Showing papers on "Laser Doppler vibrometer published in 2011"

Laser scanning system employing an optics module capable of forming a laser beam having an extended depth of focus (dof) over the laser scanning field

Abstract: A laser scanning system having a laser scanning field, and a laser beam optics module with an optical axis and including: an aperture stop disposed after a laser source for shaping the laser beam to a predetermined beam diameter; a collimating lens for collimating the laser beam produced from the aperture stop; an apodization element having a first and second optical surfaces for extending the depth of focus of the laser beam from the collimating lens; and a negative bi-prism, disposed after the apodization element, along the optical axis, to transform the energy distribution of the laser beam and cause the laser beam to converge to substantially a single beam spot along the far-field portion of the laser scanning field, and extend the depth of focus of the laser beam along the far-field portion of the laser scanning field.

Modeling and experimental verification of low-frequency MEMS energy harvesting from ambient vibrations

Abstract: Micro-fabricated piezoelectric vibration energy harvesters with resonance frequencies of 31–232 Hz are characterized and deployed for testing on ambient vibration sources in the machine room of a large building. A survey of 23 ambient vibration sources in the machine room is presented. A model is developed which uses a discretization method to accept measured arbitrary acceleration data as an input and gives harvester response as output. The modeled and measured output from the energy harvesters is compared for both vibrometer and ambient vibration sources. The energy harvesters produced up to 43 nWrms g−2 on a laboratory vibrometer and 10 nW g−2 on ambient vibration sources typically in large buildings.

High-speed nanometer-resolved imaging vibrometer and velocimeter

Abstract: Conventional laser vibrometers are incapable of performing multidimensional vibrometry at high speeds because they build on single-point measurements and rely on beam scanning, significantly limiting their utility and precision. Here we introduce a laser vibrometer that performs high-speed multidimensional imaging-based vibration and velocity measurements with nanometer-scale axial resolution without the need for beam scanning. As a proof-of-concept, we demonstrate real-time microscopic imaging of acoustic vibrations with 1 nm axial resolution, 1200 image pixels, and 30 ps dwell time at 36.7 MHz scan rate.

Multipoint laser Doppler vibrometry with single detector: principles, implementations, and signal analyses

Abstract: A 20-point laser Doppler vibrometer with single photodetector is presented for noncontact dynamic measurement. A 5×4 beam array with various frequency shifts is generated by a 1.55 μm distributed feedback laser and four acousto-optic devices, and illuminating different points on vibrating objects. The reflected beams are coupled into a single-mode fiber by a pigtailed collimator and interfere with a reference beam. The signal output from a high-speed photodetector is amplified and then digitized by a high-speed analog-to-digital converter with a sampling rate of 1 gigasample per second (1 GS/s). Several methods are introduced to avoid the cross talk among different frequencies and extract the vibration information of 20 points from a one-dimensional signal. Two signal processing algorithms based on Fourier transform and windowed Fourier transform are illustrated to extract the vibration signals at different points. The experimental results are compared with that from a commercial single-point laser vibrometer. The results show simultaneous vibration measurement can be realized on multiple points using a single laser source and a single photodetector.

Optical Doppler shift with structured light.

Abstract: When a light beam with a transverse spatially varying phase is considered for optical remote sensing, in addition to the usual longitudinal Doppler frequency shift of the returned signal induced by the motion of the scatter along the beam axis, a new transversal Doppler shift appears associated to the motion of the scatterer in the plane perpendicular to the beam axis. We discuss here how this new effect can be used to enhance the current capabilities of optical measurement systems, adding the capacity to detect more complex movements of scatters.

Analysis of the particle stability in a new designed ultrasonic levitation device.

Abstract: The use of acoustic levitation in the fields of analytical chemistry and in the containerless processing of materials requires a good stability of the levitated particle. However, spontaneous oscillations and rotation of the levitated particle have been reported in literature, which can reduce the applicability of the acoustic levitation technique. Aiming to reduce the particle oscillations, this paper presents the analysis of the particle stability in a new acoustic levitator device. The new acoustic levitator consists of a piezoelectric transducer with a concave radiating surface and a concave reflector. The analysis is conducted by determining numerically the axial and lateral forces that act on the levitated object and by measuring the oscillations of a sphere particle by a laser Doppler vibrometer. It is shown that the new levitator design allows to increase the lateral forces and reduce significantly the lateral oscillations of the levitated object.

Application of RMS for damage detection by guided elastic waves

Abstract: This paper presents certain results of an experimental study related with a damage detection in structural elements based on deviations in guided elastic wave propagation patterns. In order to excite guided elastic waves within specimens tested piezoelectric transducers have been applied. As excitation signals 5 sine cycles modulated by Hanning window have been used. Propagation of guided elastic waves has been monitored by a scanning Doppler laser vibrometer. The time signals recorded during measurement have been utilised to calculate the values of RMS. It has turned out that the values of RMS differed significantly in damaged areas from the values calculated for the healthy ones. In this way it has become possible to pinpoint precisely the locations of damage over the entire measured surface. All experimental investigations have been carried out for thin aluminium or composite plates. Damage has been simulated by a small additional mass attached on the plate surface or by a narrow notch cut. It has been shown that proposed method allows one to localise damage of various shapes and sizes within structural elements over the whole area under investigation.

Laser micro/nano processing system and method

Abstract: A laser micro/nano processing system (100, 200, 300, 400) comprises: a laser light source used to provide a first laser beam having a first wavelength and a second laser beam having a second wavelength different from the first wavelength, with the pulse width of the first laser beam being in the range from a nanosecond to a femtosecond; an optical focusing assembly used to focus the first laser beam and the second laser beam to the same focal point; and a micro mobile platform (21) controlled by a computer. Also disclosed are a method for micro/nano-processing photosensitive materials with a laser and a method for fabricating a device with a micro/nano structure using laser two-photon direct writing technology. In the system and methods, spatial and temporal overlapping of two laser beams is utilized, so as to obtain a micro/nano structure with a processing resolution higher than that of a single laser beam, using an average power lower than that of a single laser beam.

Eye-safe, single-frequency pulsed all-fiber laser for Doppler wind lidar

Abstract: A single-frequency pulsed erbium-doped fiber (EDF) laser with master-oscillator power-amplifier configuration at 1 533 nm is developed. A short-cavity, erbium-doped phosphate glass fiber laser is utilized as a seeder laser with a linewidth of 5 kHz and power of 40 mW. The seeder laser is modulated to be a pulse laser with a repetition rate of 10 kHz and pulse duration of 500 ns. The amplifier consists of two pre-amplifiers and one main amplifier. The detailed characteristics of the spectrum and linewidth of the amplifiers are presented. A pulse energy of 116 1J and a linewidth of 1.1 MHz are obtained. This laser can be a candidate transmitter for an all-fiber Doppler wind lidar in the boundary layer.

Laser processing machine

Abstract: A laser processing machine is provided which includes a chuck table adapted to hold a workpiece; and a laser beam irradiation unit for emitting a laser beam to the workpiece held by the chuck table. The laser beam irradiation unit includes a single laser beam oscillating unit for emitting a laser beam; a beam splitter which splits the laser beam emitted from the laser beam oscillating unit into a first laser beam propagating along a first path and a second laser beam propagating along a second path; a first condenser which condenses the first laser beam; and a second condenser which condenses the second laser beam.

Experimental observation of contact mode cantilever dynamics with nanosecond resolution.

Abstract: We report the use of a laser Doppler vibrometer to measure the motion of an atomic force microscope contact mode cantilever during continuous line scans of a mica surface. With a sufficiently high density of measurement points the dynamics of the entire cantilever beam, from the apex to the base, can be reconstructed. We demonstrate nanosecond resolution of both rectangular and triangular cantilevers. This technique permits visualization and quantitative measurements of both the normal and lateral tip sample interactions for the first and higher order eigenmodes. The ability to derive quantitative lateral force measurements is of interest to the field of microtribology/nanotribology while the comprehensive understanding of the cantilever's dynamics also aids new cantilever designs and simulations.

Speech measurements using a laser Doppler vibrometer sensor: Application to speech enhancement

Abstract: In this paper, we present a remote speech-measurement system, which utilizes an auxiliary laser Doppler vibrometer (LDV) sensor. When focusing on the larynx, this sensor captures useful speech information at low-frequency regions (up to 1.5–2 kHz), and is shown to be immune to acoustical disturbances. For improved speech enhancement, we propose a new algorithm for efficiently combining the signals from the LDV-based sensor and a standard acoustic sensor. The algorithm includes a pre-filtering process, to suppress impulsive noises that severely degrade the LDV-measured speech, and a soft-decision voice activity detector (VAD) in the time-frequency domain. Experimental results demonstrate the performance of the proposed system in transient noise environments.

Ultrasonic Vibration at Thermosonic Flip-Chip Bonding Interface

Abstract: Thermosonic flip-chip (TSFC) bonding is a developing microelectronic packaging technology. To provide clear understanding of bonding process and bonding mechanism, the ultrasonic vibration at bonding interface was studied. The TSFC bonding was performed on a lab bonder, the ultrasonic vibration of tool and chip were measured by using a laser doppler vibrometer, and the effect of bonding force on chip vibration velocity harmonics was analyzed. Experimental results show that the bonding strength forming process can be divided into four stages. The abrupt “amplitude dropping” of chip vibration was observed, and could be considered as a sign of bonding strength formed. The bonding strength is formed after the bonding starts 6-8 ms. This moment can be indicated by a peak of fundamental and a ramp-up of third harmonics of chip vibration velocity. The results also show that relatively small bonding force was good for forming initial bonding strength. Then the gradually increased bonding force loading is thought to be more suitable for better bonding strength and reliability.

Lamb wave excitation and propagation in elastic plates with surface obstacles: proper choice of central frequencies

Abstract: Experimental and theoretical investigations of Lamb wave excitation and sensing using piezo patch transducers and the laser vibrometer technique have been performed, aiming at the development of adequate mathematical and computer models for the interpretation of sensing data and for the choice of optimal parameters for structural health monitoring. The proposed models are validated by experimental results. Furthermore, a methodology is presented which allows for the determination of central frequencies at which maximal values of the structural response spectrum can be expected in the case of wave propagation monitoring with laser vibrometry.

Axial scanning laser Doppler velocimeter using wavelength change without moving mechanism in sensor probe.

Abstract: A scanning laser Doppler velocimeter (LDV) without any moving mechanism in its sensor probe is proposed. In the proposed scanning LDV, the measurement position is axially scanned by change in the wavelength of the light input to the sensor probe, instead of using a moving mechanism in the sensor probe. For this purpose, a tunable laser and diffraction gratings are used, and the sensor probe including the gratings is separated from the main body including the tunable laser. To demonstrate the scanning function based on the proposed concept, an experiment was conducted using optical fibers, a commercial tunable laser and a setup of the sensor probe consisting of bulk optical components. As the experimental result, it is found that the measurement positions estimated from the measured beat frequencies are in good agreement with the theoretical values. The scan ranges over a wavelength range of 30 nm are estimated to be 29.3 mm when the beam angle to the measurement position at the wavelength of 1540 nm is 10° and 20.8 mm when the beam angle is 15°. The result indicates that the scanning function by means of changing the wavelength input to the sensor probe is successfully demonstrated for the first time. The proposed method has the potential for realizing a scanning LDV with a simple, compact and reliable sensor probe.

Video-rate laser Doppler vibrometry by heterodyne holography

Abstract: We report a demonstration video-rate heterodyne holography in off-axis configuration Reconstruction and display of a one megapixel hologram is achieved at 24 frames per second, with a graphics processing unit Our claims are validated with real-time screening of steady-state vibration amplitudes in a wide-field, noncontact vibrometry experiment

Laser vibrometry from a moving ground vehicle

Abstract: We investigated the fundamental limits to the performance of a laser vibrometer that is mounted on a moving ground vehicle. The noise floor of a moving laser vibrometer consists of speckle noise, shot noise, and platform vibrations. We showed that speckle noise can be reduced by increasing the laser spot size and that the noise floor is dominated by shot noise at high frequencies (typically greater than a few kilohertz for our system). We built a five-channel, vehicle-mounted, 1.55 μm wavelength laser vibrometer to measure its noise floor at 10 m horizontal range while driving on dirt roads. The measured noise floor agreed with our theoretical estimates. We showed that, by subtracting the response of an accelerometer and an optical reference channel, we could reduce the excess noise (in units of micrometers per second per Hz1/2) from vehicle vibrations by a factor of up to 33, to obtain nearly speckle-and-shot-noise-limited performance from 0.3 to 47 kHz.

Non contact monitoring of the respiration activity by electromagnetic sensing.

Abstract: The aim of this paper is to present a novel measurement method for the detection of the respiratory activity (respiration rate and respiration period) based on the use of a continuous wave (6 GHz) microwave radar reflectometry technique. The paper aims, in particular, to explore the effect on the signal quality of distance D between the sensing apparatus and the patient. The measurement method proposed is based on the measurement of the phase variation of the reflection coefficient (S 11 ) signal measured by a vectorial network analyzer connected to a double ridge horn antenna. The S 11 signal has been compared with the synchronous acquisition made by means of a laser Doppler vibrometer (LDVi), measuring the thorax oscillations caused by the respiratory activity. Both signals have been filtered in order to eliminate the effect of high frequency disturbances (heartbeat) and noise. Results show an high correlation between respiration peaks measured with the proposed system and with LDVi; a reduction of the amplitude of the S 11 signal phase (as well as the SNR) is reported in correspondence to an increasing of the distance D (−0.11 dB/cm). Tests have been repeated for standing as well as for sitting condition of the subject confirming a better signal quality for the later. Despite the fact that S 11 phase variation and SNR are reduced by the distance D, in our experiments, it is still possible to correctly measure the respiration period up to 2.5 m. Data measured show that the reflectometeric approach can be used to monitor at distance with sufficient high SNR (18 dB at 2.5 m) the respiration activity of a subject without the need of a direct contact with the subject skin by means of electrods of sensing belts.

Modelling of in-plane wave propagation in a plate using spectral element method and Kane-Mindlin theory with application to damage detection

Abstract: This paper presents results of experimental and numerical analyses of in-plane waves propagating in a 5 mm-thick steel plate in the frequency range of 120-300 kHz. For such a thickness/frequency ratio, extensional waves reveal dispersive character. To model in-plane wave propagation taking into account the thickness-stretch effect, a novel 2D spectral element, based on the Kane-Mindlin theory, was formulated. An application of in-plane waves to damage detection is also discussed. Experimental investigations employing a laser vibrometer demonstrated that the position and length of a defect can precisely be identified by analysing reflected and diffracted waves.

Laser vibration sensing: overview and applications

Abstract: Laser vibrometry based on coherent detection allows non-contact measurements of small-amplitude vibration characteristics of objects. This technique, commonly using the Doppler effect, offers much potential for short-range civil applications and for long-range applications in defence and security. Most commercially available laser vibrometers are for short ranges (up to a few tens of metres) and use a single beam from a low power HeNe laser source (λ = 632 nm). Long-range applications need higher laser output power, and thus appropriate vibrometers typically operate at 1.5 μm, 2 μm or 10.6 μm to meet the laser safety regulations. Spatially resolved vibrational information can be obtained from an object by using scanning laser vibrometers. To reduce measuring time and to measure transient object movements and mode structures of objects, several approaches to multibeam laser Doppler vibrometry have been developed, and some of them are already commercially available for short ranges. In this paper we focus on applications in the field of defence and security such as target classification and identification, including camouflaged or partly concealed targets, and the detection of buried land mines. Some examples of civil medium-range applications are given also.

Experimental study on slider dynamics during touchdown by using thermal flying-height control

Abstract: To investigate the possibility of further lowering the clearance in head–disk interface systems, slider dynamic behavior during a touchdown sequence with a thermal flying-height control (TFC) function was investigated by using a spinstand-level evaluation utilizing an acoustic emission (AE) sensor and a laser Doppler vibrometer (LDV). Experimental results demonstrated that off-track vibration was easier to excite by head–disk contact at the beginning of head–disk contact. We then confirmed that the amplitude of pitch-mode vibration in the flying-height direction increased and sway-mode vibration in the off-track direction decreased when increasing heater power during the touchdown sequence. Moreover, we found that the peak frequency of pitch-mode vibration shifted to a higher frequency under over-push conditions. Time–frequency domain analysis results showed that the peak shift occurred at several locations during a disk rotation. The mechanism of the peak shift is attributed to the increase in stiffness at the head–disk interface (HDI) due to solid–solid contact or mode change occurred in such regions. During the touchdown sequence, the friction force at the HDI continues to increase, even though slider vibration and AE signal decrease when heater power is increased. The friction force at the HDI needs to be decreased to achieve further low-clearance HDI.

Realization of fiber-based laser Doppler vibrometer with serrodyne frequency shifting

Abstract: We demonstrate a laser Doppler vibrometer (LDV) based on the serrodyne frequency shifting technique. A proof-of-principle system is implemented on the basis of fiber-optic components but opens the way toward an ultracompact integrated LDV system on a silicon chip. With a low laser power of 50 μW, the serrodyne LDV was able to measure submicrometer vibrations with frequencies in the audio range.