Showing papers on "Laser Doppler vibrometer published in 1973"

Two-color dual-beam backscatter laser Doppler velocimeter.

Abstract: A laser Doppler velocimeter has been developed that uses two of the colors emitted from an argon-ion laser for the simultaneous measurement of orthogonal velocities. Designed for use in a 2.13-m by 3.05-m wind tunnel, it is capable of traversing its focal volume across spatially unstable flows at scan speeds of up to 1.5 m/sec. Its optical layout and principles of operation are discussed, and the data from a typical traversal of a trailing wing-tip vortex are presented.

High-resolution spectroscopy with and frequency stabilization of a cw dye laser

Abstract: The hyperfine splittings of the Na D1 and D2 lines were investigated using a single mode cw dye laser. The light of the laser was scattered by the atoms of an atomic beam and the fluorescent light was observed as the frequency of the laser was tuned across the D lines. The Doppler width of the atomic beam was reduced to about 2.5 MHz so that the absorption width of the atoms of the beam was essentially determined by the natural width of the 32P1/2 and 32P3/2 levels, which is about 10 MHz. Since the linewidth observed for the hyperfine transitions was 30 MHz, most of the hyperfine components of the D1 and D2 lines could be resolved. In another experiment the frequency of the dye laser was locked to a hyperfine transition of the D1 line. The observed variation of the output frequency of the dye laser was less than ±1.5 MHz. In addition, the intensity of the dye laser was controlled to about 10−3, using an electro-optically variable transmission filter.

Two-component dual-scatter laser Doppler velocimeter with frequency burst signal readout

Abstract: A dual-scatter laser Doppler velocimeter (LDV) system designed for measuring wind tunnel flow velocity is described. The system simultaneously measures two orthogonal velocity components of a flowing fluid at a common point in the flow. Essential single-velocity component dual-scatter concepts are presented to simplify the description of the more sophisticated two-component system. To implement the two-component system three laser beams with a 0°, 45°, and 90° polarization plane relationship are focused to a common point in the flow by the system-transmitting optics. The beams interfere to form two perpendicular sets of interference fringe planes that are orthogonally polarized. The system-receiving optics collect and separate the orthogonally polarized components of laser radiation scattered from micron-size particles moving with the flowing fluid through the fringes. The system requires no artificial seeding, since intrinsic test section aerosols are utilized for radiation scattering. The passage of each scatter particle through the interference fringes simultaneously produces two frequency-burst-type photodetected signals, the frequencies of which are directly proportional to two perpendicular components of particle velocity. The system photodetection, signal-conditioning, and data acquisition instrumentation is specifically designed to process the frequency burst information in the time domain as opposed to spectrum analysis or frequency domain processing. The system was initially evaluated in an AEDC wind tunnel operating over a Mach number range from 0.6 to 1.5. The LDV and calculated wind tunnel mean velocity data agreed to within 1.25%; flow direction deviations of a few milliradians were resolved.

Laser Doppler velocimeter with optical frequency shifting

Abstract: A bi-directional laser Doppler velocimeter utilizing optical frequency shifting has been developed. Optical frequency shifting is achieved by using a rotating radial grating. A high diffraction efficiency permits the system to be used in the fringe mode. The zero velocity frequency shift can be varied from 0.1 to 2.5 MHz with a stability better than 0.2%. Important applications are velocity measurements in reversing flows, highly turbulent flows, two-phase flows, and boundary layers. The system described may also be used for vibration analysis. Experimental results are presented.

Laser Doppler interferometry for measuring small absorption coefficients

Abstract: A laser Doppler interferometric technique for measuring small absorption coefficients of infrared‐transmitting materials is described. The interferometer is calibrated as a remote temperature sensor by monitoring optical path length changes of the sample as a function of temperature. The sample is then irradiated by a CO2 laser and the change in optical path due to absorbed laser energy is observed interferometrically as a function of time. From this measurement and the calibration data, a temperature‐vs‐time curve is generated from which the absorption coefficient of the sample at 10.6 μ may be calculated.

Confocal Backscatter Laser Velocimeter with On-Axis Sensitivity

Abstract: A confocal backscatter laser Doppler velocimeter (LDV) that measures two velocity components has been developed. This device requires only two incident beams polarized normally to one another. Moreover, the velocity components sensed are nearly orthogonal. The velocimeter employs a combined dual-scatter, local oscillator arrangement to obtain the bidirectional sensitivity. Two photodetectors are used, each sensing only one Doppler frequency proportional to one of the very nearly orthogonal velocity components. In addition, a single Bragg cell serves to frequency bias both velocity components in order to eliminate directional ambiguity. A differencing technique has also been incorporated to enhance the dual-scatter Doppler signal corresponding to the transverse velocity.

Laser Doppler instrument for measurement of vibration of moving turbine blades

Abstract: A laser Doppler instrument using a 1 mW laser has been developed for the measurement of the vibration of moving objects. Particular interest has been taken in rotating turbine blades. The design and optimisation of the instrument are described, and details given of its performance and resolution. It is shown that resolution is limited by signal/noise ratio and by Doppler broadening, both of which can be predicted. A coherence loss factor is established which enables the expected signal/noise ratio to be estimated from simple ‘noncoherent’ measurements of light scattered from a target. Experiments are described which compare the performance of photomultipliers and avalanche photodiodes. The instrument has been used successfully on a turbine having blade-tip velocities up to 300 m/s, equivalent to Doppler shifts up to 1000 MHz. At the lower frequencies, photomultipliers and avalanche photodiodes are comparable in performance, but at the top end of the range only the latter is satisfactory. Resolution in velocity of about one part in 104 is achieved with an output bandwidth of 1 kHz.

I INVERTED LAMB DIP STABILIZED He-Ne LASER

Abstract: An extremely accurately resettable He-Ne laser including a gaseous I2 129 absorption cell in the laser cavity, and a method of stabilizing a He-Ne laser on an Inverted Lamb Dip of I2 129. The laser gain tube and absorption cell are enclosed in a novel laser cavity apparatus. The cavity mirrors are positioned by novel mirror adjusting means which permits the laser to be tuned very quickly.

System for optical alignment and adjustment of a laser

Abstract: A system for optical alignment and adjustment of a machining laser, wherein an auxiliary laser, which is mounted near the principal laser provides a beam which is directed in superposition with the optical axis of the principal laser to precisely adjust a workpiece with the optical elements of the principal laser.

Analysis of Signal-to-Noise Ratio of the Laser Doppler Velocimeter

Abstract: The signal-to-shot-noise ratio of the photocurrent of a laser Doppler anemometer is calculated as a function of the parameters which describe the system. It is found that the S/N is generally a growing function of receiver area, that few large particles are better than many small ones, and that generally the ‘fringe’ or ‘differential’ mode configuration is equal to, or better than, the ‘reference beam’ mode.

Frequency sensitive laser rotary motion sensor

Abstract: A rotary motion sensor of the resonant laser bimodal optical cavity type is provided with a reference beam generating laser and feedback loop for locking the reference beam frequency to one mode. Corner modulation is provided to modulate the ring laser beams and prevent mode locking. The modulation thus introduced is demodulated out of the processed ring laser beams and the resulting signals are digitally processed to provide motion, rate and acceleration information.

A Stabilized HCN Laser for Infrared Frequency Synthesis

Abstract: Infrared frequencies have recently been synthesized in suitable diodes up to 88 THz with accuracies of parts in 109. Stabilized lasers are necessary in order to make frequency measurements of higher accuracy. The hydrogen-cyanide laser is the lowest frequency basis laser used in these synthesis schemes, and its stabilization has been the subject of recent interest. The laser is stabilized by locking it to a phase-locked microwave reference chain. Two servo loops are utilized. The first loop is a relatively slow frequency-lock loop with the correction applied to a piezoelectric-translator driver. This loop not only accommodates thermal expansion of the laser, but also serves as an acquisition aiding loop for the second servo. The latter is a phase-locked system with the correction applied to the laser discharge current controller. Data regarding the system stability are presented.

Effect of Doppler ambiguity on the measurement of turbulence spectra by laser Doppler velocimeter

Abstract: For the measurement of turbulent flows, the effect of finite transit times of scatterers to cross the scattering region in a laser Doppler velocimeter is discussed. The result shows that in the measurement of the velocity spectrum, it is equivalent to a white spectrum noise with an amplitude inversely proportional to the average transit time. Theoretically, the noise becomes negligibly small when the wavelength of the laser light becomes very short.

Doppler data processor with digital computing pulse rate filter

Abstract: A doppler data processor apparatus utilizing a digital computing pulse rate filter for processing frequency burst type data produced by a laser doppler velocimeter.

Acousto-optically Q-switched laser with modulated output

Abstract: A techinque for modulating the output signal of an acousto-optically Q-switched laser is disclosed The switch is energized with an acoustic wave having a frequency equal to the approximate frequency spacing between longitudinal lasing modes of the laser Among other advantages, the technique permits identification of lasers by length of resonator and through detection of the frequency of modulation of the laser output

Doppler ambiguity in laser Doppler velocimeters

Abstract: An analysis of the Doppler ambiguity for the laser Doppler velocimeter is presented which accounts for the signal statistics based on a Gaussian beam intensity. Results are in excellent agreement with experiment.

A two color, dual beam backscattering laser Doppler velocimeter

Abstract: A laser Doppler velocimeter has been developed which uses two of the colors emitted from an argon ion laser for the simultaneous measurement of orthogonal velocities. Designed for use in a 2.13 m by 3.05 m wind tunnel, it is capable of traversing its focal volume across spatially unstable flows at scan speeds of up to 1.15 m/sec. Its optical layout and principles of operation are discussed and the data from a typical traversal of a trailing wing tip vortex are presented.

Matched-Filter Detection of Mode-Locked Laser Signals

Abstract: The passive Fabry-Perot cavity is shown to be a good practical approach to the match-filter optimization for the sensitive detection of mode-locked laser signals. Doppler measurements of relative motion over a wide range of velocities are possible simply by measuring the cavity length for a peak output.

Processing of the laser Doppler velocimeter signals

Abstract: The laser Doppler velocimeter (LDV) is a probeless technique that provides a remote measurement of mean and fluctuating velocities. The measurement is actually obtained from small particles embedded in the flow which scatter light from an illuminating laser beam interference pattern. A portion of this scattered light is collected by a photomultiplier which yields an electronic signal whose frequency is directly proportional to the velocity of the small particles. The purpose of this paper is to describe and critically compare three techniques most used to process this electronic signal. These techniques are: (1) spectrum analyzer - a frequency scanning filter (frequency domain instrument), (2) wide-band frequency tracker - a frequency lock loop (frequency domain instrument), and (3) high-speed frequency counter - an interval timer (time domain instrument). The study determines the ability of each technique to process the LDV signal and yield velocity data to be used in determining the flow characteristics.