Showing papers on "Laser Doppler vibrometer published in 1977"

Optically pumped FIR lasers: Frequency and power measurements and laser magnetic resonance spectroscopy

Abstract: Optically pumped FIR lasers are currently in use in both frequency metrology and laser magnetic resonance spectroscopy programs in the NBS Boulder labs. The laser for use in frequency metrology is a CW 71 μm methyl alcohol waveguide laser with over 100 mW output for frequency synthesis. Another laser with an intracavity absorption cell for laser spectroscopy has been constructed and is nearly transversely pumped. The metrology technique used to measure the frequency of these lasers is briefly reviewed and a unique power meter is described.

Laser tuning with an acousto-optic lens

Abstract: A pulsed laser having a laser gain medium of broadband output is tuned to essentially monochromatic output. An acousto-optic cell is driven with an acoustic wave which varies linearly in frequency, causing the laser beam to be resolved into a number of essentially monochromatic focal points. The focused beam of the desired wavelength is reflected to the output through one or more apertures which block out the unwanted wavelengths.

Method and apparatus for sensing the position of a writing laser beam

Abstract: A system for sensing the position of a laser writing beam is provided wherein at least two laser beams of unlike frequency are superimposed on the laser writing beam. The combined laser beams are simultaneously deflected in a common optical deflection system. Once deflected, the unlike frequency laser beams are separated from the writing laser beam and focused onto a diffraction grating. The position of the focused beams of unlike frequency on the diffraction grating corresponds to the position of the writing laser beam on the writing media. The focused laser beams of unlike frequency produce a traveling interference pattern on the grating and are diffracted by the grating into a plurality of beams. By sensing the frequency of at least one of the beams diffracted by the grating, the position and movement of the writing laser beam can be determined.

Laser altimeter/profilometer apparatus

Abstract: Laser altimeter/profilometer apparatus utilizing a CW CO 2 laser transmitter mounted on an aircraft so as to transmit a narrow laser beam towards the terrain below the aircraft. The scattered laser signals are received in a receiver mounted on the aircraft, some distance along the fuselage from the transmitter, and the doppler shift in frequency of the received signal is determined. That doppler shifted frequency is utilized to calculate the altitude of the aircraft above the ground over which it is flying.

Interferential laser Doppler velocimetry in a high frequency plasma jet

Abstract: A high-frequency argon-helium plasma jet is studied by means of an interferential high-power laser Doppler velocimeter. The signal is processed by period-sampling. Mean velocities and 150 Hz, period fluctuations are measured. Coupled with dynamic pressure measurements former results allow the authors to obtain atomic and ionic temperatures.

Development of rotating diffraction gratings and their use in laser anemometry

Abstract: All-glass rotating diffraction gratings developed for use as beam splitting frequency shifters in laser Doppler anemometers are described. The gratings are produced using etching techniques and can be exposed to high power laser beams. Advantages of using these gratings in optical arrangements are that they are simple to set-up and easy to align.

Photon counting processor for laser velocimetry

Abstract: In some of the more difficult applications of laser Doppler velocimetry such as in combustion research, transonic flow, transverse wind, and others, the optical signals are too small for classical detection methods. This paper summarizes the theory, experimental prototype hardware, and test results for a new type of high-speed photon-correlation signal processor which is being developed for weak laser velocimeter signals and other photon-resolved signals. The new system may also be considered to be a statistical digital frequency discriminator. It uses full multiplication with 10-nsec delay time and has the potential for extension to the measurement of the velocity correlations and spectra.

Optical apparatus for ballistic measurements

Abstract: An optical apparatus, particularly suited for use in a ballistic measurement device, including a laser generating a laser beam, an optical means for expanding the laser beam in a plane, and an optical element, positioned between the laser and the optical means, for dividing the laser beam into a plurality of beams of substantially identical intensity so that a substantially uniform brightness results over the entire angle of divergence of the laser beam. According to another embodiment, the optical apparatus is used in a photoelectric apparatus for ballistic measurement, such as velocity of moving projectiles, further including means for deflecting the expanded laser beam to produce a second laser beam parallel to and spaced from the expanded laser beam by a predetermined distance, a photoelectric converter positioned so that the second laser beam impinges upon the converter, and an evaluation circuit responsive to an output of the converter to measure the time delay between interruptions of the first and second laser beams by a moving projectile.

Frequency modulated electrical discharge laser

Abstract: An improvement in an electrical discharge laser. The improvement consists of modulating the input power to the laser; specifically, in the case of a gas laser for instance, either (1) the power to the electron gun which produces an electron beam used for preionization of the gas, or (2) the power to the main discharge electrodes of the laser. The parameters of the modulation, i.e. its frequency and depth, are matched to the vibration energy transfer characteristics of the molecules of the lasing medium, which in the preferred embodiment is a CO mixture.

Laser Doppler, velocimeter system for turbine stator cascade studies and analysis of statistical biasing errors

Abstract: A laser Doppler velocimeter (LDV) built for use in the Lewis Research Center's turbine stator cascade facilities is described. The signal processing and self contained data processing are based on a computing counter. A procedure is given for mode matching the laser to the probe volume. An analysis is presented of biasing errors that were observed in turbulent flow when the mean flow was not normal to the fringes.

Interaction of a CO2 laser beam with a shock‐tube plasma

Abstract: The results of experimental investigations of the interaction of a CO2 laser beam with plasma produced in an electromagnetic shock tube are presented. The interaction was investigated in two different configurations: with the laser beam perpendicular to the direction of propagation of the shock wave and with the laser beam parallel to the direction of the shock wave. The laser energy was 0.3 J in a 180‐nsec pulse. The plasma density was in the range 1017–1018 cm−3 and temperature was around 2 eV. Spectroscopic methods were used in the measurement of density and temperature. Direct observation of the path of the laser beam through the plasma was made by an image‐convertor camera in conjunction with a narrow‐band interference filter. The propagation of the laser through the plasma and energy absorption are discussed. The observed maximum increase in electron temperature due to the laser in the first configuration was 0.4 eV and the estimated temperature increase in the second configuration was about 2 eV.

Laser monitor and control system

Abstract: A detector array generates a profile of the intensity of a laser beam, across the beam and in the direction of fluid flow in order to detect the spatial distribution of the laser gain. Control actions such as modifying the flow of diluent into the laser cavity is instituted to shift the beam zero-gain point out of the laser beam.

Wideband analog photorecording apparatus and method

Abstract: A wideband analog photorecorder comprises a transparent Bragg cell as an acousto-optic transducer and a pulsed laser positioned to sequentially illuminate a moving recording medium (photographic film) through the cell The acousto-optic cell is energized by an input electrical signal and the resultant acoustical wave passing through the cell diffracts the "strobelike" output beam from the laser so as to expose the film one line at a time with the optical analog of the signal The pulse repetition frequency of the laser is synchronized with the velocity of acoustic wave propagation in the Bragg cell so as to expose a new signal segment in the cell with each laser pulse and thereby form one recorded line on the film The Bragg cell utilizes an optically transparent medium, such as lithium niobate, as an acoustic wave carrier to provide optical resolution through diffraction of the laser beam This invention may be practiced utilizing either anisotropic or normal Bragg diffraction, with or without acoustic beam steeringThe method of this invention comprises converting the amplitude modulated signal of interest to an acoustic wave, causing the acoustic wave to propagate through a Bragg cell, passing a pulsed laser output beam through an active region constituting a substantial part of the length of the Bragg cell so as to diffract the beam in accordance with the acoustic signal, and optically recording the diffracted laser beam on a moving film so that one line of data is recorded on the film each time the laser is turned on in the manner of a strobe light

Radioactivity measuring apparatus

Abstract: PURPOSE:To measure the radioactivity accurately without requiring any vibration control measures even under a situation where a radioactivity detector may be subjected to vibration by disposing a vibrometer in the vicinity of a semiconductor radioactivity detector and eliminating the effect of vibration based on the correlation between both output signals. CONSTITUTION:A semiconductor radiation detector 7 is disposed in a drive mechanism, i.e., a dust capturing section 3 for hermetically sealing/opening the suction channel of dust. Radioactivity of the captured dust is then measured on-line and a measurement signal is delivered to a measuring circuit 8. A vibrometer 11, e.g. an accelerometer, is disposed at the dust capturing section 3 housing the detector 7 or on the housing 10 of a dust monitor. Vibration of the capturing section 3 or the housing 10 is monitored independently from the measurement of radioactivity and an output signal is delivered to the measuring circuit 8. A measurement signal is delivered from the vibrometer 11 through an amplifier 24 and a detector 25 to a comparator 26 monitoring the vibration. A radiation measurement and an output signal representative of presence/absence of signal are fed to a logic circuit 27 and only a radiation measurement not effected by vibration is fed to an operation control board 9.

Digital frequency meter for burst-like laser Doppler signals

Abstract: A new frequency meter employing a period-measuring method has been developed for a burst-like laser Doppler beat signal. It does not process the Doppler beat signal as a continuous signal, but detects whether each beat signal cycle is a true or a false signal, and measures the signal frequency. It not only follows the frequency changing of each cycle of the Doppler beat signal but also calculates a mean signal frequency within a certain time. The parameter setting of this frequency meter is far easier than for a system employing a bandpass filter.

The application of an optical Doppler frequency discriminator in a laser velocimeter

Abstract: A laser velocimeter is described which uses an optical Doppler frequency discriminator in the form of a spherical mirror confocal Fabry-Perot interferometer (SFPI). The SFPI pass band is made to coincide with the laser frequency using a system of external control. Experimental results are given.

Future Developments in Lunar and Satellite Laser Ranging

Abstract: Most laser ranging systems operating today are first-generation systems using either cavity dumped or Q -switched ruby lasers and time-interval counters and are capable of an accuracy of 10-100 cm. Second-generation systems using mode-locked Nd: YAG lasers and epoch timers are being developed and compact, largely automated, fully mobile stations can now be built which are capable of ranging to both high satellites and the Moon with an accuracy of a few centimetres. Even higher accuracy is possible by using streak tubes as detectors and using measurements at two wavelengths to compensate for variation in atmospheric transmission. The use of CO 2 lasers and heterodyne detection, which has the advantage that both range and velocity can be measured, is also discussed.

Optical and acoustical measuring techniques

Abstract: In recent years considerable emphasis has been placed on developing techniques to remotely measure the velocity of confined and free fluid flows (such as atmospheric, oceanic, wind tunnel, blood, pipe, and channel flows, etc.). The laser Doppler and acoustic Doppler are two techniques that will be examined in this chapter. Both techniques are based on the Doppler effect. The Doppler effect is the fact that there is a change in frequency with which energy reaches a receiver when the receiver and the energy source are in motion relative to one another. In the cases of the laser and acoustic Doppler system, energy is transmitted to a moving scatterer (tracer), which then becomes a source, and the energy is transmitted to a receiver. The Doppler systems measure the velocity of these scattering sources and, therefore, the accuracy with which the Doppler systems measure fluid velocity is dependent upon the accuracy with which the imbedded scattering source follows the true velocity of the medium. Major efforts in the development of the laser and acoustic Doppler systems have been underway since the 1960’s.(1–3) It is the intent of this chapter to skip the developmental years of these systems and present the techniques that are presently employed and the results that they are obtaining.

Versatile LDV burst simulator

Abstract: A device for generating burst signals that can be used to determine whether or not a laser doppler velocimeter is operating properly A high frequency signal which corresponds to the information frequency of the laser doppler velocimeter is modulated by a low frequency signal to provide an envelope for the high frequency signal The high frequency signal is modulated by the low frequency signal by any modulator means such as, for example, an analog multiplier The low frequency signal is added to the modulated signal to provide pedestals for the resulting series of burst pulses Then means are provided for selecting different combinations of these burst signals Also means are provided for making the burst signals asymmetrical as desired In addition, means are provided for varying the frequencies and amplitudes of the information, envelope and pedestal frequency signals in the burst signals

Laser vibration probes

Abstract: mirror is held in synchronism with the turbine wheel by a servo which compares the phase of sync pulses generated by the scan wheel with those generated by the blades themselves. The proportions and positions of this group of elements is critical in providing simultaneous synchronism in the several degrees of freedom. However, a number of basic optical formats to accomplish this task are possible. The Doppler shifted back-scatter returns along paths nearly coincident with the incident radiation, derotated in the scanner, recollimated by the initial lens, and made co-linear by the beam splitter before it is heterodyned on the detector face. A wideband photo-multiplier feeds a mixer/local oscillator, preamplifier, IF amplifier, and discriminator signal processing system. Typical signals are shown in Fig. 2. The upper trace shows the RF Doppler difference beat. The fact that the amplitude drops to near zero only two or three times over the interrogation arc (10’ in this case) shows that synchronization is within a few diffraction-limit beamwidths in rotation and translation for this period. The detected signal in the lower trace shows several cycles of blade vibration. If the number of vibration cycles per shaft revolution had been much lower, only a fraction of a cycle would have been recorded on any given pass. Several passes would then be used to reconstitute the full vibration waveform. At General Electric we have tested a system using a fourteen-inch diameter laboratory wheel at speeds up to 18,000 rpm. With near-tangential incidence vibration sensitivity corresponds to about 7 microinches rms, blade tip displacement at 1 KHz. At a viewing aspect 30 degrees from the radial, the sensitivity is poorer by a factor of about 5. Preliminary experiments in the presence of hot gases indicate little loss in sensitivity will be experienced in a hot engine.

Laser vibrometer with increased accuracy and interference immunity

Abstract: It has been established experimentally that the most interference immune of the existing laser Doppler vibrometer schemes is that in which the signal is processed at a subcarrier frequency, for example, with positive phase modulation of one of the beams in a two beam interferometer.

Correct application of beam splitters with laser beams of fluctuating polarization

Abstract: Measurements are presented which indicate that the polarization of a multimode CO2 laser beam produced in an unstable cavity varies considerably from shot to shot. This effect must be taken into account when using a dielectric beam splitter to monitor the laser beam.