Showing papers on "Laser Doppler vibrometer published in 1972"

Laser‐Induced Fluorescence: A Method to Measure the Internal State Distribution of Reaction Products

Abstract: passes through the reaction zone. As the wave­length of the dye laser is scanned, the BaO molecules are excited to the (v', 1') level whenever the laser wavelength coincides with an allowed v"i"-+v'i' transition in the A-X band system. The resulting fluorescence emitted at right angles to the laser beam is detected by a fast-response photomultiplier. The laser pulse triggers a boxcar integrator (Princeton Applied Research or Keithley Instruments) whose electronic gate (window) is maintained open from the time of the laser pulse to 2-5 times the BaO A state lifetime. This analog device permits us to average the signals resulting from many laser pulses. The scattered light from the laser beam is rejected either with a sharp cutoff Corning filter or by delaying by typically 20 nsec the opening of the electronic gate. The BaO fluorescence rate versus laser wavelength is shown in Fig.

A Laser Velocimeter for Remote Wind Sensing

Abstract: A CW carbon dioxide laser Doppler radar has been developed and applied to remote measurement of atmospheric wind velocity and turbulence. The carbon dioxide laser illuminates residual particulate matter in the atmosphere. Radiation scattered by these particles is homodyned with a local oscillator to provide the Doppler signal. The performance of the instrument is verified by comparison of wind velocity data recorded simultaneously by the laser Doppler system and a cup-anemometer wind-vane system. All data comparisons indicate very close agreement of the two systems. Data inconsistencies are within the accuracy limitations of the conventional anemometer system. The range of the laser Doppler system during these tests was confined to approximately 30 m. Laser Doppler wind velocity data were observed at ranges exceeding 300 m; however, no conventional anemometer was set up at these ranges for data comparisons.

The application of photon correlation spectroscopy to laser Doppler measurements

Abstract: A photon correlator instrument has been applied to the measurement of laser Doppler signals. In 1 s a good signal can be obtained at 1 m in backscatter from unseeded air with 10 mW of laser power.

Continuous wave radar for range and velocity measurement

Abstract: A frequency modulated, continuous wave radar range measurement system using a laser oscillator is disclosed. A ring laser, carbon dioxide (CO2), laser oscillator allows target range and velocity measurement as either an active or a passive system. Nonlinear variation of laser output power is obtained by mixing the reflected signal with the energy within the laser cavity. In an active system the resulting beat frequency is sensed by a local receiver-detector and reduced to obtain the range and velocity data at the transmission site. The beat frequency is also superimposed on the transmitted laser beam and is therefore available at the target causing the beat frequency and is available to other detectors which may be positioned to receive a portion of the reflected energy, affording a passive system.

The frequency shifting of laser light by electro-optic techniques

Abstract: In velocity measurements by the laser Doppler method, a requirement exists for changing the frequency of one light beam with respect to another derived from the same source. This is particularly useful for the determination of the sign of a velocity. The required frequency shifting or single side-band generation can be accomplished by the use of electro-optical effects and a number of possible arrangements are discussed. Trials were made using Kerr cells and electro-optic crystals and single side-band generation demonstrated by a simple Doppler beating experiment. If a low efficiency can be tolerated as in Doppler beating with a reference beam, the production of a pure frequency shifted beam is not difficult up to frequencies of a few megahertz. The differential Doppler or fringe system however requires shifted and non-shifted beams of equal intensity and hence high efficiency conversion is imperative. Radio-frequency driving power requirements may also be a limitation. The theoretical efficiencies of various multi-cell electro-optic arrangements have been calculated as a function of R.F. excitation. The possible efficiency approaches unity as the number of cells is increased.

Instantaneous Turbulence Velocity Measurement by Laser Doppler Velocimeter

Abstract: A frequency‐modulation (FM) demodulation scheme for the measurement of instantaneous velocity of turbulent flow by laser Doppler velocimeter is proposed. An analysis based on the theory of optical‐mixing spectroscopy is given.

Determination of a third orthogonal velocity component using two rotationally displaced laser Doppler velocimeter systems.

Abstract: Transformation equations are derived that may be used to determine a third orthogonal velocity component from measurements made at a common point by two rotationally displaced, two orthogonal component laser Doppler velocimeter (LDV) systems. These equations also may be used to relate velocity measurements made in a particular coordinate system to any other coordinate system. It is shown that the set of smallest angles that may be used to separate the two, two component, LDV systems is very sensitive to the relative magnitudes and directions of the individual velocity components. When the magnitudes of all velocity components are about the same, it is found that the minimum angle of separation is of the same order as the instrumental error of the LDV systems.

Method of and device for modulating directly a semiconductor laser

Abstract: A method of modulating directly a semiconductor laser such that a laser beam of one mode-order emitted from the semiconductor laser is converted into that of another mode-order by a pulse current superposed on a bias current flowing through the semiconductor laser and one of said two mode-orders is transmitted selectively through an optical guide, so that the amplitude of the laser beam can be modulated without stopping laser emission and the modulation at high repetition frequencies and with low modulating power can be obtained.

The Influence of the Atmosphere on the Wavelength of the He-Ne Laser and the Solution of Corrections of the Laser Interferometer

Abstract: A simple approximation of the wavelength of single frequency He-Ne laser as a function of temperature, pressure and relative humidity is presented. This approximation is suitable for the simple application in the portable laser interferometer. Comparison with the Owens and Edlen formulae is also presented and it is shown that the linear correction limits its accuracy.

Laser frequency fluctuations due to mechanical vibrations

Abstract: The frequency fluctuations of a single-frequency laser due to mechanical vibrations are dependent on the sound pressure and sound frequency. The two dependences have been studied with the observation of beat signals between two stable external-mirror He-Ne lasers in the free-running condition. The dependence on the sound pressure follows Hooke's law, so that the difference between the frequency fluctuations due to the acoustic noises and those due to the external vibrations was quantitatively distinguished. Some causes of mechanical instability of the laser were clarified from the dependence on the sound frequency as compared with the lowest natural frequencies of each mechanical element in the cavity. The translation of mirrors along the optical cavity axis and two rotations of mirrors about the axis perpendicular to the optical cavity axis, which give rise to the variation in the relative angle between the laser beam axis and the Brewster window plate, are the most dominant motions that cause frequency fluctuations of the laser.

Pumping of a semiconductor laser by an electron beam modulated at a microwave frequency

Abstract: Microwave modulation of the output of a GaAs laser was achieved by pumping this laser with an electron beam modulated by a microwave signal. The pulse-repetition frequency was a submultiple of the microwave-signal frequency.

A symmetrical laser Doppler velocity meter and its application to turbulence characterization

Abstract: A symmetrical method of optical heterodyning of the Doppler shifted scattered laser radiation developed for velocity measurements with a minimal instrumental spectral broadening and a high signal-to-noise ratio. The method employs two laser beams incident on the moving scatterer and does not use any reference beam for heterodyning. The Doppler signal frequency is independent of the scattering angle and the signal possesses no receiving aperture broadening. Optical alignment is simple. Typical values of the instrumental spectral broadening were approximately 0.8 percent of the center frequency of the Doppler signal, and the signal-to-noise ratio was approximately 25 dB, obtained from an air flow system using submicron dioctylphthalate scattering aerosol. Experimental and theoretical studies were made on the characteristics of the Doppler signal and the effect of system parameters in turbulent flow measurement. The optimization process involved in the beam optics and in the use of a spatial filter is described. For localized flow measurement in any direction of the three-dimensional orthogonal coordinates, the system, using uncorrected optical components, had a sensing volume which can be described by a sensitive length of 600 microns and a diameter of 100 microns.

A geometric analysis of the probe volume in a dual beam laser Doppler velocimeter

Abstract: The probe volume of the dual beam laser Doppler velocimeter is analyzed from a geometrical standpoint. The volume is graphically reconstructed and the existence of the characteristic dual beam laser Doppler velocimeter fringes is demonstrated. The fringes are analyzed and are shown to be composed of moving tubes or pockets of coherent radiation. The composition of the light is analyzed and the extent of the coherence of the laser radiation within the light pockets is considered. The scattered light from the tubes is analyzed and the coherence enhancement of the fringe structure is shown. This information is used in an attempt to resolve the controversy as to whether the dual beam laser Doppler velocimeter is governed by a fringe counting phenomena or by a beating phenomena of light from two sources of coherent radiation of different frequencies.

Doppler Q switching in a single‐mode CO2 laser by a rotating mirror

Abstract: The theory of Doppler‐effect mechanism for lasers Q switched by a rotating mirror was extended to include the case of multiple passes of the radiation in the cavity. The theoretical dependence of the pulse duration on the rotating‐mirror frequency was calculated. The experimental results obtained, using a single‐mode CO2 laser, were found to be in good agreement with the calculated values.