Showing papers on "Laser Doppler vibrometer published in 1995"

Method for controlling configuration of laser induced breakdown and ablation

Abstract: In one aspect the invention provides a method for laser induced breakdown of a material with a pulsed laser beam where the material is characterized by a relationship of fluence breakdown threshold (Fth) versus laser beam pulse width (T) that exhibits an abrupt, rapid, and distinct change or at least a clearly detectable and distinct change in slope at a predetermined laser pulse width value. The method comprises generating a beam of laser pulses in which each pulse has a pulse width equal to or less than the predetermined laser pulse width value. The beam is focused to a point at or beneath the surface of a material where laser induced breakdown is desired. The beam may be used in combination with a mask in the beam path. The beam or mask may be moved in the X, Y and Z directions to produce desired features. The technique can produce features smaller than the spot size and Rayleigh range due to enhanced damage threshold accuracy in the short pulse regime.

Sealed-cavity resonant microbeam pressure sensor

Abstract: A quasi-digital pressure sensor based on polysilicon resonant microbeams has been demonstrated. Pressure sensitivities of nearly 4000 counts per second per psi have been attained on a 10 psi device with a base frequency of 233 000 Hz. Short-term stability as low as 0.01 ppm of the base frequency is typical. The microbeams are fabricated with their own integral vacuum cavities, allowing high-Q operation in the differential pressure mode or in contact with liquids such as silicone oil. Design considerations include the effects of internal strain and lead to a push-pull layout configuration independent of microbeam strain or diaphragm thickness. Fabrication technology incorporates fine-grained polysilicon, surface micromachining, bulk micromachining, and reactive sealing. Packaging into precision avionics headers is being used for preliminary testing. Testing results indicate suitability for precision avionics, industrial, and commercial applications. Optical methods have been used to test resonant microbeam pressure sensors and verify the push-pull design methodology. Testing methods developed under this effort include electrostatic drive/piezoresistive sensing, optical drive/optical sensing, substrate piezoelectric drive/optical sensing, and electrostatic drive/laser vibrometer sensing. Wafer-level testing of 200 μm×46 μm×1.9 μm microbeams shows an average fundamental frequency of 553 150 and first overtone of 1 332 550 Hz. The standard deviations across the wafer are 0.15 and 0.10%, respectively. The internal strain and effective thickness can be determined with high resolution. Laser vibrometer measurements through the microbeam shell verify the fundamental frequency and reveal at least ten overtones up to 25 MHz.

Laser speckle velocimeter using self-mixing laser diode

Abstract: We have built a novel laser speckle velocimeter using a self-mixing laser diode (SM-LD). The speckle signal obtained by the measurement system has a waveform independent of the target velocity when the same path on the target surface is iteratively measured. The mean frequency of the speckle signal is directly proportional to the target velocity when the laser beam spot diameter is kept constant, and greater than 0.36 mm. The mean speckle signal frequency is automatically measured by using a computer. Its measurement error is inversely proportional to the scanned length, and is 2% for 100 mm. It is possible to detect velocity of the target transversely moving across the laser light beam using this compact measuring system.

Frequency and intensity noise in an injection-locked, solid-state laser

Abstract: We have calculated transfer functions for frequency and intensity fluctuations in an injection-locked solid-state laser. At modulation frequencies well below the locking frequency we find significant frequency-noise reduction, and at modulation frequencies above the locking frequency we find that the frequency noise is that of the free-running slave laser. Our intensity-noise theory predicts substantial damping of relaxation oscillations in the slave laser. To verify these results we have measured the frequency and intensity noise of a 5-W, injection-locked Nd:YAG laser.

Displacements in a Vibrating Body by Strain Gage Measurements

Abstract: An important issue in the position control of elastic systems is the correct on-line measurement of displacements. EndPoint Control of flexible manipulators, for example, requires the measurement of the manipulator's tip displacement. While this kind of measurement is relatively easy to carry out in a laboratory setting, it can be problematic in a real world environment. A novel procedure for the real-time determination of displacements at any point of a vibrating body is proposed. The procedure calls for the use of strain gauges and the knowledge of the modal properties of the structure. Advantages of the proposed method are its simplicity and low cost. To verify the method a simple test structure (a clampedend beam) was instrumented and experimented upon under different loading conditions. The displacements derived with the proposed method show good agreement with displacement measurements performed with a Laser Doppler Vibrometer.

Laser doppler velocimeter

Abstract: A laser Doppler velocimeter in which laser light whose frequency changes continuously at least for a fixed time duration is branched into at least two beams, and the at least two branched beams are respectively transmitted by at least one pair of optical fibers having an optical path difference therebetween, and are focused onto a region to be measured by at least one focusing device. The scattered light of the laser beams focused in the region to be measured are received by a light-receiving device via at least one incident device. At least one Doppler shift frequency in the region to be measured is calculated on the basis of the frequency of a signal of the scattered light received by the light-receiving device, the optical path difference, and a rate of change of the frequency of the laser light. The flow velocity and the direction of the flow velocity are calculated from the calculated Doppler shift frequency.

Process and apparatus for generating at least three laser beams of different wavelength for the display of color video pictures

Abstract: In a process for generating at least three laser beams of different wavelengths for displaying color video pictures, a pulsed laser is used to generate light. The light output of the pulsed laser is released in a pulse and is introduced into a medium with nonlinear optical characteristics for generating a laser beam. In addition to the laser beam generated by excitation, this medium releases an additional laser beam whose frequency is given by the sum or difference frequencies of the exciting laser beam and the excited laser beam due to the nonlinear optical characteristics of the medium. The laser beams generated by the medium in this way and the exciting laser beam are used directly, or after frequency conversion, to display monochromatic partial images of a color video picture. An apparatus according to the invention contains suitable devices for carrying out the process.

Tunable erbium-ytterbium fiber sliding-frequency soliton laser

Abstract: We characterize the soliton-train emission from an Er–Yb-doped fiber loop laser. We discuss the self-starting dynamics and pulse-repetition-rate control in this sliding-frequency soliton laser. We show that the laser truly self-starts after only one cavity round trip. In the steady state the laser emits a closely spaced train of solitons. We also show that the output pulse width may be controlled by the interplay of continuous frequency shifting, bandwidth-limited amplification, and nonlinear polarization rotation of the circulating solitons. The repetition rate is fixed by means of a weak intracavity feedback. The laser is tunable by shifting of the filter wavelength through the whole spectral band of the active fiber.

Amplitude and frequency stability of a diode-pumped Nd:YAG laser operating at a single-frequency continuous-wave output power of 20 W

Abstract: Amplitude and frequency noise of an injection-locked diode-pumped Nd:YAG ring laser operating at a singlefrequency output power of 20 W cw are investigated. This laser system will be used as a light source for an interferometric gravitational-wave detector.

Ultrahighly sensitive laser‐Doppler velocity meter with a diode‐pumped Nd:YVO4 microchip laser

Abstract: Extremely high sensitivity of a diode‐pumped microchip Nd:YVO4 laser to external light injection is demonstrated in optical detection of Doppler shift from a moving object. Intensity modulation induced by reflected scattered light from a rotating object at the Doppler‐shift frequency is measured for a 1‐mm‐long diode‐pumped Nd:YVO4 laser. When adjusting the diode pump power so that the relaxation oscillation frequency is resonant to the Doppler‐shift frequency, the minimum detectable intensity of scattered light injected back into the microchip laser can be <−170 dB weaker than the intracavity laser intensity.

Random pulse burst range-resolved doppler laser radar

Abstract: An laser radar system employing transmitted random and pseudo random optical pulse trains and signal processing that provides unambiguous range and Doppler velocity data. The system comprises a diode-pumped fiber laser local oscillator and an optical random pulse signal generator comprising a mode-locked fiber laser source for generating and transmitted randomly spaced optical pulses. A heterodyne output pulse monitor processes the local oscillator signals and samples the transmitted optical pulses. A heterodyne receiver detector processes the local oscillator signals and received optical pulses reflected from a target. An acoustic charge transport delay line provides a selectable signal delay for the signal provided by the heterodyne optical pulse monitor, and a mixer mixes signals provided by the heterodyne receiver detector and delay line and provides correlation output signals. A Doppler analyzer processes the correlation output signals from the mixer to provide frequency spectrum for each range bin, and an executive processor processes the frequency spectra from the Doppler analyzer to provide a range-resolved Doppler image on the display. The range-resolved Doppler output signals provided by the executive processor allow identification of a wideband weather signature and a narrow band target signature that are displayed on the display. The system employs a single waveform to provide the needed measurements, and eliminates trade-offs between range and velocity ambiguities. The present invention may be employed in vehicular collision avoidance radars and laser sensors, and in adaptive cruise control, robotic vision, and metrology-gauging applications.

Frequency locking of a Nd:YAG laser using the laser itself as the optical phase modulator

Abstract: We report frequency locking of a Nd:YAG tunable laser to a 2000 finesse 87‐cm‐long Fabry–Perot cavity in air using the Pound–Drever technique. The novelty is that the necessary phase modulation of the beam is obtained using the laser directly instead of an external phase modulator (Pockels cell). The spurious relative amplitude modulation using this method was ∼3×10−5 with a modulation index β∼1 and the spectral density of the frequency difference between the laser and the cavity is below 1 mHz/√Hz in the region 1–500 Hz.

Sensitivity enhancement in laser ultrasonics using a versatile laser array system

Abstract: Past investigators have predicted and, in limited cases, demonstrated the potential utility of using temporally and spatially modulated laser array sources as a means of increasing the signal‐to‐noise ratio in laser ultrasonic systems without causing the surface damage characteristic of a single high‐power laser point source. In an effort to develop a practical and flexible laser array source, a laser system has been designed and implemented which uses a single Nd:YAG pulsed laser and an optical delay system in which the laser pulse passes repetitively through a White cell cavity being sampled by a custom beamsplitter after each pass. Up to ten spatially separated light beams exit from the system each time a single laser pulse is introduced. Time separation between the distinct pulses can be adjusted over a range from 28 to 170 ns, corresponding to a pulse repetition rate from 6 to 36 MHz. Since individual control of the beam paths is possible, one has the flexibility to implement either a single‐element, temporally modulated array, or a multielement, ‘‘phased’’ array. From a single point, a multiple spike narrow‐band acoustic signal is generated, permitting greater detection sensitivity than can be obtained with broadband detection of a single spike. Alternatively, multielement arrays can be used to increase the acoustic signal amplitude by superposition of the signals generated from each individual element, once again enhancing detection sensitivity.

Optical measurement method and apparatus which determine a condition based on quasi-elastic-interaction

Abstract: A method and an apparatus are provided for the determination of a condition or state of an object based on quasi-elastic interaction between the object and light transmitted to the object. This light is transmitted from a light source through a diffractive optical element. The light that has interacted with the object is collected and detected. The diffractive optical element is designed in such a way that the determination of the condition or state of the object is substantially exclusively defined by the diffractive optical element and substantially independent of properties of the light source. Several diffraction patterns may be integrated in one diffractive optical element, thereby integrating several optical functions, such as lenses, beam splitters, etc. in one optical component. Use: Transit-time-velocity measurement, Doppler velocity measurement, viscoelastic measurement, differential speckle determination, differential vibrometer, distance determination apparatus, etc.

Laser vibrometry measurements of rotating blade vibrations

Abstract: One of the most important design factors in modern turbomachinery is the vibration of turbomachinery blading. There is a need for developing an in-service, noncontacting, noninterfering method for the measurement and monitoring of gas turbine, jet engine, and steam turbine blade vibrations and stresses. Such a technique would also be useful for monitoring rotating helicopter blades. In the power generation industry, blade failures can result in millions of dollars of downtime. The measurement of blade vibrations and dynamic stresses is an important guide for preventive maintenance, which can be a major contributor to the availability of steam turbine, gas turbine, and helicopter operations. An experiment is designed to verify the feasibility of such a vibration monitoring system using the reference beam on-axis laser-Doppler technique. The experimental setup consists of two flat, cantilever blades mounted on a hub attached to the shaft of a dc motor. The motor rests on a linear bearing permitting motion only in the direction of the motor shaft. The motor and blade assembly is then excited via an electrodynamic shaker at the first natural frequency of the blades. The resulting blade vibration is then detected using a laser vibrometer. The vibration frequencies and amplitudes of the two rotating blades are successfully measured.

Cavity dumped laser shock peening process

Abstract: A laser shock peening apparatus includes a cavity dumping laser oscillator including in optical serial alignment a first mirror, a Pockels cell, a polarizer, a laser rod, and a second mirror defining a cavity having an optical length between the mirrors. The laser rod is optically pumped to generate a laser beam in the cavity, and the Pockels cell has selective wave retardation to allow the beam to oscillate between the mirrors and increase energy therein, followed in turn by dumping the beam into a laser amplifier. The laser amplifier directs the amplified pulse at a target for laser shock peening thereof. The cavity length is selected to develop a substantially square-wave laser pulse for temporally shaping the resulting pressure pulse at the target.

Quasi-three-level room-temperature Nd:YAG ring laser with high single-frequency output power at 946 nm.

Abstract: Efficient room-temperature operation of a diode-pumped Nd:YAG laser is demonstrated for the quasi-three-level transition at 946 nm. Continuous-wave output powers of more than 800 mW cw in single-frequency operation are generated by application of a composite-cavity nonplanar ring laser. High amplitude and frequency stability of the emitted radiation is observed.

Laser control arrangement for tuning a laser

Abstract: In a laser control arrangement for tuning a laser a portion of the optical signal exiting a resonator is directed to the input of an optical frequency routing device. The routing device has a plurality of outputs each for receiving one of the frequencies supplied by the laser. Each output is coupled to a photodetector. When the laser emits a frequency of light corresponding to one of the resonant frequencies of the resonator, the resonant frequency will appear on one of the outputs of the routing device and will be detected by the photodetector coupled to that output.

Laser beam projection apparatus

Abstract: A laser beam projection apparatus is capable of supplying laser beams finely converged on an object at all times irrespective of a distance to the object. The laser beam projection apparatus has a laser beam generator, including a focus adjusting device, for supplying laser beams periodically swept within one plane and, at the same time, converged at a controllable distance, a light receiving element for receiving reflected laser beams from a predetermined object and generating an output signal, a distance calculating unit for calculating a distance to the object on the basis of the output signal and a controller for operating the focus adjusting device so that the swept laser beams are converged in a position corresponding to the calculated distance. The controller can change a sweep speed of the laser beam in accordance with a change in the calculated distance. The controller cna change a sweep speed of the laser beam in accordance with a change in the calculated distance.

Optical fiber accelerometer based on a silicon micromachined cantilever.

Abstract: An intensity-modulated fiber-optic accelerometer based on backreflection effects has been manufactured and tested. It uses a multimode fiber placed at a spherical mirror center, and the beam intensity is modulated by a micromachined silicon cantilever. This device has applications as an accelerometer and vibrometer for rotating machines. It exhibits an amplitude linearity of ±1.2% in the range of 0.1–22 m s−2, a frequency linearity of ±1% in the range of 0–100 Hz, and a temperature sensitivity of less than 0.03%/°C. The sensor is devoted to a wavelength-division multiplexing network.

Monitoring and adjusting laser write power in an optical disk recorder using pulse-width modulated power level checking signals

Abstract: A write-once optical disk data recorder automatically calibrates a laser during a write data operation and using write pulses focussed to a disk. First, the laser is calibrated using a non-focussed laser beam. Each sector of the disk has a laser checking or test area, such as an automatic laser power correction field (ALPC) of two byte lineal extent. During a first write operation after a power up or disk load, a pulse width modulated (PWM) laser test signal is recorded using a laser power level set using the non-focussed laser beam and an indicated desired recording power level on the disk. The recorded laser test signal is read back. The length of the read back laser test signal is measured. The measured length is then compared with a desired length of the PWM laser test signal that indicates a desired laser power level. That is, as laser recording power levels increase, a resultant recorded signal grows in size. This property is used to measure laser power for calibrating laser operation.

Method and device for measuring physical quantity, method for fabricating semiconductor device, and method and device for measuring wavelength

Abstract: Pulsed laser beams are applied to an object to be measured. A first laser beam of a pulsed laser beam having a first wavelength which is oscillated immediately after the rise of the pulsed laser beam, and a second laser beam having a second wavelength which is oscillated thereafter are used. Based on a difference between an intensity of first interfered light of reflected light of the first laser beam or transmitted light thereof, and an intensity of reflected light of the second laser beam or transmitted light thereof, temperatures of the object to be measured, and whether the temperatures are on increase or on decrease are judged. The method and device can be realized by simple structures and can measure a direction of changes of the physical quantities.

Means for controlling the power output of laser diodes in a ROS system

Abstract: The present invention provides a means and method for efficiently and automatically monitoring and adjusting the power output of either a single or a dual beam laser diode array used to provide the scanning beams in a ROS system. Collimated light outputs from a collimator lens assembly following either a single or dual beam laser are reflected along an optical path to impinge on one or two photodiodes positioned so as to intercept the reflected light. The photodiode output is proportional to the power output of the laser diode associated with the photodiode. In one multi-beam embodiment, polarization of the laser diode output is controlled so as to ensure that only outputs from an associated laser diode is sensed by an associated photodiode. In another embodiment, a half wave plate is used to rotate the polarization of one of the laser diodes.

Laser interferometry principles

Abstract: Basic theory of interference interference in laser radiation principles of constructing laser interferometers modulation laser interferometers electron processors in laser interferometers laser doppler anemometry fiber-optic laser interferometers and holographic interferometry.

Method and apparatus for analyzing the fill characteristics of a packaging container

Abstract: A system for analyzing the fill characteristics of a container. A container having a filling material therein is positioned adjacent a sound generator. Sound waves from the generator are applied to the container, causing it to vibrate. A vibration detector is used to determine the amount of container vibration. A preferred vibration detector involves a laser vibrometer which applies a reference laser beam to the vibrating container. The reference beam is reflected off of the container to generate a reflected laser beam. The reflected beam experiences a Doppler frequency shift compared with the reference beam which is caused by container vibration. The Doppler shift of the reflected beam is then compared with standardized Doppler shift data from a control container. Repeated Doppler shift measurements may also be undertaken which are converted into a vibration profile that is compared with a standardized vibration profile from a control container.

Apparatus for laser processing and monitoring

Abstract: A laser processing apparatus includes a laser beam source, a focusing optical system for focusing a laser beam from the source as a processing laser beam to a surface of an article, an illuminating optical system using a laser beam of the same frequency as that of the processing laser beam as an illumination laser beam for illuminating the surface of the article, and a photographing device for detecting the illumination laser beam reflected from the article through the focusing optical system to thereby monitor a position to be processed. Since the laser beam of the illuminating optical system is of the same frequency as that of the processing beam chromatic aberration is prevented. This allows increased monitoring accuracy so that an article such as copper foil can be processed by a laser beam with an accuracy of 10 microns to monitor and correct a processing position.