Showing papers on "Laser Doppler vibrometer published in 1984"

Laser Doppler velocimetry by modulating a CO2 laser with backscattered light.

Abstract: A portion of the Doppler shifted light backscattered from a moving diffuse target is allowed to reenter the CO2 source laser. The laser power, monitored by a photodiode through a partially transmitting mirror on the rear of the laser, is modulated at the Doppler frequency. Two regimes of operation are observed. For the laser operating well above threshold, the Doppler signal level is relatively insensitive to the laser operating point. A narrow region of enhanced sensitivity is observed, however, for laser operation at a point near threshold.

Laser Doppler velocimeter for velocity and length measurements of moving surfaces

Abstract: An industrial laser Doppler velocimeter has been developed for accurately measuring the velocity and length of moving surfaces The instrument’s advanced optical and electronic design provides a large depth of field, high SNR, and large dynamic range making it very suitable to industrial process control applications

Vibration telemetering system

Abstract: There is dislosed a vibration telemetering system capable of measuring adverse vibrations of installations or equipment in a plant such as a nuclear power plant. The system includes a vibrometer which measures the vibrations in a non-contacting fashion. The direction at which the vibrometer faces is adjusted by an angle adjusting mechanism which is mounted on a travelling unit running along a predetermined inspection route in the plant. A measured signal from the vibrometer is transmitted to a remote control room through a communication system and is used for evaluating whether or not the state of the plant is normal. The remote control room can control the travelling unit and the angle adjusting mechanism through the communication system in order to monitor the vibrations of a plurality of places along the predetermined inspection route.

Laser telemetry and Doppler measurement system with pulse compression

Abstract: Laser telemetry and Doppler measurement apparatus using pulse compression has a transmitter for providing a periodic pulsed transmission laser wave having a pair of pulses, one pulse being frequency modulated on one side of a main frequency FE, and the other pulse being frequency modulated on the other side of the main frequency FE. The transmitter also provides a reference laser wave having a frequency FL. A photomixer is adapted for superheterodyne reception of the reference laser wave and a return laser signal which has been reflected from a target. The return laser signal has a Doppler shift frequency FD. The photomixer provides a beat signal having a frequency FI plus FD, where FI is an intermediate frequency. A Doppler aquisition loop transposes the beat signal frequency and provides a transposed signal to compensate for the Doppler shift. The Doppler acquisition loop provides a coarse compensation signal having a frequency near the frequency FD. Under target tracking conditions, the Doppler acquisition loop then provides an automatic fine compensation signal which compensates for the Doppler difference ΔFD which exists between the compensation signal frequency and the Doppler shift frequency. Receiving and processing means then receive the transposed signal and provide an output signal indicative of the distance to the tracked target and the Doppler shift frequency of the target.

Signal-to-noise in a backscatter-modulated Doppler velocimeter.

Abstract: When a portion of the light backscattered from a moving diffuse target is allowed to reenter the source laser, at the Doppler frequency is observed. This paper analyzes the SNR expected modulation of the laser power laser Doppler velocity measurements. A peak in the signal-to-noise when this phenomenon is used to make is predicted for the laser operated at its threshold gain.

Laser beam scanner apparatus

Abstract: In a laser beam scanner apparatus for scanning the surface of a photosensitive drum (1) or a document to be read by deflecting a laser beam emitted from a laser diode (LD) with a deflector (50), there is provided a drive current control circuit (100) for controlling the magnitude of drive current of said laser diode on the basis of a beam deflection angle of said deflector.

Use of a laser diode and an optical fiber for a compact laser-Doppler velocimeter

Abstract: In order to make an optical-fiber-type laser-Doppler velocimeter (LDV) more compact, we propose the use of a laser diode as the coherent light source. To reduce the optical feedback on the laser source, a polarization-maintaining single-mode fiber was adopted. We have confirmed that the LDV system can be compact and also that it can have a velocity resolution of a few millimeters per second and a signal with a 20-dB signal-to-noise ratio, which are similar to the resolution and signal-to-noise ratio obtained with a He–Ne laser.

A rapidly scanning laser Doppler anemometer

Abstract: A rapidly scanning directionally sensitive fringe-type laser Doppler anemometer (SLDA) which scans the measurement volume perpendicular to the optical axis of the transmitting optics is described. Scan frequencies up to 60 Hz over scan distances of 40 cm have been used, although scan frequencies up to 150 Hz are possible. The maximum scanning velocity of the measurement volume that can be used is directly proportional to the shift frequency of the Bragg cell since each signal-producing particle must cross a minimum number of fringes to produce a valid signal. Signal-averaging bias is lower with a scanning LDA than with pointwise measurements. Results obtained for a separating turbulent boundary layer show that very good mean and RMS velocity profiles can be obtained in less than 1 min of data acquisition.

Bragg cell spectrum analyzer

Abstract: A Bragg cell RF spectrum analyzer of the type having a laser input wherein the laser input is coupled to the Bragg cell analyzer by a signal-mode optical fiber for the purpose of improving the spatial quality of the laser, thus limiting side lobes of the input laser beam; and allowing the laser input to be remote from the Bragg cell.

Characterization Of Rotating Flexible Disks Using A Laser Doppler Vibrometer

Abstract: The dynamics of a flexible disk rotating near a rigid flat plate are characterized experimentally using a laser Doppler vibrometer. The goal of the investigation was to determine the mechanism by which the gap between the disk and the base plate varies with time. A periodic averaging scheme is used to separate the instantaneous gap signal into a periodic component phased to the rotational position of the disk and an independent random component. The periodic component is shown to dominate, demonstrating that the gap variation is caused by permanent deformations of the disk moving past a fixed point. The base plate stabilizes the disk by flattening the deformations, and the gap variations can be further reduced by increasing disk rotational speed. Disks having 100 and 180 Am thickness were investigated; the thicker disks are shown to have less residual curl. The stabilizing influence of the base plate is lost as the hub height is increased, which implies that hydrodynamic forces are essential to the stabilization mechanism. The mean disk shape is generally a linear function of radius; no self-regulating region was observed.

Method of driving a semiconductor laser

Abstract: A semiconductor laser oscillating in a single mode under direct-current drive is driven by a signal in which a high frequency current is superposed on a direct current, so that the laser is brought into continuous wave pulsed oscillation at a pulse width of at most 1.2 ns, whereby laser noise is suppressed.

Fiber‐optic laser Doppler velocimeter using an external‐cavity semiconductor laser

Abstract: Influence of the spectral linewidth Δν of a semiconductor laser on the performance of a fiber‐optic laser Doppler velocimeter (FLDV) was investigated. By using an external‐cavity laser with Δν≂0.3 MHz, we could build a FLDV having an optical fiber probe length up to 25 m long.

Laser control apparatus and method for frequency offset phase locking and the minimizing of retro-reflection effects

Abstract: An active laser is slaved in an offset frequency relationship to an ultrastable reference laser, by a comparator with a substantially infinite capture range in order to preclude phase-locking on a harmonic or mirror image of the desired offset frequency. The frequency of such a slaved laser is then virtually immune to retro-reflection of its own light back into its resonant cavity, and gains a frequency stability equal to that of the reference. Further, the reference laser is preferably stabilized against retro-reflection of its own light by a method involving selective displacement of objects in the path of the reference laser beam, detection of a Doppler-shifted retro-reflected radiation, and insertion of appropriate optical controls in the beam path to eliminate such retro-reflected radiation from impinging on the reference laser cavity. Multiple laser systems are disclosed in which the active lasers are both series referenced and parallel referenced to a reference laser.

Spinning disk calibration method and apparatus for laser Doppler velocimeter

Abstract: A method and apparatus for calibrating laser Doppler velocimeters having one or more intersecting beam pairs. These velocimeters measure fluid velocity by observing the light scattered by particles in the fluid stream. Moving fluid particulates are simulated by fine taut wires that are radially mounted on a disk that is rotated at a known velocity. The laser beam intersection locus is first aimed at the very center of the disk and then the disk is translated so that the locus is swept by the rotating wires. The radial distance traversed is precisely measured so that the velocity of the wires (pseudo particles) may be calculated.

Device for frequency modulation of a laser output spectrum

Abstract: A device is provided for fast frequency modulating the output spectrum of multimode lasers and single frequency lasers that are not actively stabilized. A piezoelectric transducer attached to a laser cavity mirror is driven in an unconventional manner to excite resonance vibration of the transducer to rapidly, cyclicly change the laser cavity length. The result is a cyclic sweeping of the output wavelength sufficient to fill the gaps in the laser output frequency spectrum. When such a laser is used to excite atoms or molecules, complete absorption line coverage is made possible.

Doppler effect laser velocity measuring system

Abstract: The frequency shift of reflected laser beam from a target is transformed o an angular measurement which is proportional to the velocity of the target relative to the measurement platform. The system uses the refraction angle of the reflected beam from a movable optical grating to produce the angular measurement.

Switching laser beam apparatus for optical recording

Abstract: A switching laser beam apparatus for optical recording comprises a laser (10) for projecting a laser beam (12) in a first path, a splitter (18) for generating a read-before-write beam (22), a read beam (42) and a read-after-write beam (24) from the laser beam (12), and an acoustic optical modulator (14) between the laser (10) and the splitter (18) in the path of the laser beam (12). A drive circuit (38) generates a signal for causing the acoustic optical modulator (14) to produce a write beam (44) by diffracting a predetermined percentage of laser beam into a second path (46). A diverting device (56,62) diverts the write beam into the first path after the splitter (18).

Evaluation of the effect of variable refraction index on the path of a laser beam

Abstract: The use of a laser-Doppler anemometer to measure velocities in a fluid with a larger variation in its refractive index perpendicular to the laser beams may introduce appreciable errors. This situation is commonly encountered in convective heat transfer studies. The beams are refracted as they pass through the fluid and the distance of the intersection of the beams, control volume, from the surface may be significantly different than the distance of the beams from the surface when they entered the fluid. If the flow is unsteady or turbulent, the relative movement of the beams may be such that the size and location of the control volume is constantly changing causing appreciable errors.

Arrangement for stabilising and controlling a semiconductor laser

Abstract: A simple arrangement for stabilising the emission spectrum and for controlling the output power of a semiconductor laser is for a photodiode to be arranged at a distance in front of one of the two laser mirrors, which photodiode obtains from the optical output signal of the laser an electrical signal for controlling the laser current and/or the wavelength, the side of the photodiode (4) facing the laser mirror (6) being provided with a coating (7) which partially reflects the light emitted by the laser (2), and the distance (a) between this reflection plane (7) of the photodiode (4) and the laser mirror (6) corresponding to n times (n = 1, 2, 3, ...) half the wavelength of the longitudinal laser mode which is to be stabilised.

Fibre Optic Laser Doppler Velocimetry

Abstract: The use of fibre optics in laser Doppler velocimetry (LDV) promises increased versatility of the technique and novel optical designs. This paper discusses the properties of optical fibres and related components relevant to LDV design, and the likely limitations on system performance. The discussion is illustrated by descriptions of prototype systems constructed in these laboratories: a high power (~ 0.5 W) argon ion laser based system; a compact system incorporating a piezo-electric phase shifter to provide directional information; and a diode laser based system.

Lasers. Characterization of Laser Diodes by Measuring the Speckle Contrast at the End of a Multimode Fiber

Abstract: Summary A novel and simple method is described to characterize semiconductor lasers which may produce excessive modal noise in a transmission system. The method is based on an analysis of the speckle contrast at the end of a suitable length of multimode fiber. An optimal length of measuring fiber is calculated and verified experimentally. The qualitative dependence of speckle contrast on fiber length is shown. The measured speckle contrast allows to define an “effective” number of laser spectral lines accounting for the modal noise.

A simple interferometric vibrometer based on a laser Doppler velocimeter

Abstract: A simple heterodyne speckle interferometer can be built by slightly modifying a classical laser Doppler velocimeter (LDV) for fluid flows applications. The random character of the speckle scattered by a rough surface is considered for the statistical description of the measured signals as a function of the number of speckle grains collected by the photodetector. Three types of simple demodulators are proposed and demonstrated. In particular, it is shown that the LDV frequency counters can be used without any change to process the continuous frequency modulated signals obtained in a speckle interferometer.

Vibration detecting set of portable vibrometer

Abstract: PURPOSE:To improve detecting sensitivity, by making the length of a probe to be the minimum required length, providing a vibration insulating part between the probe and a case, to which it is attached, providing a concave spherical part corresponding to the tip part of the probe and the like in a body to be measured, thereby reducing the mass of a detector, and increasing the value of natural frequency. CONSTITUTION:The length of a probe 4 is limited to the minimum length required for measuring work, and the weight of a detecting part is decreased. In order to improve the contact spring constant of the probe 4, a convex spherical surface is formed on a tip part 43 of the probe 4. A concave spherical surface 61 corresponding to the convex spherical surface of the tip part 43 is formed in a body to be measured 6. The concave spherical surface 61 can be a concave conical surface. Instead of forming the convex spherical part at the tip part of the probe 4, a spherical body can be bonded to the tip part. A vibration insulating body 2 is provided, and the vibration detecting part and a case 1, which is a hand-held part, are separated from the viewpoint of vibration. Therefore, the mass of the detecting part can be reduced to a large extent. The tip part 43 of the probe 4 is made to be the spherical surface and the corresponding surface to be measured is made to be the concave spherical surface or the concave conical surface. Thus the contact part becomes the surface contact or line contact, and the contact spring constant can be made high.