Showing papers on "Laser Doppler vibrometer published in 2002"

Laser Doppler perfusion imaging with a complimentary metal oxide semiconductor image sensor

Abstract: We utilized a complimentary metal oxide semiconductor video camera for fast f low imaging with the laser Doppler technique. A single sensor is used for both observation of the area of interest and measurements of the interference signal caused by dynamic light scattering from moving particles inside scattering objects. In particular, we demonstrate the possibility of imaging the distribution of the moving red blood cell concentration. This is a first step toward laser Doppler imaging without scanning parts, leading to a much faster imaging procedure than with existing mechanical laser Doppler perfusion imagers.

Control of the frequency comb from a modelocked Erbium-doped fiber laser.

Abstract: We show that a stretched-pulse mode-locked fiber laser produces a well-defined frequency comb, providing a compact source of frequency combs and allowing comb-based optical frequency metrology to be extended into the 1.55 μm region. This is achieved by comparing the frequency doubled output of the fiber laser to that of a mode-locked Ti:Sapphire laser, after the two lasers are synchronized. The offset frequency of the fiber laser frequency comb is found to be highly sensitive to the pump power, which enables the implementation of a feedback loop to control the offset frequency. The resulting RMS frequency jitter of the heterodyne beat signal is 355 kHz (0.5 Hz – 102 kHz BW) for this initial demonstration.

Remote measurement of material properties from radiation force induced vibration of an embedded sphere

Abstract: A quantitative model is presented for a sphere vibrated by two ultrasound beams of frequency ω1 and ω2. Due to the interference of two sound beams, the radiation force has a dynamic component of frequency ω2−ω1. The radiation impedance and mechanical impedance of the sphere are then used to compute the vibration speed of the sphere. Vibration speed versus vibration frequency is measured by laser vibrometer on several spheres, both in water and in gel phantom. These experimental results are used to verify the model. This method can be used to estimate the material properties of the medium (e.g., shear modulus) surrounding the sphere.

Acoustic chaos and sonic infrared imaging

Abstract: We describe a quasichaotic mechanism for the generation of complex vibrations from the application of a 450 ms, 40 kHz excitation coupled nonlinearly to a sample under inspection by means of sonic infrared (SIR) imaging. Monitoring the sample vibration by means of a laser vibrometer, we find that the vibration switches from the fundamental and harmonics to several series of frequencies that are rational fractions of the fundamental driving frequency. The transition to this quasichaotic behavior is important in obtaining high-quality SIR images.

Building blocks for a two-frequency laser lidar-radar: a preliminary study

Abstract: A new principle of lidar-radar is theoretically and experimentally investigated. The proposed architecture is based on the use of an rf modulation of the emitted light beam and a direct detection of the backscattered intensity. Use of a radar-processing chain allows one to obtain range and Doppler measurements with the advantages of lidar spatial resolution. We calculate the maximum range of this device, taking into account different possible improvements. In particular, we show that use of a pulsed two-frequency laser and a spatially multimode optical preamplification of the backscattered light leads to calculated ranges larger than 20 km, including the possibility of both range and Doppler measurements. The building blocks of this lidar-radar are tested experimentally: The radar processing of an rf-modulated backscattered cw laser beam is demonstrated at 532 nm, illustrating the Doppler and identification capabilities of the system. In addition, signal-to-noise ratio improvement by optical preamplification is demonstrated at 1.06 µm. Finally, a two-frequency passively Q-switched Nd:YAG laser is developed. This laser then permits two-frequency pulses with tunable pulse duration (from 18 to 240 ns) and beat frequency (from 0 to 2.65 GHz) to be obtained.

Apparatus and method for phase control of tunable external cavity lasers

Abstract: An apparatus and method for controlling the phase of a tunable laser is provided. Stabilization of the mode of a laser beam is provided as the laser is tuned to a target frequency. For one embodiment, a laser generates a reference beam and an output beam. The power of each of beam is measured by optical detectors, and a ratio thereof is utilized to detect when a mode hop occurs as the laser is coarsely tuned. The average of the pre and post mode hop ratios is utilized as a control setpoint while finely tuning the laser to the target frequency. Wavelength lockers, optical power dividers and optical detectors are utilized to determine power levels of the reference and output beams while also monitoring frequency characteristics thereof. A control unit utilizes the outputs from the wavelength locker to control the operation of the extended cavity laser during and after tuning.

Laser machining method for precision machining

Abstract: A laser machining method and apparatus performs high-speed and high-precision machining on a thin film and a liquid crystal panel. The apparatus includes: a plurality of pulse laser generators, drivers for alternately driving a plurality of pulse laser generators to generate laser beams delayed in phase between each other; collimators for making the generated laser beams equal in quality; a wave plate for converting the laser beams into elliptically polarized laser beams; and a phase grating for dividing each laser beam emitted by each of the plurality of laser beam generators into a plurality of laser beams; such that an object to be processed is selectively illuminated with the plurality of laser beams produced by the phase grating. The phase grating has structure on its surface for dividing each of the laser beams into equal intensity beams. Open grooves are formed having uniform shape and quality.

Pico-watt and femto-watt weak-light phase locking

Abstract: Advances in laser physics and its applications have triggered the proposition and development of Laser Astrodynamics. In carrying out research projects on Laser Space Programs, it is necessary to process the laser signal sent back from remote spacecraft. After traveling an extremely long distance, the power of this signal is greatly reduced. Weak-light phase-locking is the key technique used for signal amplification in these space projects. After the returning laser beam is collected by telescope, it is used to phase-lock a local laser oscillator. The local laser then carries the phase information of the remote spacecraft laser. we used diode-pumped non-planar ring cavity Nd:YAG lasers to serve as the remote weak-light laser and the local strong-light laser. We then built an optical phase-locked loop to phase-lock them. The weak-light laser signal was simulated using ND (neutral density)-filters to decrease the light intensity. In the phase detection, we used balanced detection to eliminate laser intensity noise and improve the S/N ratio. Combining this with an appropriate loop filter, we were able to control the laser frequency and improve the phase-locking ability. We phase-locked a 2 nW weak-light beam and a 2 mW strong-light beam with a 57 mrads(rms)phase error. The locking duration was very long. Locking of a 200 pW and a 2007thinsp;μW light beam, with phase error of 200 mrad (rms) and duration of over 2 hours was achieved. The phase error for locking a 200 μW to a 20pW light beam was 160 mrad (rms). The locking duration was also longer than 2 hours. the last locking performed was carried out with a 2 pW and a 200 μW light beam. The phase error and the locking duration were 290 mrad(rms) and 1.5 min respectively.

Absolute distance interferometer with grating-stabilized tunable diode laser at 633 nm

Abstract: In this paper an absolute distance interferometer for measurements in the range of several metres is presented. A grating-stabilized tunable extended-cavity diode laser without AR coating and a stabilized helium–neon laser are used to generate a variable synthetic wavelength. The phase distortion of analogue filters in the presence of non-linear laser tuning is discussed. A linearization of the laser frequency course and a suppression of sound-induced spectral line-broadening is achieved by electronic feedback from a homodyne fibre interferometer. The first experimental distance measuring results are shown.

Disc drive servo track writer gas leak detector and method

Abstract: A method of detecting a leak of low-density gas from an enclosure containing a rotating storage disc includes monitoring a signal which varies in response to variations in the concentration of the low-density gas in the enclosure. Additional quantities of the low-density gas are added to the enclosure when the monitored signal indicates that the concentration has dropped below a predetermined level. Monitoring the concentration of the low-density gas may include monitoring actual concentration levels of the gas using a pressure sensor located within the enclosure or monitoring disc mode vibration amplitudes using an accelerometer attached to the enclosure. Indirect monitoring of the vibration amplitudes may also be accomplished using sensors such as a laser Doppler vibrometer, a capacitance probe or the servo head position feedback circuitry found within a disc drive or a dedicated servo track writer.

Laser Doppler velocimetry by optical self-mixing in vertical-cavity surface-emitting lasers

Abstract: In this letter, we analyze experimentally the responsivity of vertical-cavity surface-emitting lasers (VCSELs) to backscattered Doppler radiation. Their responsivity is strongly enhanced by at least 10 dB with the selection of one polarization state in the detection beam and by the presence in VCSELs of polarization bistability. When the VCSELs are operated inside the bistable region, they emit square wave pulses at the Doppler shift frequency in both polarization states. When they are operated outside the bistable region, they emit the normal saw-tooth pulses in both polarization states. These unique features, absent in conventional edge-emitting lasers, make them suitable for new applications in laser Doppler velocimetry (LDV).

Glass-fiber self-mixing intra-arterial laser Doppler velocimetry: signal stability and feedback analysis

Abstract: We have developed a blood velocimeter based on the principle of self-mixing in a semiconductor laser diode through an optical fiber. The intensity of the light is modulated by feedback from moving scattering particles that contain the Doppler-shift frequency. Upon feedback the characteristics of the laser diode change. The threshold current decreases, and an instable region may become present above the new threshold. The amplitude of the Doppler signal turns out to be related to the difference in intensity between situations with and without feedback. This amplitude is highest just above feedback. The suppression of reflection from the glass-fiber facets is of paramount importance in the obtaining of a higher signal-to-noise ratio. Using an optical stabilization of the feedback, we optimized the performance of the laser-fiber system and the Doppler modulation depth and clarified its behavior with a suitable physical model. We also investigated the effect of the finite coherence length of the laser. We tested the efficiency of the self-mixing velocimeter in vivo with the optical glass fiber inserted in the artery with endoscopic catheters, both in upstream and in downstream blood flow conditions. For the latter we used a special side-reflecting device solution for the fiber facet to allow downstream measurements.

Frequency shift in a fiber laser resonator

Abstract: A new model that describes the influence of a laser intracavity frequency shifter is proposed. Laser line broadening that is due to the presence of the frequency shift is derived. The theoretical model is validated by experimental results. Multiwavelength operation of this laser is explained.

NDE of smart structures using multimode fiber optic vibration sensor

Abstract: There are many conventional methods which may be used to monitor the health of structures — of which vibration monitoring is effective and can be used for special structures such as aircraft wings. A smart structure system based on fiber optic vibration sensors has been developed to monitor structural damage. A method based on the detection of spatial speckle of a multimode optical fiber is described. A multimode optical fiber with a diameter of 200/230 μm is used in the present experiment. The theoretical analysis and the experimental tests are described. The fiber optic sensors have been embedded in the carbon/epoxy composite specimens and surface mounted on the surface of aluminum specimens. Furthermore, experiments to examine the different types of damage to these specimens have been carried out. The results show that this kind of multimode fiber optic sensor can be used in the non-destructive evaluation of smart materials.

Comparison between Accelerometer and Laser Vibrometer to Measure Traffic Excited Vibrations on Bridges

Abstract: The use of accelerometer based measurement techniques for evaluating bridge forced vibrations or to perform bridge modal analysis is well established. It is well known to all researchers who have experience in vibration measurements that values of acceleration amplitude can be very low at low frequencies and that a limitation to the use of accelerometer can be due to the threshold parameter of this kind of transducer. Under this conditions the measurement of displacement seems more appropriate. On the other hand laser vibrometer systems detect relative displacements as opposed to the absolute measures of accelerometers. Vibrations have been measured simultaneously by a typical accelerometer for civil structures and by a laser vibrometer equipped with a fringe counter board in terms of velocity and displacements. The accelerations calculated from the laser vibrometer signals and the one directly measured by the accelerometer has been compared.

Laser alignment tool

Abstract: A laser alignment tool includes a line laser assembly including a laser and emitting a laser beam line and an electronic target assembly including a photo detector array. The laser beam line of the laser is pointed toward the center of the photo detector array with the longitudinal axis of the laser beam line being perpendicular to the centerline of the photo detector array. The line laser assembly includes a laser diode having a line-generating lens attached in front using a lens holder and being mounted onto a tilt stage. Using a line laser provides a certified accuracy of +/−0.005 inches at a distance of 10 feet. The electronic target further includes a narrow band pass filter and a discrete LED display that is controlled by a microprocessor. The microprocessor may be programmed with a variable range for each LED of the discrete LED display according to the alignment task.

Automotive components vibration measurements by tracking laser Doppler vibrometry: advances in signal processing

Abstract: This paper describes the application of a tracking laser Doppler vibrometer (TLDV) for vibration measurements on automotive components and the advances in data processing. A Lagrangian approach is adopted: the target point moves continuously and it is tracked by the measurement laser spot, thus no relative velocity between the target and the laser spot exists. By eliminating the relative motion of the laser beam on the surface, speckle noise is drastically reduced. Measuring on a moving reference frame opens an innovative way to look at automotive component vibrations, because the space-time evolution of the vibration can be measured under simulated operative conditions. After a presentation and discussion of the measurement technique, attention is focused on three different applications in the automotive field: the side-wall vibration of a rolling tyre, the out-of-plane vibration of a timing belt and the rubber blade vibration of complex motion windscreen wipers. The development of optimized TLDV measurement systems for each specific application is shown and some results are presented. Vibration data have been analysed in both the time and frequency domains. Moreover, in order to point out time-dependent frequency features in signals, vibration data have been further processed by means of joint time-frequency algorithms.

Broadband vibration measurements using a continuously scanning laser vibrometer

Abstract: In this paper, a novel methodology is introduced to measure the vibration behaviour of structures using a scanning laser Doppler vibrometer. The laser beam continuously scans the surface of the device under test, while a broadband excitation signal is applied. In particular, the method developed adapts itself automatically to the (spatial) complexity of the vibration patterns and to the (frequency domain) modal density. The measurement time is compared with classical measurement procedures.

Dual-stage servo with on-slider PZT microactuator for hard disk drives

Abstract: A dual-stage servo system with an on-slider piezoelectric (PZT) microactuator is developed for future high-density hard disk drives. The parallel control scheme is used in servo design with considerations over the stroke limitation and the hysteresis of the microactuator. Experimental results on a spin-stand with a laser Doppler vibrometer as the position sensor demonstrate that the system has a servo bandwidth higher than 4 kHz and has disturbance rejection capability up to near 5 kHz.

Laser annealing device and thin-film transistor manufacturing method

Abstract: A laser annealing device (10) comprises a laser beam oscillator (12) for emitting pulse laser beam of predetermined cycle, and an irradiation optical system (15) for applying pulse laser beam to an amorphous silicon film (1). The irradiation optical system (15) controls a laser spot to move so that the pulse light is applied onto the same position on the amorphous silicon film (1) a plurality of times. The laser beam oscillator (12) emits laser beam of the pulse generation cycle shorter than the reference cycle. The reference cycle is the time interval from the timing of laser beam emission to the timing when the substrate temperature which has been raised by the irradiation of the laser beam is returned to the original substrate temperature by one pulse laser beam applied onto the surface of the film (1).

Vibration and noise reduction of an optical disk drive by using a vibration absorber

Abstract: Vibration and noise reduction of an optical disk drive by using a vibration absorber is presented in this paper In optical disk drives such as CD-ROM or DVD drives, vibration may become one of serious problems to be resolved as the rotating speed and the storage capacity increase It is practically difficult to overcome the off-track and off-focusing errors by only the servo control without reducing vibration In particular, when an optical disk drive has a wavy spinning disk, it may suffer from severe vibration Based on the vibration signals measured by a laser vibrometer and an accelerometer, the resonance frequency of the disk drive is identified For reduction of the vibration around the resonance frequency, a vibration absorber is designed and it is equipped on the feeding deck of the drive The proposed vibration absorber consists of a steel plate ring and a rubber bobbin, which play roles of inertia mass and a spring, respectively During the design procedure of the absorber, a finite element model is used to determine the required fundamental natural frequency of the absorber The test results of vibration and noise after being equipped with the absorber show that vibration and noise of the optical disk drive are considerably reduced

Laser beam multimeter

Abstract: The objective of the present invention is providing a method and a simple instrument that can be used on a routine basis to accurately and quickly measure the focus position, waist radius, divergence, quality, power and power density of a laser beam. The measurement is performed by scanning a thin film of a nonlinear optical material in the focal region along the propagation direction of the beam and registering the variation of the on-axis intensity of the laser beam by a photodetector.

Three Component Planar Doppler Velocimetry Using Imaging Fibre Bundles

Abstract: This paper describes a planar Doppler velocimetry (PDV) technique that is capable of measuring the three, instantaneous and time average components of velocity over two dimensions using a single pair of signal and reference cameras. PDV can be used to measure the instantaneous 3-D velocity of a fluid by using an absorption line filter to determine the Doppler shifted frequency of a narrow line-width, pulsed laser that has been scattered from particles seeded into the flow. In the technique presented here the three views required to obtain three component velocity information are guided from the collection optics to a single imaging plane using flexible fiber imaging bundles. These are made up of a coherent array of single fibers and are combined at one end as the input plane to the measurement head. A fourth leg of the imaging fiber bundle is used to image the individual laser pulses and allow correction for pulse-to-pulse frequency variations. As part of the development phase of the technique measurements are presented of the velocity field of a rotating disk using both an Argon Ion laser giving time average results and a frequency doubled Nd:YAG laser to give an instantaneous result. A typical vector map of the computed velocity is shown in Figure 1. In this figure vectors are shown at only 1 in 10 data points.