Laser Ultrasound for NDT: investigation of the generation beam shape

Usually laser ultrasonic systems use an infrared laser operating at 10 µm to generate the ultrasound in CFRP objects. However this system suffer from an important drawback: optical fiber cannot be used to transport the high energy beam from the laser output to the target which limits the flexibility of the system to investigated complex shaped objects. To overcome this issue, visible light can be used to generate ultrasound. In our case we use a fiber-coupled laser operating at 532 nm. The output end of the optical fiber is placed on an industrial robot arm. The investigated object remains stationary while the optical fiber mounted on the robot arm scan the object. This system offers large flexibility but laser generation at 532 nm is known to be less efficient than CO 2 system emitting at 10 µm. Increasing the visible pulse power is one of the options but optics and CRFF object will be damaged before the same echo level as CO2 generation can be obtained. An alternative solution consists in the investigation of the generation beam size and shape. We recently developed an optical design composed of an axicon lens able to generate annular beam. The diameter of the beam can be adapted. If the axicon lens is removed the optical system provides a disk whose diameter can also be adapted onto the investigated object. Ray-traycing simulations performed with ASAP and experimental beam profile investigated with a beam profiler show good agreement between theoretical and experimental optical design. An experimental comparison between ring and disk shapes will be presented. The comparisons take into account the beam diameter and the beam intensity but also the effect of the angle of incidence will be presented.TECCOM

Development of a full fiber-coupled laser ultrasound robotic system using two-wave mixing 1064 nm detection and 532 nm YAG generation

An all-fibered laser ultrasonic system for complex shape composite parts is presented. It is based on two-wave mixing detection and a long pulse laser working at 1064 nm and generation by a fibered YAG Q-switch laser working at 532 nm. A compact optical head combining both beams is interfaced to a robot system for scanning. Some practical issues of this system are studied.ECOTA

Holography in the invisible. From the thermal infrared to the terahertz waves: outstanding applications and fundamental limits

peer reviewedSince its invention, holography has been mostly applied at visible wavelengths in a variety of applications. Specifically, non-destructive testing of manufactured objects was a driver for developing holographic methods and all of their parents based on the speckle pattern recording. One substantial limitation of holography non-destructive testing is the setup stability requirements directly related to the laser wavelength. This observation has driven some works for 15 years: developing holography at wavelengths much longer than visible ones. In this paper, we will first review researches carried out in the infrared, mostly digital holography at thermal infrared wavelengths around 10 micrometers. We will discuss the advantages of using such wavelengths and show different examples of applications. In non-destructive testing, large wavelengths allow using digital holography in perturbed environments on large objects and measure large deformations, typical of the aerospace domain. Other astonishing applications such as reconstructing scenes through smoke and flames were proposed. Going further in the spectrum, digital holography with so-called Terahertz waves (up to 3 millimeters wavelength) has also been studied. The main advantage here is that these waves easily penetrate some materials. Therefore, one can envisage Terahertz digital holography to reconstruct the amplitude and phase of visually opaque objects. We review some cases in which Terahertz digital holography has shown potential in biomedical and industrial applications. We will also address some fundamental bottlenecks that prevent fully benefiting from the advantages of digital holography when increasing the wavelength

Use of specklegrams background terms for speckle photography combined with phase-shifting electronic speckle pattern interferometry

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Electronic speckle pattern interferometry (ESPI) is combined with digital speckle photography (DSP) to measure out-of-plane deformation in the presence of large in-plane translation or rotation. ESPI is used to measure out-of-plane displacements smaller than the speckle diameter. In-plane displacements larger than the speckle size are obtained by DSP using artifacts images computed from the phase-stepped specklegrams. Previous works use the specklegram modulation for that purpose, but we show that this can lead to errors in the case of low modulation. In order to avoid this, a simple averaging of phase-stepped specklegrams allows obtaining the average irradiance, which contains information on the speckled object image. The latter can be used more efficiently than the modulation in DSP and is simpler to compute. We also perform a numerical simulation of specklegrams, which show that the use of background terms is much more stable against some error sources as compared to modulation. We show experimental evidence of this in various experiments combining out-of-plane ESPI measurements with in-plane translations or rotations obtained by our DSP method. The latter has been used efficiently to restore phase loss in out-of-plane ESPI measurements due to large in-plane displacements. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)Electronic speckle pattern interferometry (ESPI) is combined with digital speckle photography (DSP) to measure out-of-plane deformation in the presence of large in-plane translation or rotation. ESPI is used to measure out-of-plane displacements smaller t548FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)sem informaçãoRedouanne Zemmamouche would like to acknowledge the Ministry of Higher Education and Scientific Research of Algeria, the University of M’SILA, and the Institut National d’Optique et Mécanique de Precision of Sétif University for providing the grant for

Digital Holographic Interferometry in the Long-Wave Infrared for the Testing of Large Aspheric Space Reflectors

Digital holographic (DH) interferometry has been developed in the long-wave infrared spectral range with CO2 lasers and microbolometer arrays. This application has been driven by the European Space Agency’s constant need of techniques for monitoring large displacements of large structures. Here the study focuses on the case of aspheric mirrors, like parabola and ellipses. Usually they are tested through interferometric wavefront error measurements which require expensive null-lenses matching each of the reflectors considered. In the case of monitoring deformation a holographic technique can be considered where the wavefront is compared with itself at different instant. Therefore the optical can be quite simple and easily reconfigurable from one reflector to another. The advantage of using long wavelength is that large deformations can be measured at once, in addition to being more immune against environmental perturbations. Another advantage of DH at such wavelengths is that the ratio between the wavelength and the pixel size allows reconstructing objects 5 to 10 times larger than with DH in the visible. In this project we considered first the case of a 1.1 meter diameter parabola for submillimeter range observations. Such specimen shows strong specular reflectivity. We have developed several set-ups with different ways to illuminate the object and to collect rays to form the object beam: either through point source or through and extended diffuser working by reflection of the laser beam. Both possibilities have been compared in terms of fringe quality as well as measurement range. We selected the diffuser illumination for applying the set-up into a large vacuum facility for measuring the deformation of the parabola between 224 and 107 K. Results of this measurement campaign are presented. A further application has been shown by observation of off-axis ellipse. In this case interferometric testing is difficult to achieve and LWIR DHI with diffuser illumination is found quite simple to implement and gives straightforward results.GSTP HOLODI

PARTICIPATION OF UNDERGRADUATE STUDENTS TO THE SOLAR PANELS DESIGN OF THE EDUCATIONAL SPACECRAFT ESEO

The objective of this presentation is to discuss the participation of students at the University of Liège to the educational spacecraft project SSETI-ESEO proposed by the European Space Agency

Development of full-field deflectometry for characterization of free-form mirrors for space applications

We demonstrate that full-field deflectometry is a viable alternative to interferometry for the characterization of free-form mirrors. Deflectometry does not require the use of a CGH. Instead of measuring the surface height map, the deflectometer measures the surface slopes in two orthogonal directions using the phase-shifting Schlieren method [1]. The surface height map is then reconstructed by integration of the slope maps. We present two instruments. The first one can be mounted in the lathe for in situ measurement. The second is adapted for the characterization of large concave mirrors

Status of the SALTO demonstrator: project overview and first on-sky operations

SALT

Interférométrie de speckle en infrarouge thermique

L’utilisation de l'interférométrie de speckle (ESPI) sur des objets opaques diffusants à une longueur d’onde de 10 µm en utilisant une caméra thermographique commerciale est présentée pour la première fois à notre connaissance. L’idée d’utiliser une longueur d’onde plus grande que les longueurs d’onde visibles habituelles est de rendre les mesures de déplacements par des techniques holographiques moins sensibles aux perturbations extérieures. Dans cet article, nous discutons certains aspect particuliers dûs à l’accroissement de la longueur d’onde dans le domaine thermique de 10 µm. Nous montrons ensuite les résultats de mesures de rotations dans le plan d’une plaque métallique. Nous avons utilisé la technique du décalage de phase pour obtenir des mesures quantitatives qui sont corrélées par des mesures menées en parallèle avec un théodolite