speeds up to several km/s. Velocity measurements are important in a wide range of research in fundamental fluid mechanics, and in development and testing of fluid machinery (compressors, turbines, jet engines, pumps, furnaces, etc.). This invention can also be used to measure gas temperature in flows with no particle contamination and could be adapted for LEDAR measurements from ground stations or satellites.
  • Interference from laser beam scattering off windows can be a problem in the practice of this invention. For high speed flows, this can be countered by tuning the laser to a peak absorption so that the unshifted scattering from the windows is completely blocked, while the Doppler shifted scattering from the flow is only partially attenuated and provides the desired signal.
  • This invention has no limitations beyond those inherent in the existing FRS technique, and is a substantial improvement over the current state-of-the art.
  • the electronics and optics required to implement the scheme for point measurements are available off-the shelf.
  • FIG. 1 is a schematic diagram of the arrangement.
  • the basic filtered Rayleigh scattering scheme is well known.
  • the laser is set to a convenient frequency near an atomic or molecular absorption line and the Rayleigh scattered light is filtered by a cell containing the atom or molecule in question.
  • I have used rubidium, and tune the diode laser to a hyperfme component of the D 2 transition of rubidium at 780 nm.
  • FIG. 2 shows the absorption spectrum of the D 2 line a mixture of the two naturally occurring Rb isotopes as recorded with the diode laser.
  • the absorption pattern of the reference beam and the scattered beam are compared.
  • the offset of the two patterns provides data that is used to compute the velocity of the flow by use of formula
  • V is the gas velocity
  • ⁇ 0 is the wavelength of the laser
  • the factor containing unit vectors in brackets arises from the geometry of the experiment and is of order unity.
  • Ambient light interferences can be a problem because the Rayleigh signal is weak particularly when using a cw solid-state laser. Indeed, through practice of this invention it is possible to calculate two velocity vectors using a single laser beam. This is noteworthy in view of the state of the art where two lasers are used to obtain a single velocity vector.
  • this invention provides a measurement that is sensitive to the second derivative with respect to frequency of the scattered light. Synchronous detection provides very high rejection of interferences, so weak signals and ambient light are much less of a problem. Finally, the modulation of the laser frequency that is needed also provides a simple means of stabilizing the laser frequency and improving the measurement precision still further.
  • the diode laser is a particularly good source because it is tunable and the laser operating frequency is set by its temperature and current. The frequency is readily modulated by modulating the input current, and the laser absolute frequency is stabilized in a feedback loop by adjusting the input bias current.
  • the diode laser linewidth is typically very narrow, which allows very high precision velocity measurements, and the effective linewidth is further narrowed by feedback stabilization. Frequency precisions of 1 part in 10 9 are readily achievable, and stability of 1 part in 10 12 has been demonstrated. Diode lasers are much cheaper, and more compact, rugged, and reliable than the lasers used in other schemes (excimer, argon-ion, doubled YAG, or ion-pumped Ti: sapphire lasers).
  • Diode lasers can be easily modulated at rates approaching 1 GHz by modulating the drive current while most other laser sources are harder to modulate.
  • the GHz modulation capability in combination with radio frequency lock-in amplifiers, permits tracking of MHz velocity fluctuations so the system can follow the highest speed turbulent fluctuations.
  • the modulation scheme can be used to stabilize the absolute frequency of the laser. Diode lasers have been routinely frequency stabilized to about 100 kHz corresponding to an absolute frequency stability of 1 part in 10 10 , and the best stability is several orders of magnitude better than this. Thus I anticipate no difficulty stabilizing the laser to the level desired. I contemplate that velocity resolution better than 1 m/s in high speed flows (500- 2500m/s) may be achieved.
  • FIG. 3 shows the experimental data corresponding to a slow (20 Hz) sweep of the laser frequency across three of the four hyperfme components while simultaneously modulating at 20 kHz. Because I use lock-in detection at the second harmonic of the modulation frequency the shape of the traces corresponds approximately to the second derivative of the absorption profiles. The Doppler shift between the two traces provides a mean gas velocity of 408 m/s ( ⁇ 6%). An improvement in precision of at least one and possibly two orders of magnitude is possible.
  • FIG. I depicts a non-limiting representative configuration of the apparatus of this invention.
  • FIG. 2 depicts the hyperfme structure of the D 2 transition of rubidium (Rb) at 780 ran recorded in a 6 cm absorption cell containing a normal isotopic mixture of 85 Rb and 87 Rb at 35 OK. Warming the cell to 400K gives much steeper edges to the absorption features because the rubidium vapor pressure increases exponentially with temperature and the absorption coefficient increases exponentially with vapor pressure.
  • FIG. 3 depicts a record showing a shifted hyperfme structure from scattered light relative to a reference structure.
  • FIG. 4 depicts another embodiment of an apparatus of this invention.
  • FIG. 5 depicts a graph showing simultaneous measurement of stream-wise and cross- stream velocities of a condensing, supersonic carbon dioxide jet.
  • FIG. 1 A representative system of this invention is depicted in FIG. 1.
  • the schematic diagram in FIG. 1 thus depicts a system that employs a modulated filtered Rayleigh scattering configuration that is used for point velocity measurements.
  • a laser 12 preferably a diode laser, emits a light beam 14 to be cast upon a flow, depicted by arrow 44.
  • the frequency and the intensity of the beam 14 may be adjusted through inputs to the laser 12 by laser controller 10 and current controller 32.
  • a portion of the beam 14 is deflected by mirror 34 to serve as a reference standard.
  • the reference beam 18 may be pass through a cell 20 containing a molecular vapor or an atomic vapor such as rubidium.
  • the cell 20 is devoid of any substance other than the vapor and is typically under a vacuum except for the partial pressure of the vapor.
  • rubidium for example, solid rubidium within the cell is often heated to 50 degrees Centigrade to 100 degrees Centigrade in order to create vapor which provides a small fraction of an atmosphere of pressure.
  • Rubidium has an absorption pattern as shown in FIG. 2. Accordingly, depending on the frequency of the light, a portion of the reference beam 18 will be absorbed by the rubidium.
  • the filtered light exiting the cell 20 may then be detected by light detector 22 which provides an output of data to lock-in amplifer 24.
  • the data from the lock-in amplifier 24 is then optionally fed to an inverting amplifier 26 which performs the function of inverting the same and adjusting the magnitude of the reference output, which may be used for feedback stabilization in frequency- locked mode.
  • Output from the inverting amplifier 26 is then provided to a summing amplifier which combines the feedback signal and the modulation signal from oscillator 28.
  • Output from the summing amplifier 30 is then sent to the laser current controller 32 which assesses the data and sends a signal to the diode laser to control the current dependent on the data received from the summing amplifier 30.
  • the light beam 14 it may be directed at the flow 44 under study, optionally being directed by angle-adjustable mirror 36 and optics 40 for beam positioning and focussing.
  • the mirror 36 may be adjusted to allow the beam 14 to strike the flow 44 at any desired angle.
  • a portion of the light beam is scattered by the flow to provide scattered light beam 48, with the balance of the light beam 14 passing through the flow 44.
  • the scattered light 48 may optionally be collected by optics 42 designed to collect the Rayleigh scattering, and focussed upon a mirror 38, which is optional (or, the optics 42 may focus the scattered light 48 directly toward cell 50.
  • cell 50 contains a substance which absorbs a portion of the scattered light 48, with the light that passes through the cell 50 being detected by detector 52.
  • Output from the detector 52 is sent to lock-in amplifer 54 which serves to detect signals at the fundamental or second harmonic of the reference frequency provided by the high frequency oscillator 28.
  • Output from lock-in amplifier 54 may then be forwarded to a high speed analog/digital ("A D") converter and computer 56.
  • the high frequency oscillator 28 provides the reference modulation frequency signal that is used by the laser current controller 32 to modulate the laser frequency.
  • the same reference signal is used by the lock-in amplifiers 54 and 24 which use it to internally generate the second harmonic frequency at a fixed relative phase that is used in "lock-in" detection.
  • Data on the reference beam is also fed to the computer via a second channel on the analog/digital converter board.
  • a diode laser system used in the practice of this invention may consist not only of a laser 102, but also a power supply 104 and temperature controller 106.
  • the current source 104 for the laser 102 should be very stable, to avoid unwanted fluctuations in laser wavelength due to electronic noise. It should also be capable of accepting an electronic modulation signal to control its current, so that modulation spectroscopy can be used.
  • the temperature controller 106 should also be very stable, because the diode laser emission wavelength also depends on temperature.
  • a pair of voltage function generators and a summing amplifier 110 may be employed to generate the current modulation for the laser. Both techniques (frequency scan mode and frequency lock-in mode) require one of the function generators to feed a low- amplitude, high- frequency modulation into the laser current to provide the rapid wavelength dithering. It also provides the source for the lock-in amplifiers 112, 114, 116 to use as a reference frequency and phase.
  • the other function generator 118 is used to generate the larger amplitude, low frequency ramp that scans the laser across a wider wavelength interval. It is used in both modes of operation to tune the laser into the correct frequency bandwidth such that it locates the alkali, for example rubidium, absorption features.
  • Rubidium is preferred because of its high vapor pressure relative to other alkali metals. In the frequency-locked operation, however, it is damped out and removed from the laser current modulation. This is done so that the laser is running with only the dither about one particular wavelength on the edge of one of the absorption lines.
  • Another useful feature, available on many summing amplifiers, or implemented independently, is the ability to filter the signal for unwanted noise which is useful in keeping the laser modulation as smooth and noise-free as possible.
  • a feedback system is used to form a reference-arm that takes light coming directly coming from the laser.
  • a fraction of the light output of the laser is extracted with a beam splitter 120.
  • the light is passed through a Rb absorption cell 122 and finally into a fast, but not necessarily sensitive detector 124, due to the need to rapidly follow the strong light source.
  • filters may be needed to reduce the light input to the detector to a level that will not damage or saturate it.
  • the detector signal is then sent to a lock-in amplifier 116 whose output goes to the data acquisition system 126.
  • the remainder of the laser light passes into the flow to be probed.
  • Light scattered by the point of the flow to be analyzed is collected by a large numerical aperture (low f-number) optics 128 and directed into a detection-arm.
  • This arm also contains a Rb absorption cell 130 and another photodetector 132.
  • This detector 132 may be sensitive enough within the bandwidth of the laser to be able to detect the weak intensity of flow scattered light.
  • a second detector 134 within the same arm can be used in a ratioed detection scheme. That detector accepts the scattered light without passing through the vapor cell 130 and is used to monitor and compensate for fluctuations in scattering intensity.
  • Absorption signal from the detection-arm is sent to another lock-in amplifier 112. If an orthogonal velocity component is desired, an additional identical detection-arm, including all equipment and lock-in amplifiers must be placed to collect light scattered in the opposite direction.
  • Reflections from surfaces are a concern. These reflections may be from windows or body surfaces whose boundary layers are being explored. As with other optical flow diagnostic techniques, light scattering from the flow will be of significantly lower power than that scattering from a solid surface. Therefore, care must be taken either to avoid illuminating surfaces that can be seen by the detection-arm or to implement the use of spatial filtering. One way of doing this is to image the received light to a focal point through an iris to keep nearly all other stray reflections away from the detector. Depending on the geometry involved, apparatus blocking the light path can also make imaging a second velocity axis opposite the first impractical or impossible.
  • the frequency-scanning mode of operation compares the frequency displacement of the scattered light signal versus that of the reference signal, and thereby gives a direct measurement of Doppler Shift, and with it, velocity.
  • the reference- arm scan serves as a static reference, showing the frequency of the laser at a given point in the scan. From the displacement between the smooth peaks, one can also get a time (or data element) to frequency conversion factor. Next, the phase displacement between the reference and detection arm signals is measured, accounting for any lag that may be due to electronic sources. With the phase to frequency conversion and the Doppler shift to velocity conversion that is established by the physical layout and collection lens numerical aperture, one arrives at a velocity measurement from the phase displacement of the signal acquired.
  • Frequency-scanning mode of operation has both advantages and disadvantages relative to the frequency-locked mode. Being a phase measuring technique rather than being an absolute amplitude measuring technique, the frequency-scanning mode is not susceptible to fluctuations in scattering intensity of the same or greater period than the scan time.
  • the technique is significantly limited in temporal resolution, because it must make a complete scan through the absorption features in order to make a velocity measurement. Because it is relatively slow, it effectively averages over high frequency scattering fluctuations as well. Therefore, this technique can be performed without a ratioed detection scheme with reasonable accuracy, although significant scattering fluctuations at approximately the same frequency as the scanning frequency can cause error in finding the phase displacement.
  • Another concern for the accuracy of the technique is the stability of the laser within periods shorter than the scan that can also distort the phase difference measurement. The system automatically accounts for slow laser frequency drifts by comparing at the relative phase displacement of two signals acquired simultaneously.
  • the laser In the frequency-locked mode of operation, the laser is locked to a single frequency on the edge of one of the sharp second harmonic features.
  • the signal from the reference-arm is used as part of a feedback loop to stabilize the laser. Not only does this keep the laser at one set frequency, but it can also suppress electronic fluctuations in the power supply.
  • the shape and size of the absorption feature at the scattered light level must be determined to establish a frequency to signal voltage conversion. This is why the use of a ratioed detection scheme is critical in frequency-locked mode operation. A fluctuation in scattering intensity will lead to a change in input power to the detector, which will result in an amplitude change in signal that will be reflected in all further derivatives of the signal.
  • a frequency band of the second harmonic signal from the absorption hyperfme structure must be selected that goes from a maximum to minimum value, such that there will be a one-to-one correspondence between signal and frequency.
  • One criterion for the choice is a large change in signal for a small change in frequency, which improves the frequency, and therefore velocity, resolution.
  • another concern is that the local slope of the absorption line must be appropriate for the frequency locking, given the laser tuning characteristic (decrease in emission frequency with increasing input current) or else the feedback loop must be inverted.
  • any change in signal amplitude will be a reflection of a change in velocity.
  • the temporal response of the technique is only limited by the modulation frequency of the laser or the output time constant of the lock- in amplifier-whichever is slower-leading to a significant improvement over the frequency- scanning mode.
  • Its primary disadvantage is its sensitivity to scattering fluctuations and reliance on a more complex frequency measurement system.
  • the scattered signal / scattered reference ratio system also affects temporal response in that it must respond much faster than the fluctuation frequency such that it can correctly ratio the modulated signal without distortion, or if the compensation is done in post-processing, it adds a delay to the measurements.
  • the diode laser selected was compatible with the alkali metal for the absorption cell. For instance, several high-powered single diode lasers with peak emissions near the 780nm wavelength band are available. Among the alkali metals, rubidium has very strong absorptions for its D, and D 2 electronic transitions at 795nm and 780nm respectively. This convenient proximity and large optical depth lead to it being chosen as the atomic filter.
  • the laser was the Hitachi HL7851G which lists among its rated attributes 50mW continuous wave (cw) power for a current input of 140mA and a typical emission wavelength of 785nm.
  • This GaAlAs diode is only about 1mm square in a canister 9mm in diameter and only 5mm tall.
  • the diode was placed in an ILX Lightwave LDM-4420 head mounting.
  • the diode laser head allowed access for the various laser support systems.
  • the first was the ultra- low noise ILX Lightwave LDX-3620 current source to power the diode. Cooling and thermal stabilization were provided by an ILX Lightwave LDT-5910 thermo-electric temperature controller.
  • dry nitrogen was bled into the laser head to prevent moisture from condensing on its sensitive electronics.
  • a wave-meter was constructed and was used to characterize the HL7851G diode. Wave-meters measure the optical beat frequency between an unknown source and a very stable laser whose wavelength is known precisely in order to determine the unknown source frequency. The effects of temperature and input current on output wavelength of the laser diode were studied. It was noticed that at the extremes of temperature and current, and especially when adjusting those qualities rapidly, it suffered from "mode hop.” Within a range of wavelength, the diode responds linearly to changes in temperature and current. However, it can be made to "hop" to a different emission wavelength for the same input current and temperature, after which its behavior is again quasi-linear. It was decided that the laser should be run chilled to extend its life.
  • the laser was operated at the wavelength of the D 2 transition in Rb rather than the D, transitions.
  • the laser could be consistently made to emit at 780J4nm, the center of the D 2 hyperfme structure of Rb, at 5.0 degrees Centigrade and a nominal current input of 114mA. With these settings, the laser's output power was
  • the D 2 transition in rubidium is found to be composed of six hyperfme components, three in each of two clusters over a span of 7GHz.
  • the occurrence of two stable rubidium isotopes leads to further complexity in the absorption spectrum of a cell containing the isotopes in normal abundance (12.2% 85 Rb and 27.8% 87 Rb).
  • the 10cm long rubidium cells used in this experiment exhibit four distinct features because Doppler broadening near room-temperature make the clusters of three hyperfme components appear as a single peak each (the narrow laser line-width is negligible).
  • the absorption record shows a pair of less intense peaks spaced 6.837GHz apart on either side of a pair of more intense peaks 3.035 GHz apart as the laser is scanned through the hyperfme structure.
  • the laser modulation is accomplished by a pair of function generators and a pair of summing amplifiers; their output is fed into the current source modulation input on the LDX- 3620.
  • the modulation input converts a IV signal into a 100mA change in current output.
  • An Exact 200MSP function generator produces the 50kHz sinusoidal modulation frequency with an amplitude of 5mV as well as a TTL synchronization pulse fed to the lock-in amplifier's locking frequency inputs.
  • This frequency was chosen, because the lock-in amplifiers used could only lock to signals up to 100kHz, and the second harmonic frequency was used in these experiments.
  • the slow frequency scan is accomplished by Stanford Research Systems DS345 function generator putting out a 10Hz triangle wave with an amplitude of 40mV. Both of these modulations are then input to a Stanford Research Systems SR560 summing amplifier, summed with unit gain, and are low-pass filtered at 300kHz.
  • the ramp is passed through a variable attenuator composed of a 5kOhm potentiometer, which is used to gradually decrease the amplitude of the scanning signal to zero. This is needed because sudden deactivation of the ramp may result in a laser mode hop.
  • the output of the first summing amplifier is then fed into another SR560 to be used in the frequency-locking feedback technique and again filtered for high frequencies before being input as the modulation signal for the current source.
  • the modulations result in a laser that is tuned in wavelength around the center on the absorption.
  • the 5mV dither results in a modulation dither bandwidth of 1.05GHz.
  • the absorption peaks are found to have a HWHM of 0.523GHz at operating conditions. Therefore the modulation depth is 2.01, which is close to the suggested optimal modulation depth of about 2J.
  • the plus or minus 40mV ramp results in a scanning bandwidth of 16GHz that is sufficient to scan completely through the D 2 hyperfme structure.
  • the modulated laser light is first collimated, then guided into and through the instrument by mirrors chosen for their high infrared reflectivity.
  • the beam can be split by a pellicle, but in the acoustically noisy environment generated by the supersonic carbon dioxide jet, a microscope slide was used because of pellicle vibrations.
  • Finally, the laser is targeted onto a "flow" for study.
  • a simple static target was inserted. Low velocity measurements are accomplished with a spinning disk, painted with flat black lacquer to simulate weak scattering and attached to a voltage-regulated electric motor. The speed of the probe point on the disk is measured independently with a timing strobe.
  • a gas jet is used to produce higher velocities, at the cost of also generating a great deal of turbulence and velocity fluctuations.
  • the beam paths were set up at right angles for reasons of precision and simplicity. With the jet aimed at a 45 degree angle to the optical bench grid, the result is a beam path 135 degree from the flow direction while the x-axis, or stream-wise detection-arm is at 45 degrees.
  • the y-axis, or crosswise detection-arm was placed at 225 degrees, opposite the other.
  • a FORTRAN numerical simulation of the received light was generated from an absorption profile modeling code adapted to account for large collection optics.
  • the experiment has more recently used a pair of Hamamatsu photomultiplier tubes (PMTs) and the APD has been moved to the reference arm.
  • PMTs Hamamatsu photomultiplier tubes
  • the R636-10 PMT detectors were designed to have very broad peak spectral sensitivity from 300 to 800nm (2.8 x 10 4 A/W radiant anode sensitivity across the band). They are powered by an Oriel 70705 high voltage power supply at a voltage, depending on scattering intensity, so that the detectors supply good signal but without risking overload. While able to detect fainter scattering than before, a more powerful laser source is needed in order to detect light scattering from pure molecular scattering.
  • Signal from the filtered signal detecting photodetectors is routed to the lock-in amplifiers for 2F-detection.
  • the lock-ins being used for detecting absorption filtered light are locked to the same 50kHz reference signal from the modulation source. All have their output time constants set to the minimum value practical: 1ms for the SR510 and SR530, and 300ms for the SR830s.
  • the SR830 lock-ins are faster, and therefore have even lower available time constants, however with such small time constants, electronic noise beings to degrade the signal.
  • the lock-in output is then amplified to arrive at a signal that is approximately 5-10V peak to peak for analysis.
  • the signals from the lock- ins are sent directly to the data acquisition equipment.
  • the output of the reference arm is used as the source of a feedback loop to control for laser frequency.
  • the laser is first carefully tuned to one edge of a strong harmonic absorption feature chosen to be the frequency the laser is locked to. Its slope should be opposite that of the laser response: increasing in voltage for increasing frequency, or the feedback signal must be inverted.
  • the tuning point is either where the reference lock- in output is nearly zero, or else an offset may be applied to the lock-in signal and the laser tuned to point to return the output to a value of zero.
  • the output from the reference-arm lock-in is then attenuated almost completely by a lOKOhm variable potentiometer and fed into the second summing amplifier to be added in with the modulation signal. If the laser begins to drift in frequency, that will be reflected in the absorption through the reference cell and from there to the output of the lock-in amplifier will depart from OV, which will then generate a signal to the laser current supply to change its current such that the attenuation is reduced, increasing the feedback gain, to improve the wavelength stabilization without resulting in a feedback resonance.
  • one of the detectors receives the unfiltered light.
  • a chopper is used to modulate the laser light heading for the probe volume.
  • Light from the unfiltered photodetector is measured by a lock-in at the first harmonic of the chopper frequency, and in this case the signal has a value proportional to the absolute laser power. This is because the light is simply being turned on and off giving the signal harmonic content that can be locked to, instead of scanning back and forth in frequency across an absorption line which actually dithers the signal received.
  • the system can be used for real-time or postprocessing extraction of velocity from the data.
  • the virtual instrument is triggered by the frequency-scanning ramp and first collects a record of the lock-in signal from the reference and detection-arms. Using peak fitting algorithms, it measures the distance between the two 85 Rb peaks of the reference-arm in terms of data elements. Then, it conducts a numerical cross-correlation on detection-arm signals versus the reference-arm. A cross-correlation evaluates how well correlated, or matched, two functions are as one is shifted in phase relative to the other.
  • the element location of the cross correlation maximum gives the phase displacement between two similar functions. Since the measurement takes into account the entire signal, it is significantly more accurate than simply matching one or two similar points and measuring the phase difference between them. This element phase difference is converted to Doppler frequency shift by the previous peak to peak element spacing and the known frequency displacement between the peaks. This automatically corrects for any changes in the slope of the frequency-scanning ramp. The next component was a system to account for scattering intensity fluctuations so that frequency-locked measurements could be made.
  • the scattered signal level is divided by the scattered reference signal. Since signal fluctuation is linear with received light, this should be sufficient to make scattering fluctuation effects negligible.
  • the signal is normalized and compared to a polynomial fit to the 2F detection edge slope that is being used as the frequency reference to determine the Doppler frequency shift.
  • the laser is modulated with a chopper and detected with a lock-in amplifier set to the chopper frequency.
  • Interference sources include stray room light and broad-band electronic noise in the detector.
  • the chopping frequency must be set to a frequency much higher than the modulation frequency of the laser.
  • the feedback loop was established from the lock-in amplified signal, attenuated, and fed into the current modulation input.
  • the attenuation is adjusted to be the point where the attenuation made the fluctuations a minimum and before increasing the feedback gain results in an increasing feedback signal.
  • Under external power the average fast jitter in was kept down to a bandwidth of around 190mV, which translates to an uncertainty of 3.3MHz ((1.9m/s), while under pure battery power, it was nearly a quarter of that at only 50mV, translating to an uncertainty of 0.89MHz ((0.5m/s). For the 100 second period scans, however, the results were very similar.
  • the lock-in amplifier time constant was set from 1ms to 10ms to remove much of the high frequency jitter while retaining a temporal response more than fast enough to lock the laser.
  • the externally powered laser was held to a standard deviation of only 17mV in the period shown, which translates to an uncertainty of 0.60MHz or (0.34m/s.
  • the laser run off internal batteries only deviated by 21mV in the period shown and so the fluctuation of the Doppler shift would be 0J5MHz, or a velocity uncertainty of (0.42m/s.
  • the laser running on internal DC power alone is more stable on small time scales, but its overall drift is comparable with running with external power being supplied to the current source.
  • Measurements on the condensing jet of carbon dioxide were used to demonstrate the velocimetry system of this invention's ability to make simultaneous, two-component velocity measurements in the frequency-scanning mode of operation.
  • This test flow was chosen for its high velocity, simplicity, and the ability to highlight the frequency-scanning system rejection of scattering fluctuations.
  • 4000 data elements were taken at a data rate of 80kHz. These values were chosen such that a single data element displacement translated to approximately lm/s, placing a lower bound on the velocity resolution. This value was chosen to be roughly equal to the uncertainty in each measurement due to fluctuations in laser wavelength of the same order period of the scan as evidenced by scattering from a static target. The static target was employed to measure and compensate for any inherent electronic phase lag in the system between the detection arms.
  • FIG. 5 shows average values of one experimental run with measurements taken at radial positions across the jet as measured from what was believed to be the jet center. Some care was taken to make the stagnation pressure behind the jet the same in each run. However, it was found that a proper pressure regulator prevented the carbon dioxide from condensing into a strongly scattering fog, and so it was controlled with the gas tank valve by hand, monitored and corrected during the experimental run. At each position, 25 velocity measurements were taken. The error bars on the plot are equal to the error of the measurements (2(/nl/2).
  • the mean standard errors in measuring mean jet velocity were 4.5m/s and 2Jm/s respectively for the X and Y-axis scattering arms.

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Abstract

L'invention porte sur un système et un procédé de mesure de la vitesse d'un fluide, tel qu'un jet de gaz supersonique, recourant à la lumière laser modulée d'une diode laser accordable, la modulation consistant à faire vibrer le laser. La lumière diffusée une fois recueillie traverse une cellule contenant par exemple du rubidium. Le motif de la lumière diffusée est comparé à un modèle de référence. Le décalage Doppler de la lumière diffusée par rapport à la référence fournit des données servant à calculer la vitesse du fluide.
PCT/US1999/000691 1998-01-12 1999-01-12 Diffusion de rayleigh modulee et filtree WO1999035519A2 (fr)

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AU24556/99A AU2455699A (en) 1998-01-12 1999-01-12 Modulated filtered rayleigh scattering

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WO2006134103A1 (fr) * 2005-06-14 2006-12-21 Forschungsverbund Berlin E.V. Procede et dispositif pour generer et detecter un spectre de raman
US7400385B2 (en) 2002-08-02 2008-07-15 Ophir Corporation Optical air data systems and methods
US7564539B2 (en) 2002-08-02 2009-07-21 Ophir Corporation Optical air data systems and methods
DE102009042404B3 (de) * 2009-09-16 2011-04-14 Technische Universität Dresden Verfahren zur Bestimmung der Geschwindigkeit eines bewegten Fluids unter Einsatz einer Eigenkalibrierung eines Doppler-Global-Velozimeters mit Laserfrequenzmodulation
US8072584B2 (en) 2002-08-02 2011-12-06 Ophir Corporation Optical air data systems and methods
US9334807B2 (en) 2014-05-13 2016-05-10 The Boeing Company Methods and apparatus to determine airflow conditions at an inlet of an engine
GB2532585A (en) * 2013-06-30 2016-05-25 Wind Farm Analytics Ltd Turbine fluid velocity field measurement
CN113176582A (zh) * 2021-04-27 2021-07-27 厦门大学 一种基于双频泵浦的流速探测激光雷达和流速探测方法
EP3443360B1 (fr) * 2016-04-15 2024-04-10 Deutsches Zentrum für Luft- und Raumfahrt e.V. Aéronef et procédé de détermination de grandeurs caractéristiques importantes pour le vol

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US4919536A (en) * 1988-06-06 1990-04-24 Northrop Corporation System for measuring velocity field of fluid flow utilizing a laser-doppler spectral image converter

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US8072584B2 (en) 2002-08-02 2011-12-06 Ophir Corporation Optical air data systems and methods
US7400385B2 (en) 2002-08-02 2008-07-15 Ophir Corporation Optical air data systems and methods
US7564539B2 (en) 2002-08-02 2009-07-21 Ophir Corporation Optical air data systems and methods
US7760339B2 (en) 2002-08-02 2010-07-20 Ophir Corporation Optical air data systems and methods
US7894045B2 (en) 2002-08-02 2011-02-22 Ophir Corporation Optical air data systems and methods
US8068216B2 (en) 2002-08-02 2011-11-29 Ophir Corporation Optical air data systems and methods
US7864311B2 (en) 2005-06-14 2011-01-04 Forschungsverbund Berlin E.V. Method and device for producing and detecting a Raman spectrum
WO2006134103A1 (fr) * 2005-06-14 2006-12-21 Forschungsverbund Berlin E.V. Procede et dispositif pour generer et detecter un spectre de raman
DE102009042404B3 (de) * 2009-09-16 2011-04-14 Technische Universität Dresden Verfahren zur Bestimmung der Geschwindigkeit eines bewegten Fluids unter Einsatz einer Eigenkalibrierung eines Doppler-Global-Velozimeters mit Laserfrequenzmodulation
GB2532585A (en) * 2013-06-30 2016-05-25 Wind Farm Analytics Ltd Turbine fluid velocity field measurement
GB2532585B (en) * 2013-06-30 2018-04-25 Wind Farm Analytics Ltd Turbine fluid velocity field measurement
US9334807B2 (en) 2014-05-13 2016-05-10 The Boeing Company Methods and apparatus to determine airflow conditions at an inlet of an engine
US10161773B2 (en) 2014-05-13 2018-12-25 The Boeing Company Methods and apparatus to determine airflow conditions at an inlet of an engine
EP3443360B1 (fr) * 2016-04-15 2024-04-10 Deutsches Zentrum für Luft- und Raumfahrt e.V. Aéronef et procédé de détermination de grandeurs caractéristiques importantes pour le vol
CN113176582A (zh) * 2021-04-27 2021-07-27 厦门大学 一种基于双频泵浦的流速探测激光雷达和流速探测方法

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