Showing papers on "Laser Doppler vibrometer published in 1971"

Self-induced thermal distortion in the near field for a laser beam in a moving medium

Abstract: A theoretical model is developed that predicts the steady-state self-induced thermal distortion of a laser beam in an absorbing medium flowing transverse to the direction of propagation A general perturbation solution is obtained that is appropriate when the thermal distortion effects are small A computer iteration of this solution is performed to obtain results that are applicable over an increased range of distortion conditions The effects of diffraction are neglected, and therefore the solution is limited to the near-field range of the laser beam or to a range where a caustic or infinite intensity is predicted as a result of the self-focusing associated with the thermal distortion effects of the flow The thermal distortion of an initially Gaussian collimated laser beam causes a shift of the beam into the direction of the flow and, at the outset of the thermal effects, a decrease in the peak intensity As the thermal distortion becomes more severe, the self-induced effects cause an increase in the peak intensity, ie, a focusing to occur Experimental results are presented for the thermal distortion of a CW CO 2 laser beam propagating in CS 2 that moves transverse to the propagation direction The data verify the predicted beam deflection and the defocusing as well as focusing effects of the self-induced thermal distortion

Differential Doppler Heterodyning Technique

Abstract: Measuring velocity without disturbing the moving object is possible by use of the laser doppler heterodyning technique. Theoretical considerations on the doppler shift show that the antenna property of the photodetector can solve an apparent conflict between two different ways of calculating the detected doppler frequency. We found that the doppler frequency for this particular setup is independent of the direction of detection. Investigations of the signal-to-noise ratio (SNR) show that the maximum SNR—considering the optical setup—is obtained by measuring the frequency difference between two doppler-shifted beams rather than by measuring the shift of a single beam by comparing it with a reference beam. Measurements seem to be in agreement with the theoretical consideration.

Analysis of Atmospheric Laser Doppler Velocimeters

Abstract: Fundamental relationships between backscattered power, range, wavelength, and number of scatter centers in the probe volume for the self-aligning, dual-scatter, laser doppler velocimeter are developed. It is shown that not all power scattered from the velocimeter probe volume contributes to a doppler signal. This fact leads to significant deviations in calculations involving signal-to-noise power ratios as compared to the case when only gross backscattered power is considered.

Method of stabilizing a gas laser

Abstract: The output radiation of a gas laser is directed into a cell containing a gas with a resonant absorption line. The gas is subjected to a standing wave field of the laser radiation. A saturation resonance is detected in the emitted spontaneous radiation from an energy state collisionally coupled to an interacting energy level of the absorption transition when the laser is tuned near the center frequency of its Doppler broadened line profile. This resonance is utilized as a reference in a feedback system to frequency lock the laser to a heretofore unknown degree of stability.

Directional Laser Doppler Velocimeter

Abstract: A directional laser doppler velocimeter for localized directional measurements of fluid flow is described Doppler-shifted radiation is scattered and received from small particles in the fluid. The frequency shift is proportional to the velocity of the scatterer and can be detected by heterodyning the scattered signal with an appropriate reference. Directional sensitivity is achieved by shifting the frequency of the reference laser beam, using an acoustical diffraction cell. Results of steady and time-varying flow velocity measurements in a perturbed circular flow section are presented.

Laser doppler velocimeter detector

Abstract: A laser doppler velocimeter detector for determining velocity information in flow fields in which a laser beam utilized in the flow field is split into a pair of beams, one of which has its plane of polarization rotated 90 DEG . A portion from each of the oppositely polarized beams is mixed to produce the desired doppler information. Polarization beam splitting is provided producing two beams, each containing doppler shifted information. Each of the beams is detected to provide a pair of representative electrical signals, one of which is subsequently inverted and added to the uninverted to result in a signal having only the frequency content of the original doppler information.

Multipath laser moving target indicator

Abstract: A moving target velocity indicator including a multipath laser which operates simultaneously as an oscillator for producing a laser beam which is directed onto both a moving target and a detector and as an amplifier of the Doppler-shifted laser beam reflected from the target. The optical return signal from the target is amplified while passing through the multipath laser system and is directed, after the final pass through the laser medium, onto the detector, This amplified signal can be combined at the detector with the transmitted signal to provide a beat signal at the frequency of the Doppler shift which can be applied to an oscilloscope display calibrated in frequency. Knowing the frequency of operation of the laser, the velocity of the target can then be calculated.

Doppler jitter stabilization of a CO2 laser

Abstract: A CO2 laser oscillating on the P(18) transition of the 10. 6‐μ band was electronically stabilized without internal modulation to an inverted Lamb dip generated in an extra‐cavity SF6 absorption cell. Laser modulation was avoided by employing an external moving mirror to Doppler jitter the light entering the absorption cell. The resultant stabilization point was shown to be independent of laser characteristics. The frequency stability so achieved was 1 part in 109 over a 15‐min period.

Passive q-switch and modulator for co2 lasers

Abstract: A cell containing a gaseous bleachable dye is positioned in the optical feedback cavity of a carbon dioxide laser to Q-switch the laser. An acoustic wave generated through the gas will cause the output of the laser to be modulated at the acoustic frequency.

Laser Frequency Stabilization Techniques and Applications

Abstract: A review of progress in laser stabilization techniques and laser frequency measurement is given. Methods for relating laser frequencies to the time standard and methods for absolute laser frequency stabilization are described. Experimental information on reproducibility and noise characteristics is reported. Application to frequency and wavelength standards is discussed.

Beam scanner for high power laser

Abstract: Beam scanning of high power laser beam is accomplished by sequentially deflecting successive thin layers of the beam in such a way that each thin strip of deflected laser light passes across a narrow aperture mask. The laser light passing through the aperture is effectively a scan of that layer of the laser beam. It is subsequently focused onto an appropriate laser beam detector. A spiral arrangement of radial reflector rods mounted on a rotatable drum support structure is used to achieve sequential deflection of successive layers of the laser beam.

Energy converter, surface laser and methods for their operation

Abstract: Laser action is developed in a minute crystal volume by direct excitation of an absorption line with a beam of a tuneable wavelength laser. Raising the energy intensity of the exciting beam causes an initial surface laser mode to shift into a penetration mode having an output aligned with the exciting beam. Energy converters are disclosed based upon the above.

Frequency modulation of a self-mode-locked dual-polarization CO 2 laser

Abstract: Simultaneous active stabilization and frequency modulation of a mode-locked dual-polarization CO 2 laser is reported. Mode locking was maintained during frequency modulation through use of a self-regenerative feedback technique.

Frequency Broadening in Reference Beam Laser Doppler Velocimeter Data

Abstract: : The laser Doppler velocimeter (LDV) provides velocity data in the form of a frequency spectrum. Because of the inherent optical geometry of the laser velocimeter and the effects of light scattering particles passing through the probe volume, a frequency distribution density function representing these combined effects is found in the readout device. Hence, interpretation of data must be effected for frequency-to-velocity conversion. This report deals with this frequency broadening effect associated with the LDV reference beam type. Six frequency broadening effects are discussed in terms of their magnitude and relative importance.

Signal broadening in the laser Doppler velocimeter.

Abstract: Critical review of a recent paper in which Denison, Stevenson, and Fox (1971) discussed the sources of spectral broadening in the laser Doppler velocimeter. It is pointed out that, in their discussion, the above-mentioned authors indicated that the spread in wave vectors of the incident and detected fields and the finite length of time a scattering center stayed in the sample volume each contributed separately and independently to the observed spectral width of the scattered radiation. This statement is termed incorrect, and it is shown that the two effects are one and the same.

On the mechanical impedance of human body surface

Abstract: Several methods for measuring the mechanical impedance of human body surface are discussed and their results are also presented. The purposes of these measurements are mainly to obtain the fundamental data for designing a phonocardiographic transducer (microphone). On the other hand, several measurements were performed for the purpose of investigating the mechanical behavior of human body to vibratory energy of jet and rocket engine.Generally, three types of measuring methods are available; the precise measurement by comparing output voltages and their phase angles of a vibrometer at each frequency, the automatic measurement by sweeping an oscillator at required frequency range and the simpler method for measuring the stiffness and mass of human body surface at a few frequencies. The merits and the demerits of each method are also discussed.Finally, some theoretical considerations concerning to human body impedance are described.

Nomograms for the Doppler Shift Velocimeter

Abstract: : The preliminary design of a laser Doppler velocimeter (LDV) for a particular application requires the determination of the frequency-to-velocity conversion constant and the spacial resolution of the focal volume. These parameters give the frequency range required of the signal processing electronics and the size of the region in which velocity measurements are being made. These values are determined from the laser wavelength, the focal length of the focusing lens, the angle of the LDV beam pair intersection, and the diameter of the focused beams at the intersection. A set of nomographic scales has been developed to allow rapid and accurate determination of these parameters. They are also particularly useful in evaluating the effects of parameter variation and optimization of the LDV.

Optical Readout of Laser Velocimeter Signals

Abstract: An optical readout system for the analysis of laser Doppler velocimeter (LDV) signals is described and test results are given. The signal from the LDV is displayed on a TV screen and recorded photographically. The photographic negative is placed in a Fourier transform optical system which displays the frequency spectrum of the negative on a screen or photographic material. Flow velocities are then calculated from the frequency values displayed by the system and these velocities are shown to agree very closely to velocity measurements made simultaneously using a Pitot tube.