Double-pulsed Ruby Laser Research Articles

Location of delamination in laminated composite plates by pulsed laser holography

The location of delamination in composite laminates based on their vibration characteristics is presented in this paper. Composite materials are widely used as structural materials for aerospace engineering, because of its excellent mechanical properties such as light weight, high stiffness and anti-corrosion characteristics. This paper uses pulsed electronic speckle pattern interferometry (Pulsed-ESPI) to perform non-destructive evaluation of carbon fiber reinforced plastic (CFRP) laminate plates containing various sizes of artificial defects located at the centre of the specimen, based on vibration characteristic using double-pulsed Ruby laser with a pulse duration of 30 ns. The pulsed separation can be adjusted in between 2 and 800 μs. The main advantage of this technique over conventional ESPI is the ability to carry out measurements even under harsh environmental conditions. From the results, it seems to be effective for the detection of defects in various kinds of composite materials. In this paper, the basic principles of the technique are described briefly.

Measurement of shock waves using phase shifting pulsed holographic interferometer

A phase shifting pulsed holographic interferometer was applied to the experimental study of the propagation of laser-induced shock waves over metal plates. A double-pulsed ruby laser was used to generate the shock waves and to make a holographic interferogram of the wave fields. The phase shifting method with a dual-reference beam solved the sign ambiguity problem in holographic fringe patterns and allowed a quantitative evaluation of the phase of the interference patterns. The transient surface profile and propagation behavior of the shock wave over plates were investigated from the holographic fringe patterns.

Pulsed digital holographic interferometry for dynamic measurement of rotating objects with an optical derotator

A method for measuring dynamic deformations of rotating objects with pulsed digital holography is described. An optical derotator is used to compensate for the rotation. A CCD camera is used to record two holograms with a short time separation (20 mus). Results of deformations between the recordings are obtained after subtraction of the phase distribution between the two digital holograms. Fringe phase maps of the phase subtraction of two holograms compensated by the derotator and recorded with a Q-switched double-pulsed ruby laser are presented. A flat disk and the blades of a fan were investigated. We used an optical arrangement that allowed us to improve laser illumination and energy efficiency. Experimental results on quantitative evaluation of dynamical out-of-plane deformations are presented.

Comparative study of various endoscopes for pulsed digital holographic interferometry

A comparison of several endoscopes as object image carriers in pulsed digital holography is presented. Three multicore flexible fiber endoscopes of different spatial resolution and one rigid endoscope are investigated. The four endoscopes are integrated in a setup for the recording of digital holograms on a CCD camera. A double-pulsed ruby laser is used as the light source. A spatial carrier is introduced by an off-axis reference beam, which permits quantitative evaluation of the phase difference between two holograms recorded with a short time separation (5-600 micros). From reported studies it may be inferred that the quality of the phase maps so derived from digital holographic interferometry has a strong correlation to the spatial resolution of the multicore fiber used in these endoscopes. With the endoscopic technique combined with pulsed digital holography a number of useful applications (in areas such as medical endoscopy, micromechanics, and microelectronics) are envisaged for which access to the objects of interest is otherwise difficult.

Holographic Interferometry Using a Double-Pulse Ruby Laser with Nonsimultaneous Flashes of Pump Lamps

A method for extending the interval between paired laser pulses is investigated. This method is based on pumping an active medium by two groups of flash lamps with nonsimultaneous discharges, each of which excites a giant pulse. It is established that, at asymmetric pumping with respect to the cavity axis, a difference in the pulse propagation paths is achieved because of the thermooptic distortions of ruby rods, which result in the impossibility of recording holographic interferograms for a pulse separation exceeding several milliseconds. An unlimited interval between pulses is achieved when using a four-lamp laser pump chamber ensuring axially symmetric pumping.

Highly sensitive pulsed digital holography for built-in defect analysis with a laser excitation

A highly sensitive method is presented for noninvasive defect analysis on thin structures with a Q-switched double-pulsed ruby laser with frequency doubling (347 nm). In our research we feature an all-optical arrangement, where a focused laser pulse derived from the same ruby laser (694 nm) acts as a built-in synchronous excitation source for digital holographic interferometry. The recordings are made with a CCD camera for capturing two holograms (two states of the specimen) corresponding to the two UV laser pulses with a short time separation (10-50 mus). Subtraction of the phase distribution in two digital holograms gives a fringe phase map that shows the change in deformation of the specimen between the recordings. The advantage of the proposed method is two fold. First, the use of a shorter wavelength results in a higher sensitivity. Second, owing to the induced synchronous built-in optical excitation, the specimen is not subjected to any external physical excitation devices. Experimental results are presented on identification and evaluation of defects in thin metal sheets.

Pulsed digital holography for deformation measurements on biological tissues

Digital holograms are recorded of biological tissues by use of a Q-switched double-pulsed ruby laser. An image-plane digital holography setup is used with a CCD camera for capturing two holograms with a short time separation (20-800 micros). Subtraction of the phase distribution in two digital holograms yield a fringe phase map that shows the change in deformation of the tissue surface between the recordings. Experiments are performed on tissue from a pig that was excited by a short-shock pulse and on a human hand that was excited by sinusoidal stimulation. Results when the object is imaged through an endoscope are also presented. The technique could be an approach for measuring parameters like elasticity on biological tissues.

Simultaneous three-dimensional dynamic deformation measurements with pulsed digital holography

The three deformation components x, y, z of a vibrating object are measured simultaneously by use of digital holography with a double-pulse ruby laser source. The object is illuminated from three different directions, each optically path matched with three reference beams such that three independent digital holograms are formed and added incoherently in one single CCD image. The optical phase difference between the two recordings taken for each hologram is quantitatively evaluated by the Fourier-transform method so that a set of three phase maps is obtained, representing the deformation along three sensitivity vectors. The total object deformation is obtained as a vector resultant from the data of the three phase maps. To give the full three-dimensional (3-D) description, the shape of the object is measured by the two-wavelength contouring method. Experiments are performed with a cylinder as the test object, transiently and harmonically excited. The 3-D deformation and shape measurement results are presented graphically.

Tomographic reconstruction of transient acoustic fields recorded by pulsed TV holography

Pulsed TV holography combined with computerized tomography (CT) are used to evaluate the three-dimensional distribution of transient acoustic fields in air. Experiments are performed with an electrical discharge between two electrodes as the sound source. Holograms from several directions of the acoustic field are recorded directly onto a CCD detector by use of a double-pulsed ruby laser as the light source. Phase maps, representing projections of the acoustic field, are evaluated quantitatively from the recorded holograms. The projections are used for the CT reconstruction to evaluate the pressure-field distribution in any cross section of the measured volume of air.

Transient wave response of a cymbal using double-pulsed TV holography

Using an electronic system for pulsed TV holography with a double-pulsed ruby laser, transient wave propagation during intervals from 30 to 480 μs after impact is recorded. The first observable bending waves, having wavelengths of about 5 mm, propagate at about 1700 m/s, and reach the edge of the cymbal in about 60 μs. These are quickly followed by waves of longer wavelength which scatter at the outer edge of the cymbal and also the central dome and result in standing waves. A phase unwrapping procedure is used to obtain a three-dimensional map of the wave field. Holographic film recordings similarly show scattering of transient bending waves at the central dome.

Double pulse Thomson scattering system at RTP

In this article a double pulse multiposition Thomson scattering diagnostic, under construction at RTP, is discussed. Light from a double pulsed ruby laser (pulse separation: 10–800 μs, max. 2×12.5 J) is scattered by the free electrons of the tokamak plasma and relayed to a Littrow polychromator for spectral analysis. The spectrally resolved light is recorded by two ICCD detectors. Simulations show that the system sensitivity will be such that electron temperatures in the range of 100 eV–7 keV can be determined with an accuracy as good as 2%–3% for electron densities of 1020 m−3, with a spatial resolution down to 2.6 mm. With this diagnostic the dynamics of small scale structures in the electron temperature profile will be studied.

GENERATING AND RECORDING TRANSIENT BENDING WAVES IN PLATES BY PULSED LASERS

We have presented a method to generate bending waves in a plate by focusing a Q-switched Nd:YAG-laser pulse on its surface. These waves are recorded by double pulsed hologram interferometry. Evaluation of the interferograms show that the bending wave pattern is very similar to what is predicted by the Kirchhoff plate equation assuming a point impact of infinitesimally short duration. This indicates that a short Nd:YAG-laser pulse may be considered as a Dirac pulse in space and time. Future investigations will be performed with a more powerful and “cleaner” laser to get a higher energy and impulse transfer to the plate thus giving bending waves of larger amplitude. Promising preliminary experiments have also begun using a double pulsed ruby laser both for generating and recording of bending waves.

Holographic interferometry measurements of transient bending waves in tubes and rings

Propagating bending waves are studied in a tube of steel and in a ring of aluminum. The waves are generated by the impact of a ballistic pendulum. Holographic interferometry, with a double-pulsed ruby laser as light source, is used to record the waves. A conical mirror is placed axially inside the tube. Axial illumination and axial observation directions, make it possible to view all sides of the tube simultaneously with a high sensitivity to radial deformation. The interferograms, which have an unusual perspective, are captured with a CCD-camera and then spatially transformed into an unwrapped strip of the tube wall. This makes the interpretation of the measurements simpler. The geometry of the tube causes the wave pattern to propagate with different speed and amplitude along and across the tube, even when the material itself is isotropic. A finite-element simulation of the impact is compared to the corresponding experiment. An impact on a ring with a defect is performed in order to study the effect on the wave pattern. The proposed method could be used in nondestructive testing of pipes.

Implementation of double-pulsed holography in evaluation of whole-body vibration

This study used a unique holographic technique to evaluate the effects of vibration on soft tissues and bones. It was possible to record forced whole-body vibration in humans by holograph interferometry using a double-pulsed ruby laser. The study investigated the manner in which the muscles of the back and vertebral column are affected by vibrations applied to the human buttocks in the sitting position. The subject was exposed to vibration at two frequencies: 40 and 60 Hz (vertical Z axis). Transmission of the vibrations along the subject's back was recorded by means of double-pulsed holography and electromyography. Evaluation of the vibration pattern showed that the vibrations are transmitted along the back all the way up to the neck and head. The pattern of vibration in the muscles of the back and vertebral column showed that the greatest effect was exerted on the lumbar region of the back and the area of transition between the thoracic and cervical regions.

How to get an exact impact on the object in double-pulsed ruby laser holographic interferometry-new instrumentation

A new instrumentation for non-destructive testing of metals is described. In the course of the test an external impact is given to the object and simultaneously a double-pulsed ruby laser interferogram is taken of the object. The difficulty with these kinds of NDT method is to get a good, well controlled impact. The problem is solved by using a CO2 pistol and a timer based on a fast hardware multiplier. The apparatus designed for working outside the laboratory, and some results, are described.

A new method of resolving transient stresses in dynamic holophotoelasticity

A method which allows the simultaneous separation of the isochromatic and isopachic fringes for transient plane-stress problems is presented. A double pulsed ruby laser and a terbium glass Faraday rotator is employed to resolve the transient isochromatic and isopachic patterns. Separated fringe patterns for a structural component are recorded at nine different instants after impact loading. The dynamic material-fringe values of isochromatics and isopachics are obtained within the experiment. Finally, dynamic stresses distributed along a section at different time intervals are resolved.

A study of stress propagation under impact loading using double exposure speckle photography and finite element analysis

This paper presents a solution to the problem of displacement fields in a plane stress model during the propagation of a stress wave using double exposure speckle photography and the finite element method. Double exposure speckle photography at different time delays was recorded utilizing a double-pulse ruby laser and the reflective speckle interferometry method. The instantaneous loading after impact was recorded experimentally. By setting this load-time relation as the force boundary condition, the displacement was calculated using the finite element method.

Transient bending waves in plates studied by hologram interferometry

Propagating bending waves are studied in plates made of aluminum and wood. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as the light source, is used to record the out of plane motion of the waves. Elliptic-like fringes visualize differences in wave speed for different directions in the anisotropic plate and circular ones are obtained for the isotropic plate. The experimental data for the isotropic plate compare favorably with analytical results derived from the Kirchhoff-plate equation with a point impact of finite duration. A similarity variable is found when starting conditions are modeled as a Dirac pulse in space and time, that brings new understanding to the importance of specific parameters for wave propagation in plates. A formal solution is obtained for a point force with an arbitrary time dependence. For times much larger than the contact time, the plate deflection is shown to be identical to that from a Dirac pulse applied at the mean contact time. A method for determining material parameters, and the mean contact time, from the interferograms is hence developed.

Transient bending waves in anisotropic plates studied by hologram interferometry

Propagating bending waves are studied in plates made of glass-fiber reinforced polyester. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as light source, is used to record the out of plane motion of the waves. The interferograms have an elliptic-like symmetry for an orthotropic plate, while the wave pattern for a symmetric angle-ply reinforced plate has a symmetry about the axes of reinforcements. Experimental data are compared on one hand to analytical results obtained by assuming that the orthotropic plate can be described as if isotropic along the main axes, and on the other hand to numerical results from calculations using the finite-element method. The effective Young's modulus raised to power 1/4 is shown to be an important parameter for the description of the dispersive wave pattern. A defect in the plate alters the wave pattern in the interferograms significantly. This may have technical use.

Transient wave propagation in wooden plates for musical instruments

Transient bending waves in three wooden plates are studied. A rectangular flat spruce plate (from a guitar top plate) and two arched violin plates (of spruce and maple, respectively) are impacted by a small ballistic pendulum. The impact point is at the center of the guitar plate and the bridge position of the violin plates. A sequence of holographic interferograms of the events is recorded using a double pulsed ruby laser as the light source. The holographic setup is a standard one. The elliptical shape of the interference fringes shows that wood is strongly anisotropic. The shape of the propagating pulse changes with time, which shows the dispersive behavior of bending waves in plates. A solution of the isotropic Euler plate equation for a flat plate is compared to experimental results of the guitar plate, assuming that this description is adequate along the main axes of this orthotropic plate. The shape of the propagating wave pattern and the elliptical rather than circular shape of the violin imply an interesting coincidence.