Showing papers on "Laser Doppler vibrometer published in 2014"

Laser Doppler and Phase Doppler Measurement Techniques

Abstract: Preface.- Introduction.- Basic Measuring Principles.- Fundamentals of Light Propagation and Optics.- Light Scattering from Small Particles.- Signal Generation in Laser Doppler and Phase Doppler Systems.- Signal Detection, Processing and Validation.- Laser Doppler Systems.- Phase Doppler Systems.- Further Particle Sizing Methods Based on the Laser Doppler Technique.- Fundamentals of Data Processing.- Processing of Laser Doppler Data.- Processing of Phase Doppler Data.- Choice of Particles and Particle Generation.- System Design Considerations.- Appendix.- Bibliography.- Index.

Noncontact detection of fatigue cracks by laser nonlinear wave modulation spectroscopy (LNWMS)

Abstract: Nonlinear wave modulation spectroscopy (NWMS) has been used to detect nonlinear ultrasonic signatures produced by fatigue cracks in materials. It is done by generating ultrasonic waves at two different frequencies and measuring their modulation. A proper choice of two distinct frequencies for a given structure plays a significant role in NWMS. This paper, instead of using input signals at two distinct frequencies, takes only one broadband pulse signal as the driving input, which can be generated by a laser beam. With a broadband excitation, material nonlinearity exhibits modulation at multiple peaks in a spectral plot due to interactions among various input frequency components of the broadband input. A feature called sideband peak count (SPC), which is defined as the ratio of the number of sideband (modulation) peaks over a moving threshold to the total peak number in the specified frequency band, is extracted from the spectral plot to measure the degree of material nonlinearity. The basic premise of the proposed laser nonlinear wave modulation spectroscopy (LNWMS) is that this SPC value will rise as the level of material nonlinearity increases. A noncontact laser ultrasonic system has been built for LNWMS measurement by integrating and synchronizing a Q-switched Nd:YAG laser for ultrasonic wave generation and a laser Doppler vibrometer for ultrasonic wave detection. The proposed LNWMS technique has been successfully tested for detecting fatigue cracks in metallic plates and aircraft fitting-lugs having complex geometries.

Laser printing system

Abstract: The invention describes a laser printing system (100) for illuminating an object moving relative to a laser module of the laser printing system (100) in a working plane (180), the laser module comprising at least two laser arrays of semiconductor lasers and at least one optical element, wherein the optical element is adapted to image laser light emitted by the laser arrays, such that laser light of semiconductor lasers of one laser array is imaged to one pixel in the working plane of the laser printing system, and wherein the laser printing system is a 3D printing system for additive manufacturing and wherein two, three, four or a multitude of laser modules (201, 202) are provided, which are arranged in columns (cl, c2) perpendicular to a direction of movement (250) of the object in the working plane (180), and wherein the columns are staggered with respect to each other such that a first laser module (201) of a first column of laser modules (cl) is adapted to illuminate a first area (yl) of the object and a second laser module (202) of a second column (c2) of laser modules is adapted to illuminate a second area (y2) of the object, wherein the first area (yl) is adjacent to the second area (y2) such that continuous illumination of the object is enabled

Structural Modal Identification Through High Speed Camera Video: Motion Magnification

Abstract: Video cameras offer the unique capability of collecting high density spatial data from a distant scene of interest They could be employed as remote monitoring or inspection sensors because of their commonplace use, simplicity, and relatively low cost The difficulty is in interpreting the video data into a usable format that is familiar to engineers such as displacement A methodology called motion magnification, developed for visualizing exaggerated versions of small displacements, is extended to modal identification in structures Experiments in a laboratory setting on a cantilever beam were performed to verify the method against accelerometer and laser vibrometer measurements Motion magnification is used for modal analysis of cantilever beams to visualize mode shapes and calculate mode shape curvature as a basis for damage detection Suggestions for applications of this methodology and challenges in real-world implementations are given

Improved acoustic coupling of air-coupled micromachined ultrasonic transducers

Abstract: Phased array imaging with micromachined ultrasound transducer (MUT) arrays is widely used in applications such as ranging, medical imaging, and gesture recognition. In a phased array, the maximum spacing between elements must be less than half of the wavelength to avoid large sidelobes. This places a limit on the maximum transducer size which is not attractive since the acoustic coupling drops rapidly for MUT diameters less than a wavelength. Here, we present a new approach to increase the acoustic coupling of small radius MUTs using an impedance matching resonant tube etched beneath the MUT. Impedance, laser Doppler vibrometer (LDV), and acoustic burst measurements confirm a 350% increase in SPL and 8x higher bandwidth compared to transducers without the impedance matching tube, enabling compact arrays with high fill-factor and efficiency.

Experimental study of the oscillation of spheres in an acoustic levitator

Abstract: The spontaneous oscillation of solid spheres in a single-axis acoustic levitator is experimentally investigated by using a high speed camera to record the position of the levitated sphere as a function of time. The oscillations in the axial and radial directions are systematically studied by changing the sphere density and the acoustic pressure amplitude. In order to interpret the experimental results, a simple model based on a spring-mass system is applied in the analysis of the sphere oscillatory behavior. This model requires the knowledge of the acoustic pressure distribution, which was obtained numerically by using a linear finite element method (FEM). Additionally, the linear acoustic pressure distribution obtained by FEM was compared with that measured with a laser Doppler vibrometer. The comparison between numerical and experimental pressure distributions shows good agreement for low values of pressure amplitude. When the pressure amplitude is increased, the acoustic pressure distribution becomes nonlinear, producing harmonics of the fundamental frequency. The experimental results of the spheres oscillations for low pressure amplitudes are consistent with the results predicted by the simple model based on a spring-mass system.

Runout Tracking in Electric Motors Using Self-Mixing Interferometry

Abstract: In this paper, a self-mixing interferometry sensor has been used as a proximity probe to measure possible runout in permanent magnet synchronous motors, for fault diagnosis. A general procedure for the measurement of the 2-D trajectory of the motor shaft is described in detail, including procedures for the characterization of the uncertainty due to the shape of the shaft, and the management of speckle noise. The performance of the proposed sensor has been compared to that of a commercial Polytec laser vibrometer, for validation purposes. Results show inaccuracies in the order of ±6 μm, which agree well with the measured uncertainty introduced by shaft surface imperfections.

Nonlinear characterization of a single-axis acoustic levitator.

Abstract: The nonlinear behavior of a 20.3 kHz single-axis acoustic levitator formed by a Langevin transducer with a concave radiating surface and a concave reflector is experimentally investigated. In this study, a laser Doppler vibrometer is applied to measure the nonlinear sound field in the air gap between the transducer and the reflector. Additionally, an electronic balance is used in the measurement of the acoustic radiation force on the reflector as a function of the distance between the transducer and the reflector. The experimental results show some effects that cannot be described by the linear acoustic theory, such as the jump phenomenon, harmonic generation, and the hysteresis effect. The influence of these nonlinear effects on the acoustic levitation of small particles is discussed.

Holographic laser Doppler imaging of microvascular blood flow.

Abstract: We report on local superficial blood flow monitoring in biological tissue from laser Doppler holographic imaging. In time-averaging recording conditions, holography acts as a narrowband bandpass filter, which, combined with a frequency-shifted reference beam, permits frequency-selective imaging in the radiofrequency range. These Doppler images are acquired with an off-axis Mach–Zehnder interferometer. Microvascular hemodynamic components mapping is performed in the cerebral cortex of the mouse and the eye fundus of the rat with near-infrared laser light without any exogenous marker. These measures are made from a basic inverse-method analysis of local first-order optical fluctuation spectra at low radiofrequencies, from 0 Hz to 100 kHz. Local quadratic velocity is derived from Doppler broadenings induced by fluid flows, with elementary diffusing wave spectroscopy formalism in backscattering configuration. We demonstrate quadratic mean velocity assessment in the 0.1-10 mm/s range in vitro and imaging of superficial blood perfusion with a spatial resolution of about 10 micrometers in rodent models of cortical and retinal blood flow.

Feasibility of Using a Seismic Surface Wave Method to Study Seasonal and Weather Effects on Shallow Surface Soils

Abstract: The objective of this paper is to study the feasibility of using a seismic surface wave method to investigate seasonal and weather effects on shallow surface soils. In the study, temporal variations of subsurface soil properties were measured and monitored by using a combination of a new seismic surface method and an existing acoustic probe system. A laser Doppler vibrometer (LDV)-based multi-channel analysis of surface wave (MASW) method is developed to measure the soil profile in terms of shear wave velocity at depths up to a few meters. While the conventional MASW uses geophones as surface vibration sensors, the present MASW uses a moving LDV as a non-contact sensor to detect Rayleigh wave propagation generated by an electromagnetic shaker operating in chirp mode. A long-term survey was conducted by measuring the S-wave velocity profile with the MASW along with measurements of soil temperature, water content, water potential, and P-wave velocity with the acoustic probe system. The results from the two methods are compared and interpreted with the concept of the effective stress, governed by soil suction stress for top unsaturated soils and by overburden pressure for deeper layers of soils. This study demonstrates that the S-wave velocity images obtained from the LDV-MASW method can effectively monitor and visualize the temporal and spatial variations of subsurface soil properties caused by seasonal and weather effects.

A simple laser system for atom interferometry

Abstract: We present here a simple laser system for a laser-cooled atom interferometer, where all functions (laser cooling, interferometry and detection) are realized using only two extended cavity laser diodes, amplified by a common tapered amplifier. One laser is locked by frequency modulation transfer spectroscopy, the other being phase locked with an offset frequency determined by an field-programmable gate array-controlled direct digital synthesizer, which allows for efficient and versatile tuning of the laser frequency. Raman lasers are obtained with a double pass acoustooptic modulator. We demonstrate a gravimeter using this laser system, with performances close to the state of the art.

Optical three-dimensional vibrometer microscope with picometer-resolution in x, y, and z

Abstract: The state-of-the-art technique for optical vibration analysis of macroscopic structures is laser-Doppler vibrometry in which a single-laser beam measures the motion in the beam direction. Thus, three laser beams are necessary to investigate three-dimensional(3-D) motions. The laser spots canbe separated on macroscopicspec- imens with scattering surfaces to prevent optical crosstalk between the measurement beams, but such separation is impossible for a microscopic scatter point. We demonstrate a solution for this problem: an optical 3-D vibrometer microscope with a single-impinging laser beam, which collects scattered light from at least three directions. We prove that it is possible to realize a small laser focus of <3.5-μm diameter on a proper scatter point such as an etch hole of a microelectromechanical-systems device to obtain real-time, 3-D vibration measurements with megahertz vibration bandwidth and picometer amplitude resolution. A first measurement of operational-deflection shapes is presented. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in

Method for monitoring the energy density of a laser beam by image analysis and corresponding device

Abstract: The invention relates to a method for monitoring (S) the energy density of a laser beam (3) using parameters of the laser beam (3) including the following steps: regularly applying the laser beam (3) to a reference substrate and measuring (S4), with each application, the resulting light intensity; identifying (S6, S7, S8) a change in the light intensity on the reference substrate between at least two measurements; and, when the change in the light intensity is higher than a predetermined threshold, determining the unstable parameter or parameters of the energy density of the laser beam (3) (S8).

Single-frequency polarized eye-safe all-fiber laser with peak power over kilowatt

Abstract: An all-fiber, single-frequency, linearly polarized, high peak-power, pulsed laser at 1,540 nm for Doppler wind lidar is presented. This laser is composed of a single-frequency, narrow-linewidth external cavity diode laser, and multistage fiber amplifiers. A peak power of 1.08 kW and a pulse width of 500 ns at 10 kHz repetition rate are achieved, which is the highest peak power with a linewidth of 800 kHz in erbium-doped silica fiber to our knowledge. The beam quality of M 2 < 1.3 and a polarization extinction ratio over 16 dB are obtained. This laser will be employed in a compact long-range coherent Doppler wind lidar.

Compact Stabilized Semiconductor Laser for Frequency Metrology

Abstract: We report on the development of a frequency modulatable 795 nm semiconductor laser based on self-injection locking to a high quality factor whispering gallery mode microresonator. The laser is characterized with residual amplitude modulation below -80 dB and frequency noise better than 300 Hz/Hz^(1/2) at offset frequencies ranging from 100 Hz to 10 MHz. The frequency modulation (FM) speed and span of the laser exceed 1 MHz and 4 GHz, respectively. Locking of the laser to Doppler-free saturated absorption resonance of 87Rb D1 line is demonstrated and frequency stability below 10^(-12) is measured for integration time spanning from 1 s to 1 day. The architecture demonstrated in this study is suitable for realization of frequency modulatable lasers at any wavelength.

3-D Vibration Measurement Using a Single Laser Scanning Vibrometer by Moving to Three Different Locations

Abstract: 3-D vibration measurement is achieved using a single laser scanning vibrometer (LSV) and laser scanner (LS) by moving them to three arbitrarily different locations from the principle that vibration analysis based on the frequency domain is independent of the vibration signal based on time domain. The proposed system has the same effect as using three sets of LSVs, and has an advantage of reducing equipment costs. Analytical approach of obtaining in-plane and out-of-plane vibration of surface is introduced using geometrical relations between three LSV coordinates and vibrations measured at three different locations. The proposed algorithm is verified by comparing the experimental results obtained by a three-axis accelerometer and a developed optical system with an LSV and an LS combined together.

Monitoring of pipelines in nuclear power plants by measuring laser-based mechanical impedance

Abstract: Using laser-based mechanical impedance (LMI) measurement, this study proposes a damage detection technique that enables structural health monitoring of pipelines under the high temperature and radioactive environments of nuclear power plants (NPPs). The applications of conventional electromechanical impedance (EMI) based techniques to NPPs have been limited, mainly due to the contact nature of piezoelectric transducers, which cannot survive under the high temperature and high radiation environments of NPPs. The proposed LMI measurement technique aims to tackle the limitations of the EMI techniques by utilizing noncontact laser beams for both ultrasound generation and sensing. An Nd:Yag pulse laser is used for ultrasound generation, and a laser Doppler vibrometer is employed for the measurement of the corresponding ultrasound responses. For the monitoring of pipes covered by insulation layers, this study utilizes optical fibers to guide the laser beams to specific target locations. Then, an outlier analysis is adopted for autonomous damage diagnosis. Validation of the proposed LMI technique is carried out on a carbon steel pipe elbow under varying temperatures. A corrosion defect chemically engraved in the specimen is successfully detected.

Characterizing laser output in a HAMR device

Abstract: At least one laser input current is applied to a laser in a heat assisted magnetic recording device. Laser output power of the laser is measured at the at least one applied laser current. A relationship is characterized amongst temperature, applied laser input current and laser output power. Laser current is set to an optimal laser current as determined at manufacturing. A metric of recording performance is measured to determine if the relationship is acceptable.

Frequency agile LADAR

Abstract: In one aspect, a frequency agile LADAR (laser detection and ranging) sensor includes a transmitter configured to provide laser pulses towards a target, a receiver configured to receive a reflected signal from the target and control circuitry configured to tune an optical frequency of a first laser pulse of the laser pulses to be different from an optical frequency of a second laser pulse of the laser pulses and tune an optical frequency of the receiver to be different than an optical frequency of a laser pulse most recently transmitted by the transmitter.