Showing papers on "Laser Doppler vibrometer published in 1976"

Principles and practice of laser-Doppler anemometry

Abstract: Keywords: anemometrie a : laser ; anemometrie ; optique ; laser : Doppler Reference Record created on 2005-11-18, modified on 2016-08-08

Length controlled stabilized mode-lock Nd:YAG laser

Abstract: A method and apparatus for stabilizing the amplitude and repetition rate of mode-locked Nd:YAG laser pulses by controlling the laser length through a feedback loop is described. The end mirror of the laser is mounted on a piezoelectric crystal which is dithered at a low frequency. A portion of fundamental 1.06 micrometer laser radiation is converted into its second harmonic frequency and the average power of the second harmonic frequency is detected by an integrating detector. The amount of the power of the second harmonic frequency depends on the match between the optical length of the laser cavity and the mode-lock frequency. The length is controlled by a feedback loop which phase compares the output of the second harmonic detector to the piezoelectric crystal dither signal.

Remote measurement of wind speed using a dual beam backscatter laser Doppler velocimeter.

Abstract: Measurements of atmospheric wind speeds at distances up to 60 m using a dual beam laser Doppler velocimeter with digital autocorrelation analysis, an unfocused argon ion laser (0.10–0.35 W) at 5145 A, and a 20-cm collecting telescope are reported. Criteria affecting the field performance of this system and an expression giving the power and range dependence of the data accumulation rate are presented. Major improvement of the system’s performance can be made by focusing the laser beams and properly restricting the field of the collecting telescope. The potential of this technique for remote wind measurement at ranges of several hundred meters is discussed.

Tunable frequency laser

Abstract: A method and apparatus for stabilizing the frequency of a laser beam is provided wherein the laser beam from a laser cavity is sampled and dispersed. A pair of detectors detect any energy sharing of the laser beam at the desired output frequency with laser beams at a frequency above and below the desired frequency to produce an error signal. The error signal is fed back to the laser cavity to correct the distance between opposite ends of the cavity to adjust the wave length of the laser beam to emit amplified light at the desired frequency.

Visual modeling of laser Doppler anemometer signals by moiré fringes

Abstract: This report describes the employment of moire patterns to model visually interference phenomena in general and laser Doppler anemometer signals in particular. The modeling includes signals created in dual beam and reference beam anemometers by both single particles and particle pairs. The considerations are extended to visual modeling of multiparticle signals and the decay of signal quality in the presence of many particles. The fringe model of the laser Doppler anemometer is also considered, and moire patterns are employed to demonstrate the interference fringes in the crossover region of two intersecting laser beams. Gaussian beam properties are taken into account to allow the effects of improperly designed optical systems to be studied. Instructions for using computer generated transparencies to produce the different moire patterns are provided to allow the reader to study in detail the various interference phenomena described.

Laser control circuit

Abstract: The operating current level of an injection laser is stabilized at a point of maximum linearity. A test signal modulates the laser current, and the operating current level of the laser is automatically adjusted until the harmonic distortion component in the output light at twice the test signal frequency is minimized.

Method and apparatus for controlling the intensity of a laser output beam

Abstract: The output beam of a laser tube is modulated between a maximum intensity (lase) and minimum (no-lase) intensity levels by varying the laser tube current between corresponding values. The device for controlling the output beam comprises a single transistor provided in the current path of the laser tube, the current therethrough being controlled, or modulated, to provide the modulated laser beam. The device is capable of operating in a plurlity of modes. In the preferred mode, an analog signal is coupled to the transistor control circuitry whereby the laser beam intensity is linearly modulated between the maximum and minimum levels to provide a laser beam having grey scale capabilities. In a second operating mode, a digital signal is coupled to the transistor control circuitry whereby the laser beam intensity is modulated to a constant value between minimum and maximum levels.

The Doppler effect in a reflecting system

Abstract: The Doppler shift is derived for light incident at any angle upon a perfectly reflecting mirror moving in any arbitrary direction. The resulting equations confirm that there should be no Doppler shift for light reflected from a transversely moving mirror. This result disproves several previous theoretical predictions.

Laser beam transmitter system for laser beam rider guidance systems

Abstract: A laser beam rider guidance system having an improved launcher based laser transmitter subsystem for illuminating a laser beam receiver subsystem on board a moving carrier is disclosed. The improved launcher based laser transmitter subsystem includes a boresight compensation mechanism having x and y scan lasers, a synchronization laser, a scanning means for the x and y scan lasers, a first beamsplitter for directing the x and y scan laser beams through a zoom lens for focusing approximately at the missile's range, a second beamsplitter for focusing a portion of the x and y scan laser beams to a third beamsplitter for reflecting the portion of the x and y scan laser beams through a telescope sight to the reticle of the sight, reflecting means on the center of the sight reticle to reflect the portion of the x and y scan laser beams to a fourth beamsplitter, a light detector responsive to the reflected light for producing an electrical signal, when the x and y scan laser beams cross the line of sight to target for a signal processor, to trigger the synchronization laser. The signal processor can compute a delay in activating the synchronization beam to compensate for gravity, target movement, and wander of the zoom lens optical axis due to the motion of the zoom lens components to change the focal length.

Scanner system for laser beam rider guidance systems

Abstract: A laser beamrider guidance system having an improved launcher based laser transmitter subsystem for illuminating a laser beam receiver subsystem on-board a moving carrier is disclosed. The improved launcher based laser transmitter subsystem includes a boresight compensation mechanism having L-shaped x and y scan diode lasers and a synchronization laser, a rotating scanning means for producing two rotations of the laser beam images for each rotation of the scanning means, a zoom lens in the optical path of the x and y scan rotating lasers for focusing the laser beams within the maximum target range, a first beamsplitter for reflecting a portion of the x and y scan laser beams to a telescopic sight mechanism adapted to determine when the x and y scan laser beams cross the line of sight to target and activate a signal for activating the synchronization laser beam.

Pseudo-backscatter laser doppler velocimeter employing antiparallel-reflector in the forward direction

Abstract: A laser Doppler velocimeter for measuring the velocity of a flowing fluid comprising laser means for providing first and second beams of collimated coherent monochromatic electromagnetic radiation which are focused to intersect at a predetermined location in a flowing fluid, the movement of the particles of the fluid serving to scatter radiation in substantially all directions, the scattered radiation being shifted in frequency from that of the first beam by an amount corresponding to the velocity of the fluid at the predetermined location, a corner cube disposed generally forward of the predetermined location, the corner cube responding to a portion of the radiation scattered in the forward direction and being operative to reflect the portion in a direction substantially antiparallel to the forward direction, and a collector responsive to the frequency of the first beam and to the frequency of the reflected portion of radiation and operative to provide a frequency modulated signal representative of the velocity of the fluid at the predetermined location. In alternative embodiments, a cat's eye lens and a bank of corner cubes replace the corner cube.

A simple two-component laser Doppler anemometer using a rotating radial diffraction grating

Abstract: A direction-sensitive two-component laser Doppler anemometer is described. The basic element in this system is a rotating radial diffraction grating functioning both as a beam splitter and as a frequency shifter. Two different arrangements are discussed. One arrangement gives two non-orthogonal velocity components from which the on-axis velocity component could be derived. The second arrangement directly gives two orthogonal velocity components perpendicular to the optical axis. Experimental results are reported from measurements of the orbital velocities below artificially generated surface waves in a water flume.

Remote intensity fluctuation measurements with a laser Doppler radar

Abstract: A coaxial focused CW scanning laser Doppler velocimeter (SLDV) radar equipment applying heterodyne detection at 10.6 microns can measure intensity fluctuations under field conditions. The set includes a 20 W CO2 laser, a coaxial Cassegrainian telescope, standard heterodyne equipment, and a SAW spectrum analyzer with 100 kHz signal resolution. Operation of the equipment and techniques for taking remote measurements are described briefly. Applications to remote measurements of transverse component of wind speed, as a complement to the traditional Doppler method of determining axial velocity, are under study. SLDV equipment has been used in detection, tracking, and measurements of atmospheric turbulence associated with aircraft wing-tip vortices or with dust devils, and in measurement of general atmospheric wind profiles.

A laser Doppler velocimeter which measures the three components of velocity

Abstract: A device for simultaneously measuring the three orthogonal components of a velocity vector is described. The system uses a laser beam passing twice through the region of flow under investigation. This allows one to determine the third component using only forward scattering. Vector diagrams which explain the mode of operation and experimental signals are given.

Investigation of the spectral content of a cw CO laser output using a high-resolution scanning Fabry-Perot interferometer

Abstract: The use of a scanning Fabry-Perot interferometer of variable length (variable resolution) for investigating the output of a cw CO laser is reported. Under experimental conditions required to achieve maximum power it was found that the laser output consisted of two or more simultaneously occuring transitions due to the overlapping nature of the vibration- rotation bands. The frequency differences between these transitions were measured to an accuracy of ≲ 0.01 cm -1 . Operation of the CO laser on a single vibration-rotation transition was obtained by controlling the laser gain with the experimental variables of discharge pressure, gas temperature, and optical alignment. An alternative approach of obtaining monochromatic radiation, by using a Fabry-Perot interferometer as a frequency selective filter external to the CO laser, is reported.

Laser engraving cylinders for photogravure - using picture scanner and logic circuits for automatic modulation of the laser beam

Abstract: Process for controlling the engraving of a surface for photogravure, or other engraved surface, using a laser beam modulated by a control signal. The novelty is that a laser energy sensor is employed to pick up the laser energy reflected from a point on the engraved surface, and the sensor part of a control circuit which responds when the sensor signal alters from below the threshold value of laser beam intensity at which engraving occurs to a level above this value; the signal at the threshold value is compared with a reference signal and any deviation is compensated so the laser beam directed onto the surface has the desired power. The laser used is pref. a CO2 laser or a neodymium-YAG laser, and the pref.appts. is included in the claims. Esp. for engraving surfaces possessing cells filled with plastic which is easily engraved by the laser beam.

Higher order harmonics of CO 2 and N 2 O laser difference frequencies

Abstract: The generation of the fourth and fifth harmonics of the difference frequency between two 28-THz laser lines, by a tungsten-nickel diode, was observed.

Results from a two-dimensional laser Doppler anemometer

Abstract: Results obtained using a two-dimensional laser Doppler system are compared with those from the conventional one-dimensional system. The results show good agreement, but attempts to measure simultaneously from two channels with the three-beam system were abandoned due to the small, but significant variations in the two tracker systems. An alternative superposition fringe mode procedure is discussed.

Frequency Error of Doppler Beat Signals due to Extended Scattering Particles

Abstract: The dependence of the frequency error of Doppler beat signals on the size of scattering particles is theoretically investigated by assuming the probing area to be finite, i.e. by assuming beat signals to have a burst form. The effect of the particle size on output beat signals can not be ignored when the laser Doppler flowmeter is applied to measure the flow velocity in a microscopic area of the order of several microns in one dimension. The present analysis reveals that the frequency error occurs in some cases as the frequency deviation of more than 6% from the ideal Doppler beat frequency due to point-like scattering particles.

Polarization Properties Of Internal Mirror He-Ne Lasers

Abstract: The states of polarization of the output beam (632.8 nm) of internal mirror He-Ne lasers have been investigated using a scanning Fabry-Perot Interferometer and linear polarizers. It was found that during the initial warmup period of the laser, the output power fluctuates periodically and the axial modes sweep rapidly across the Doppler gain profile. A thermal analysis shows that the sweeping rate is in good agreement with the calculated rate of expansion of the laser cavity. During certain periods of time which recur with a frequency that is related to the mode motion the output beam is linearly polarized in two preferred orthogonal directions. The states of polarization of the output beam are due to the tendency of axial modes to be linearly polarized, with an orthogonal relationship existing at times between adjacent modes. However, the polarization states of the total beam and of the individual modes also vary periodically and in a complicated manner as the laser warms up. It was found that the simple description of orthogonally polarized alternate modes is inadequate to characterize the beam during a large fraction of the time corresponding to a fluctuation cycle. The implications of these polarization properties on laser scanning applications are discussed.

Laser modulator for producing dark-pulses

Abstract: A dark pulse is manifested when the intensity of a laser beam, for example, a continuous wave laser beam drops to zero and returns to its original intensity in a very short time period. A modulator for producing dark pulse outputs is disclosed which includes a two-photon absorption cell provided with two input laser beams: a continuous wave beam having a frequency ω 1 and a mode-locked laser beam having a frequency ω 2 selected such that ω 1 + ω 2 = Ω 12 where Ω 12 is also the two-photon absorption frequency of the vapor contained in the cell. The mode locked beam modulates the continuous wave beam to produce the desired dark pulses.

Directional acoustic measurements by laser Doppler velocimeters

Abstract: Laser Doppler velocimeters (LDVs) were used as velocity microphones to measure sound pressure level in the range of 90-130 db, spectral components, and two-point cross correlation functions for acoustic noise source identification. Close agreement between LDV and microphone data is observed. It was concluded that directional sensitivity and the ability to measure remotely make LDVs useful tools for acoustic measurement where placement of any physical probe is difficult or undesirable, as in the diagnosis of jet aircraft noise.

Measurement of Earth Strains with a Laser Interferometer

Abstract: A newly designed laser earth strainmeter has been installed in the observation vault of the Matsushiro Seismological Observatory in Nagano, and earth strains have been measured over several days. The record of the laser strainmeter was compared with the record of a quartz strainmeter of the Observatory. The laser strainmeter is a modified Michelson type corner-cube interferometer having a 25m long airtight pipe and a high resolution analogue system for recording displacement of the interference fringe. The errors caused by a wavelength variation, by an analogue recording of the fringe dis-

Remote measurement utilizing NASA's scanning laser Doppler systems. Volume 1. Laser Doppler wake vortex tracking at Kennedy Airport

Abstract: Test operations of the Scanning Laser Doppler System (SLDS) at Kennedy International Airport (KIA) during August 1974 through June 1975 are reported. A total of 1,619 data runs was recorded with a totally operational system during normal landing operations at KIA. In addition, 53 data runs were made during cooperative flybys with the C880 for a grand total of 1672 recorded vortex tracks. Test crews were in attendance at KIA for 31 weeks, of which 25 weeks were considered operational and the other six were packing, unpacking, setup and check out. Although average activity equates to 67 recorded landing operations per week, two periods of complete runway inactivity spanned 20 days and 13 days, respectively. The operation frequency therefore averaged about 88 operations per week.

Multipurpose vehicle vibrometer

Abstract: PURPOSE:To record vibration of a vehicle and stresses of each part of a vehicle easily and automatically with travelling conditions simultaneously.

On the Feasibility of Detecting Low Level Acoustic Signals in the Ocean by Use of a Laser Heterodyne Detector

Abstract: : This report provides a basis for assessing the feasibility of using a laser doppler velocimeter for the measurement of low amplitude acoustic particle velocity in the sea at very low frequencies.

The delta Doppler technique for LDV measurements at long distances

Abstract: A technique for measuring velocity, referred to as a Delta Doppler technique, was presented. This technique determines scattering source velocities by measuring the difference in Doppler shifts of two different frequencies. By transmitting the two frequencies along the same path, a moving fringe pattern is established such that a nonmoving scatterer at the sensing volume would see an intensity variation exactly equal to the difference in the transmitted frequencies. If the particle has a velocity component along an axis which bisects the angle formed by the transmitter and receiver axes, a Doppler shift in the difference frequency can be measured and the velocity component computed. The frequency measured would correspond to the difference in Doppler frequencies that two laser Doppler velocimeters using separate frequencies (the same frequencies as used previously) would have measured, thus the term Delta Doppler.