Photoacoustic/photothermal spectroscopy is an established technique for detection of chemicals and explosives.
However, prior sample preparation is required and the analysis is conducted in a sealed space with a high-sensitivity
sensor coupled with a lock-in amplifier, limiting the technique to applications in a controllable laboratory environment.
Hence, this technique may not be suitable for defense and security applications where the detection of explosives or
hazardous chemicals is required in an open environment at a safe standoff distance. In this study, chemicals in various
forms were excited by an intensity-modulated quantum cascade laser (QCL), while a laser Doppler vibrometer (LDV)
was applied to detect the vibration signal resulting from the photocoustic/photothermal effect. The photo-vibrational
spectrum obtained by scanning the QCL’s wavelength in MIR range, coincides well with the corresponding spectrum
obtained using typical FTIR equipment. The experiment in short and long standoff distances demonstrated that the LDV
is a capable sensor for chemical detection in an open environment.
Photoacoustic/photothermal spectroscopy is an established technique for trace detection of chemicals and explosives. Normally high-sensitive microphone or PZT sensor is used to detect the signal in photoacoustic cell. In recent years, laser Doppler vibrometer (LDV) is proposed to remote-sense photoacoustic signal on various substrates. It is a highsensitivity sensor with a displacement resolution of <10pm. In this research, the photoacoustic effect of various chemicals and explosives is excited by a quantum cascade laser (QCL) at their absorbance peak. A home-developed differential LDV at 1550nm wavelength is applied to detect the vibration signal at 100m. A differential configuration is applied to minimize the environment factors, such as environment noise and vibration, air turbulence, etc. and increase the detection sensitivity. The photo-vibrational signal of chemicals and explosives on different substrates are detected. The results show the potential of the proposed technique on detection of trace chemicals and explosives at long standoff distance.
Photoacoustic spectroscopy (PAS) is a useful technique that suitable for trace detection of chemicals and explosives. Normally a high-sensitive microphone or a quartz tuning fork is used to detect the signal in photoacoustic cell. In recent years, laser Doppler vibrometer (LDV) is proposed to remote-sense photoacoustic signal on various substrates. It is a high-sensitivity sensor with a displacement resolution of <10pm. In this research, the photoacoustic effect of various chemicals is excited by a quantum cascade laser (QCL) with a scanning wavelength range of 6.89μm to 8.5 μm. A home-developed LDV at 1550nm wavelength is applied to detect the vibration signal. After normalize the vibration amplitude with QCL power, the photoacoustic spectrum of various chemicals can be obtained. Different factors that affect the detection accuracy and sensitivity have also been discussed. The results show the potential of the proposed technique for standoff detection of trace chemicals and explosives.
Laser Doppler vibrometry (LDV) is a well-known interferometric technique to measure the motions, vibrations and mode shapes of machine components and structures. The drawback of commercial LDV is that it can only offer a pointwise measurement. In order to build up a vibrometric image, a scanning device is normally adopted to scan the laser point in two spatial axes. These scanning laser Doppler vibrometers (SLDV) assume that the measurement conditions remain invariant while multiple and identical, sequential measurements are performed. This assumption makes SLDVs impractical to do measurement on transient events. In this paper, we introduce a new multiple-point laser coherent detection system based on spatial-encoding technology and fiber configuration. A simultaneous vibration measurement on multiple points is realized using a single photodetector. A prototype16-point laser coherent detection system is built and it is applied to measure the vibration of various objects, such as body of a car or a motorcycle when engine is on and under shock tests. The results show the prospect of multi-point laser coherent detection system in the area of nondestructive test and precise dynamic measurement.
Generally there are two categories of noncontact laser interferometric methods commonly used in dynamic measurement, camera-based full-field interferometry and photo-sensor-based laser Doppler interferometry. The two methods have different advantages and disadvantages thus are suitable for different applications. The camera-based interferometry enjoys the valuable merit of full-field observation and measurement. In this paper, one typical full-field interferometry, digital holography, is employed to monitor the growth process of aqueous sodium chlorate crystals. The phase proportional to the solution concentration is retrieved from the holograms captured by CCD camera in real time. There exist no phase ambiguity problem in holography compared with other optical interferometric methods. On the other hand, laser Doppler interferometry is usually a point-wise measurement but with a very high temporal sampling rate. A multipoint laser Doppler interferometer is proposed for modal parameter measurement in this paper. The multiple transient vibration signals of spatially separated points on a beam structure subjected to a shock excitation are recorded synchronously. The natural frequencies and mode shapes are extracted in the signal processing stage. This paper shows that laser interferometry is able to contribute more to the practical applications in dynamic measurement related fields.
Laser Doppler vibrometry(LDV) is a precise and non-contact optical interferometry used to measure vibrations of
structures and machine components. LDV can only provide a point-wise measurement, or a scanning measurement via
moving the laser beam rapidly onto the vibrating object which is assumed to be invariant in the scanning course.
Consequently, LDV is usually impractical to do measurement on transient events. In this paper, a new self-synchronized
multipoint LDV is proposed. The multiple laser beams are separated from one laser source, and different frequency shifts
are introduced into these beams by a combination of acousto-optic modulators. The laser beams are projected on
different points, and the reflected beams interfere with a common reference beam. The interference light intensity signal
is recorded by a single photodetector. This multipoint LDV has the flexibility to measure the vibration of different points
on various surfaces. In this study, two applications in experimental mechanics area are presented. Firstly, the proposed
system is used to measure the resonant frequencies of structure in a shock test. Secondly, The proposed multi-point LDV
is also used to measure the mode shape of a beam with an artificial crack. Compared with the original vibration mode
shape, the crack location can be identified easily.
Optical coherent detection is a precise and non-contact method for measurement of tiny deformation or movement of an
object. In the last century, it can only be used on the static or quasi-static measurement of deformation between two
statuses. Recently it has been applied on dynamic measurement with the help of high-speed camera. The advantage of
this technique is that it can offer a full-field measurement. However, due to the limited capturing rate of high-speed
camera, its capability in temporal domain cannot meet the requirements of many applications. In this study, several
issues in high-speed-camera-based optical interferometry are discussed. For example, introduction of carrier in temporal
and spatial domain, signal processing in temporal-frequency domain, and the introduction of dual-wavelength
interferometry in dynamic measurement. The discussion leads to a clue to select suitable technique to fulfill whole-field
dynamic measurement at different ranges.
Shearography is a whole-field, noncontact optical technique that allows the direct measurement of first-order derivatives of deflection on spatial coordinates, depending on the measurement setup. In many cases, the curvatures and twists of an object provide more interesting parameters, as they are directly related to the induced stresses when an object is subjected to external loads. We describe the use of digital shearography for the measurement of these stress-related parameters through phase retrieval when an object is undergoing continuous deformation. A sequence of shearograms is captured by a high-speed camera during the deformation. To avoid the problem of phase ambiguity, either a spatial or temporal carrier is introduced. A comparison of spatial and temporal carrier is also presented. The obtained three-dimensional matrix is then analyzed by Fourier and windowed-Fourier transform in a spatial and temporal domain and a high-quality spatial distribution of the deflection derivative, curvature and twist are extracted at any instant.
This article presents a novel fiber-based multi-beam laser Doppler vibrometer (LDV). In this design, a single
wavelength laser source at 1550 nm combined with several acousto-optic modulators (AOM) form the transmitter head
of the LDV. At the receiver side, one single high-speed photo-detector is employed, instead of multiple detectors
according to other reported multi-beam laser Doppler vibrometer.1, 2 Utilization of spatial encoding technique allows us
to produce transmitted laser beams with different frequency shifts. In this work, a laser source passes through a sequence
of totally four AOMs at different regimes, producing a 4×5 laser beam matrix which is then sent onto different points of
vibrating targets for measurement. The backscattered light signals are collected back into a single mode fiber by a fiber
collimator and combined with a common reference beam. This mixture of optical signals passes through an Erbium
Doped Fiber Amplifier (EDFA) before it is detected by a high-speed fiber-based InGaAs photo-detector. With a digital
demodulation algorithm implemented in Labview, the phase variations and thus the vibrations of different testing points
can be extracted separately from their corresponding frequency bands. The experimental results show it is possible to do
a precise vibration measurement on twenty testing points simultaneously using this novel multi-beam LDV.
In recent years, optical interferometry based on high-speed imaging has been applied to full-field, non-contact
measurement of low-frequency vibration or continuous-deformation. Retrieving dynamic phase values from a sequence
of interferogram leads to a precise measurement of different kinematic and deformation parameters of the testing object.
However, the temporal measurement range of this type of 2-D method is still limited by the imaging rate of the camera.
On the other hand, laser Doppler vibrometer (LDV) significantly extend measurement capabilities in time axis, but most
of the present vibrometers are based on pointwise measurement. A scanning system is normally employed to generate a
2-D measurement. This will dramatically increase the measurement time and limit the system to study the repeatable
events. In this paper, two new optical dynamic testing methods are introduced to increase the measurement range in
temporal and spatial axes. One is based on high-speed digital holography from which the instantaneous phase can be
retrieved spatially to avoid the phase ambiguity problem in temporal analysis. Another is a double-beam Doppler
vibrometer which can measure the vibration on different points simultaneously. The results of these two methods show
the trend that the optical interferometry will meet various requirements of dynamic measurement with different temporal
and spatial resolutions.
A method for whole-field non-contact measurement of displacement, velocity and acceleration of a vibrating microobject
based on digital holographic microscopy is presented. A micro-beam is excited by a fluctuating voltage with a
sinusoidal configuration. A series of digital holograms are captured using a digital holographic microscope with a highspeed
camera. The result of reconstruction is a three dimensional complex-valued matrix with noises. In this paper,
Fourier analysis and windowed Fourier analysis are applied in both the spatial and temporal domains to extract the
kinematic parameters. The instantaneous displacement is obtained by temporal phase unwrapping of the filtered wrapped
phase map, while the velocity and acceleration are evaluated by windowed Fourier analysis along the time axis. The
combination of digital holographic microscopy and temporal Fourier analyses is able to study the vibration without a
phase ambiguity problem, and the instantaneous kinematic parameters on each point are obtained.
In recent years, optical interferometry has been applied to
whole-field, non-contact measurement of vibrating or
continuously-deforming objects. In optical dynamic measurement, an interferogram sequence is obtained by a highspeed
camera. Retrieving dynamic phase values from this interferogram sequence leads to a precise measurement of
different kinematic and deformation parameters of a
continuously-deforming or vibrating object. In this paper, this
interferogram sequence is classified into two types: (i) intensity variation; and (ii) exponential phase signal. Different
temporal phase retrieving techniques, such as Hilbert transform, Fourier transform, windowed Fourier transform and
wavelet transform are applied to extract the phase from a simulated signal. The advantage and drawback of each
algorithm are discussed. In addition, a new method based on the combination of Fourier transform and windowed
Fourier transform is proposed and the simulation shows it can eliminate the noise and evaluate the phase more accurately.
This paper describes feasibility study of temporal phase analysis techniques using wavelet transform. In electronic speckle pattern interferometry (ESPI), a series of speckle patterns is captured during the deformation or vibration of the test specimen. The intensity variation on each pixel is analyzed along time axis. Phase values are evaluated point by point using complex Morlet wavelet transform. To demonstrate the validity of the proposed method, two
experiments based on ESPI are conducted. These include instantaneous velocity and displacement measurement on continuous deformed objects; and absolute displacement measurement on vibrating objects using temporal carrier technique. Compared to temporal Fourier transform, wavelet analysis detects the optimized instantaneous frequency and performs an adaptive band-pass filtering of the measured signal, thus limits the influence of noise sources and increases the resolution of measurement significantly. It was observed that continuous wavelet
transform (CWT) on each pixel generates a smoother spatial displacement distribution at different instants compared to a Fourier transform. The maximum displacement fluctuation due to noise is around 0.04 μm in Fourier transform, but only 0.02 μm in wavelet analysis. The wavelet transform proposed in this paper demonstrates a high potential for robust processing of continuous image sequences. The deep exploration on wavelet phase analysis techniques will broaden the applications in optical and non-destructive testing field.
Measuring continuous deformation of specimens whose dimensions are in the range of sub-millimeter introduces a number of difficulties using laser speckle interferometry. During deformation, the speckle patterns recorded on a camera sensor change constantly. These time-dependent speckle patterns would provide the deformation history of the object. However, compared to large objects, noise effect is much more serious due to the high magnification. In this study, a series of speckle patterns on small objects are captured during deformation by high speed camera and the temporal intensity variation of each pixel is analyzed by a robust mathematical tool --- complex Morlet wavelet transform instead of conventional Fourier transform. The transient velocity and displacement of each point can be retrieved without the need for temporal or spatial phase unwrapping process. Displacements obtained are compared with those from temporal Fourier transform, and the results show that wavelet transform minimize the influence of noise and provide better results.
When a continuously deforming object is measured by electronic speckle pattern interferometry (ESPI), the speckle pattern recorded on a camera sensor changes constantly. These time-dependent speckle patterns would provide the deformation history of the object. Various objects are applied with both linearly and nonlinearly varying loads and speckle patterns are captured using a high-speed CCD camera. The temporal intensity variation of each pixel on the recorded images is analyzed by a robust mathematical tool—Morlet wavelet transform instead of conventional Fourier transform. The transient velocity and displacement of each point can be retrieved without the necessity of the temporal or spatial phase unwrapping process. The displacements obtained are compared with those from a temporal Fourier transform, and the results show that the wavelet transform minimizes the influence of noise and provides better results for a linearly varying load. System error in the wavelet analysis for nonlinear load is also discussed.
In recent years, optical fringe-projection and other optical interferometric techniques for surface profiling have received much attention because they are whole-field and non-contacting; very high data processing speeds can be achieved using computer image-processing techniques. These advantages over many other mechanical probe-based techniques are particularly useful for the measurement of large surfaces as well as for micro-systems at the sub-micron level. In the fringe-projection technique, a reference optical grating is first generated and then projected onto the surface of interest. For a given optical set-up, the distribution of the reference grating is perturbed in accordance with the profile of the test surface, thereby enabling direct derivation of surface profiles from measurements of the perturbed fringe distribution. The reference gratings are readily generated with a Michelson interferometer, which uses a beam-splitting cube and mirrors - these optical elements are readily available in all laboratories. A major drawback of this technique is the need for good vibration isolation, as otherwise unstable fringes will be generated. Alternatively, beam-splitting cubes with coated reflective surfaces can be used, but this would not allow adjustment of the frequency of the generated fringes. This paper describes a very simple method of generating and projecting optical grating for surface profiling. The working principle is based on the reflection-refraction of a commercial beam-splitting cube. By carefully adjusting the orientation of the laser beam, the frequency of the grating can be varied. A distinct advantage of this method over the Michelson interferometer lies in its ability to generate stable carrier fringes under lax vibration isolation conditions.
For measuring the angle of rotation of flat objects using projected fringes, the method of point-of-light triangulation and the method of line-of-light triangulation will breakdown when the grating lies on the axis of rotation. Therefore, a grating other than a point or linear lines is preferred. In this paper, a simple Michelson interferometer-based method for the generation and projection of circular gratings is described. The basic optical element in a Michelson interferometer is a beam- splitting cube. With this Michelson interferometer, a circular grating is observed when the screen is placed normal to the line containing the two point-light sources produced by the beam splitter. By placing an expander between the beam-splitter and the laser source, and by carefully adjusting the two mirrors beside the beam- splitting cube, the frequency of the circular grating can be adjusted. This paper also describes the use of the generated circular optical grating for measuring the amount of rotation of flat surfaced that are either diffuse or specularly reflective - the method is based on relating the distortion of the circular grating to the angular rotation of the surface.
In this paper, a simple method is described for the measurement of small angles of rotation of a flat surface using a circular optical grating, which may be generated and projected onto the test surface using either a standard Michelson interferometer or a computer-controlled LCD projector. In view of its availability in most laboratories, a Michelson interferometer is used in this paper. The diameter of the grating that is generated can be easily magnified or be reduced to suit the size of the test surface. With circular grating, the angular rotation of both diffuse and specularly reflective surface about any axis of rotation can be measured from the distortion of the grating. The distorted grating that is diffracted from a specularly reflective surface may be recorded in two different ways using a CCD camera. In the first method, the distorted grating is recorded off the m mirror surface as though it were a diffuse surface. In the second method, the distorted grating is recorded off the miro surface as though it were a diffuse surface. In the second method, the distorted grating is specularly reflected onto an opaque screen and the CCD camera subsequently records the grating image off this screen.
Foam-adhesive bonding is a common fabrication process in the aerospace, the automobile and the electronic industry. During service, the change with time in the behavior of components fabricated by this process is often caused by degradation of the foam layer within. As it is impractical, and at times impracticable, to dismantle the component for the purpose of testing and examining the foam layer, the need arises to develop methods of assessing the properties of the foam layer in situ. Many foam-adhesive bonded components comprise segments of strips bonded along their lengths to a rigid support. Additionally, these strips are secured with mechanical fasteners to prevent detachment from the support should the bonding fail. In this paper, a method is proposed which permits estimating the stiffness of the foam layer when the adherend (i.e., the strip) is subjected to point loads representing the forces exerted by the fasteners. The surface of the adherend is of mirror-like finish, as is found in the disk-drives industry. From the Moire fringe pattern generated with the use of the mirror-image method, the deflection of the adherend due to the point loads is deduced. Treating the foam layer as of the Winkler type, the magnitude of its stiffness is iterated using the theory of beams on elastic foundation.