Showing papers on "Laser Doppler vibrometer published in 1978"

An Instrument to Measure Cutaneous Blood Flow Using the Doppler Shift of Laser Light

Abstract: A system utilizing the Doppler shift of monochromatic laser light has been developed to measure blood flow in skin. Light from a low power (S mW) He-Ne laser is coupled into a quartz fiber and transmitted to the skin. This light is reflected from both the nonmoving tissues (reference beam) and moving red blood cells (Doppler shifted beam), received by a plastic fiber, and transmitted back to a photodiode where optical heterodyning occurs. The heterodyned output signal, which is proportional to the Doppler shift frequency, is amplified and both RMS and dc values obtained. The RMS value is weighted against the backscattered light intensity using the measured dc value as an index of total received power. This is used as the output flow velocity value.

Frequency determination of visible laser light by interferometric comparison with upconverted CO(2) laser radiation.

Abstract: A servocontrolled 1-m plane-parallel Fabry-Perot interferometer has been developed at NPL for the precise intercomparison of laser wavelengths. This instrument has been used to measure the wavelength ratio of a 679-nm radiation and that from a 633-nm iodine-stabilized He–Ne laser, achieving an accuracy of 2.9 parts in 1011. The 679-nm light was derived from a stabilized CO2 laser radiation by upconversion, and the wavelength of this 9.3-μm laser radiation can be calculated from the visible wavelength result. Frequency measurements on the same CO2 laser radiation have already been made in this laboratory, so that the experiment reported here leads to a precise value for the speed of light in vacuum and to the value of 473, 612, 380.5 ± 0.3 MHz for the absolute frequency of the visible radiation from a He–Ne laser stabilized to component d of127I2.

The fibre-optic Doppler anemometer

Abstract: A laser Doppler anemometer is described in which an optical fibre is used to transmit the light to and from the measurement zone. An account is given of the various types of measurement that have been made with the instrument, particularly those involving Brownian motion. An analysis is made of backscattering into a fibre which shows that the back-scattered power is confined to a region extending only a few tens of core diameters from the fibre end.

Wave guide for surface wave transmission of laser radiation

Abstract: Apparatus and method for transmission of laser radiation in the infrared portion of the spectrum. A 10.6 μm laser beam is focused tangentially upon an inner surface of an elongated synclastic wave guide having a generally oval cross-section, and an output end of the wave guide is directed upon a target. Preservation of spatial mode coherence in the laser beam is demonstrated both theoretically and experimentally.

Continuously variable laser output coupler

Abstract: The output power of any laser is optimized by providing an output mirror with a transmission which varies as a function of position on the mirror and transmitted wavelength. The transmission which corresponds to the optimum output for a given laser wavelength is chosen by monitoring laser output and moving the output mirror in at least one direction essentially perpendicular to the laser beam axis while maintaining the orientation of its reflecting surface perpendicular to the laser beam axis, and locating the mirror position corresponding to optimum laser output.

Method and apparatus for Doppler frequency modulation of radiation

Abstract: A method and apparatus for frequency modulating radiation, such as from a laser, for optoacoustic detectors, interferometers, heterodyne spectrometers, and similar devices using two oppositely reciprocating cat's-eye retroreflectors to Doppler modulate the radiation By reciprocally moving both retroreflectors, the center of mass is maintained constant to permit smooth operation at many Hertz By slightly offsetting the axis of one retroreflector relative to the other, multiple passes of a light beam may be achieved for greater Doppler shifts with the same reciprocating motion of the retroreflectors A Doppler shift of 2(v/c)ν occurs for each pass where v is the velocity of each retroreflector, c is the speed of light, and ν is the frequency of radiation

Working method by laser

Abstract: PURPOSE: To oscillate laser beam perpendicularly to the scanning direction of laser and modulate the power of laser beam in synchronization with this oscillation to irradiate uniformly portions to be worked. CONSTITUTION: Horizontal parallel laser beam 1 oscillated from a laser oscillator is reflected from a plane mirror 2, then condensed by a condenser 3 and irradiated on the working line X-X' of workpieces 4a, 4b. Then said plane mirror 2 is swung about a fulcrum 5 by a cam mechanism 8 to change cyclically the reflecting angle and oscillate the laser beam 1 to the direction Y-Y' perpendicular to the scanning direction X-X'. Simultaneously the power of laser beam 1 is modulated in response to this oscillation to reduce the temperature gradient in the direction Y-Y' and irradiate uniformly the butt portion of workpieces 4a, 4b with laser beam for welding them integrally. COPYRIGHT: (C)1979,JPO&Japio

Multicomponent frequency shifting self-aligning laser velocimeters.

Abstract: A design using simple lenses and frequency shift by scattering disk has been developed, which allows easy construction, and alignment of one‐, two‐, and three‐orthogonal‐component, frequency shifting laser Doppler velocimeters. A series of instruments designed on this basis have proved to be inexpensive to construct, precise and accurate to use, adaptable to a wide range of applications, and have proven particularly advantageous in studies of turbulent phenomena and complex flows. In this paper the design concept is explained, construction and alignment procedures are outlined for the simplest assembly of the one and two component versions, and performance and application examples are presented. The extension of the system to measure the third orthogonal component is discussed.

GaAlAs diode sources for laser‐Doppler anemometry

Abstract: A laser‐Doppler anemometer based on a continuous GaAlAs diode laser source is described. It is shown that the coherence length of the diode output requires that the optical design incorporate equal path lengths for good fringe visibility. The results show that Doppler signals of processable quality may be obtained using this compact laser source.

Laser Doppler velocity simulator

Abstract: A method and apparatus for inducing a Doppler frequency shift in a reference beam laser velocimeter light beam to simulate target velocity wherein the light beam is passed through a rotating refractive transparent block positioned between two reference points along the beam resulting in a continuous change in the light beam optical path length between the reference points according to a known function. The velocity indicated by the laser velocimeter is compared to the known simulated velocity function for velocimeter testing and calibration.

Laser cutting of tubular articles

Abstract: Method and apparatus for cutting tubular material with two separate laser beams having a constant power ratio by using a laser source having a single output beam of random polarization. A beamsplitter is positioned so that the output of a laser source strikes it at a near normal angle of incidence. The beamsplitter divides the laser beam into two components whose power ratio remains relatively constant due to the near normal angle of incidence of the laser beam onto the beamsplitter. The two components are then directed to strike the tube. The constant power ratio permits the most efficient use of the power of the laser source.

Beam waist location and measurement in a dual-beam laser Doppler anemometer

Abstract: A method is described for measuring Gaussian laser beam diameter and, in particular, determining the size of the beam waist in a dual-beam laser Doppler anemometer. Results of measurements carried out by different techniques and theoretical predictions are presented as a check to the validity of that method. The associated problem of the beam waist location is also discussed. The method, called here the 'power ratio method' offers itself as a simple and reliable way of measuring beam diameters and locating beam waists, which, otherwise, are cumbersome and time-consuming.

Laser plasma isolator for a retropulse in a CO2 laser amplifier chain

Abstract: A laser plasma isolator was developed to attenuate a retropulse in a high‐power CO2 laser system. An attenuation ratio of 1/20 of a main laser beam (∼1‐J 3‐nsec pulse‐width FWHM) was obtained by blowing off a carbon plasma induced by a plasma production laser beam (∼0.5‐J 3‐nsec pulse‐width FWHM) onto the waist of the main laser beam.

Stabilization of a Complex Resonator He–Ne Laser

Abstract: A complex resonator He–Ne laser at 633 nm was constructed as a local oscillator for measuring the absolute wavelength of a Co2 laser. It is stabilized by a dual servo-control system. One is a frequency offset-lock system, by which the frequency of the laser is stabilized so as to hold an offset frequency from an iodine stabilized He–Ne laser as a master laser. The other is a sub-resonator control system, by which the output power of the laser is stabilized. The laser oscillates at a single frequency with a maximum output power of 10 mW. The oscillation frequency can be turned over a 1.5 GHz spectral range and locked without modulation to any frequency over a range of ±15 MHz of the offset frequency from the master laser. A frequency stability of ±1×10-11 and an output power stability of ±4×10-4 are obtained.

Stark cell noise reduction of a CO 2 laser beam

Abstract: A technique is demonstrated for reducing the rapid intensity fluctuations in the beam of a single frequency, polarized CO 2 laser tuned to the P(20) transition at 10.6 μm. The method uses a Stark-tunable absorption line in NH 2 D to compensate for the fluctuations of the laser output. Substantial reductions in laser beam intensity noise have been obtained. The method may be applied to other lines of the CO 2 laser and indeed to other types of lasers.

Broadband gas laser

Abstract: A gas laser having a bandwidth which substantially exceeds the normal doppler bandwidth. To this end the lasing beam is made to propagate at an angle to the direction of flow of the stream of lasing gases. The gases should move at a supersonic speed so that the normal doppler bandwidth of the laser is shifted up or down in frequency by a predetermined amount. By utilizing two or more gas flows at different angles to each other, the normal amplification band of the laser medium can be shifted up and down in frequency by different amounts to obtain an overall bandwidth on the order of 1 - 2 gigahertz (ghz).

Amplitude stability improvement of CW submiltimeter wave lasers with phase modulated optical pumping

Abstract: An experimentally measured reduction of 4:1 in the amplitude fluctuation of an optically pumped CW-submillimeter wave laser was achieved using a mirror, vibrating at audio frequencies, to decouple the pump beam reflected back to a CO 2 pump laser. An expression is derived for the peak frequency deviation of the pump laser due to a phase modulated feedback signal.

Design and Evaluation of a Laser Doppler Velocimeter for the AEDC-PWT 4-Foot Transonic Tunnel

Abstract: : The design details and initial performance evaluation of a two- component Laser Doppler Velocimeter (LDV) system for a large transonic wind tunnel are presented The two-color, backscatter LDV was specially developed for free-stream, pitch plane flow angle and velocity magnitude measurements using only intrinsic particulate matter as light scatterers An environmental control system and remote optical alignment techniques were developed since the LDV was located in the hostile environment (vibration, vacuum, inaccessibility, etc) of the tunnel plenum region A new-type LDV signal processor was developed to process the low-level signals produced by the small intrinsic particles A versatile, computer-based system was developed for high-speed data acquisition LDV mean velocity and flow angle data are compared to free-stream velocity data and aerodynamic flow angle probe data