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Marc GEORGES, Centre Spatial de Liège

High Resolution Dynamic Holography with Photorefractive Crystals : Principles and Applications to Vibrations Measurement. Marc GEORGES, Centre Spatial de Liège. Holographic Interferometry. Full-field, non-contact technique Displacements measurement : 10 nm - 25 microns (one shot)

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Marc GEORGES, Centre Spatial de Liège

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  1. High Resolution Dynamic Holographywith Photorefractive Crystals :Principles and Applications to Vibrations Measurement Marc GEORGES, Centre Spatial de Liège

  2. Holographic Interferometry • Full-field, non-contact technique • Displacements measurement : 10 nm - 25 microns (one shot) • Higher resolution compared to Speckle-based • Needs of potential user • Easy to set up • Quantified data, easy to interprete • Transportable/portable, compact, robust, flexible, … • Configuration adaptable • Cheap, low consumption

  3. I(x,y)=I0(x,y) [1+m(x,y) cos(Df(x,y))] Holographic Interferometry • Real-time Holographic Interferometry Interferogram

  4. Holographic Interferometry • Critical segment for applicability : Holographic medium • Fast • Homogeneous (optical quality) • Processable in-situ • Erasable, reversible • Low diffusion noise (high signal-noise ratio) • No or fewest operations possible for obtaining information

  5. Photorefractive crystals 1.Fringe pattern created by interference between 2 waves 2.Charges generated byphoto-excitation in illuminated area, migrate and are trapped in dark area Local space charge field created between dark and illuminated area

  6. Photorefractive crystals 3. Electro-optic effect (Pockels) Refractive index n is modulated by space-charge field Recording of a volumic refractive index grating (thick hologram) 4. Processus is dynamic and reversible In-situ recording Erasure possible = Re-recording

  7. Dn = Dnsat (1-exp(-t/t)) Photorefractive crystals • Crystal families • Sillenites : Bi12SiO20 (BSO), Bi12GeO20 (BGO), Bi12TiO20 (BTO) • Ferroelectrics : LiNbO3, BaTiO3, KNbO3, KTN, SBN,… • Semiconductors : CdTe, ZnTe, AsGa, InP,… • Figures of merit • Recording energy at saturation : Es = t.I • Diffraction efficiency : h = Idiff/Iref ~ (Dn)2

  8. Photorefractive crystals • Particular properties : depend on crystal cut Anisotropic diffraction Isotropic diffraction Interferogram contrast depends on the analyser orientation Interferogram contrast depends on the product : -coupling constant -crystal thickness

  9. Photorefractive crystals • Sillenites : BSO - BGO - BTO Sensitive in blue-green, red with dopants ES ~ 1-10 mJ/cm2, h~ 0.1 %, G~ 0.5 cm-1 • Ferroelectrics : LiNbO3 - KNbO3 - BaTiO3 - SBN ... Sensitive blue-green, red-near IR with dopants ES ~ 1-10 J/cm2, h~ 100 %, G ~ 1 - 40 cm-1 • Semiconductors : CdTe - ZnTe - CdZnTe .… Sensitive in near IR ES ~ 0.1-1 mJ/cm2, h~ 1 %, G ~ 0.5 cm-1

  10. Cw Holographic Camera • Developed by CSL : 1993-1998 • Optical head : L=25 cm, diam=8 cm 1 kg • Laser : DPSS, VERDI 5W • Laser light brought by optical fiber • Specialty fiber developed (5 m, Transmission 80%, 5W injected) • Mobile rack including • laser + power supply • camera, piezo,.. electronic controls

  11. Cw Holographic Camera • Applications : static measurements • NDT (defect detection) : impacts-delamination in CFRP Interferogram obtained after thermal stimulation (40X55 cm2) Calculated phase image Unwrapped image with vertical differentiation • NDT (defect detection) : lack of soldering in flat cables (10 x 5 cm2)

  12. Cw Holographic Camera • Displacement metrology : • calibration of piezoelectric sheets (40x25 cm2) • sensor-actuators for smart structure control • High fringe density

  13. Cw Holographic Camera • Displacement metrology : • Determination of CTE of carbon fiber rods or assemblies • Observe top of object and baseplate • After DT : Measure difference of displacements betw. • top of object : piston effect • baseplate : piston effect

  14. Frequency scan Acquisition Cw Holographic Camera • Acousto-optic shutter synchronized with sinusoidal excitation • Open at maximum object displacement • Displacement btw. average & maximum positions • Duty cycle : 0.15 - 0.2 • Compromise between fringe contrast - image intensity • Applications : Stroboscopic Real-Time

  15. Stroboscopic system • Applications • Academic demonstration : Metallic plate excited with loudspeaker (M. Georges, Ph. Lemaire, Optics Comm. 98) • Recent tests (by Optrion) : Compressor blades for new aircraft engine • Certification predicted resonance frequencies and mode shapes • Several modes found were not predicted

  16. Positive • High quality results • Convenient for mode shape visualization • Convenient for comparison with predicted frequencies / mode shapes • Userfriendly device, indefinitely reusable • Limits : • Displacements : from 15-20 nm to 30 microns • Stroboscope • loss of light (80 % with 0.2 duty cycle) • small objects (25x25 cm2) with 500 mW laser

  17. Pulsed system • Motivations • Luminous Energy concentrated over a few nanoseconds • One can deal with perturbed environment • No more illumination constraints at the readout step (like in the case of stroboscopic readout with cw laser) • 2 pulses with variable delay • High vibration amplitudes • Fast transient events

  18. Pulsed system • First works • LCFIO (group of G. Roosen-G. Pauliat) • Labrunie et al., Opt. Lett. 20 (1995) • Labrunie et al., PR ’95 • Labrunie et al., Opt. Comm. 140 (1997) • Ruby Laser at 694 nm PR crystal weak sensitivity BSO - BGO 488 - 514 - 532 nm New crystal BGO:Cu (J-C. Launay, ICMCB Bordeaux) • Quality of results (vibration mode of turbine blade) was average, tough acceptable

  19. Pulsed system • New developments since 1998 (CSL and LCFIO) • Use Q-switch YAG laser (COHERENT Infinity) frequency doubled : 532 nm (adapted to sillenite crystals) pulses : 3 ns energies : 0 to 400 mJ/pulse repetition rate : 0,1 to 30 Hz • Additional equipment for energy balance between pulses • Application in vibration measurement

  20. Pulsed system • Pulse 1 : all energy used for the recording • Pulse 2 : readout • decrease Eobj to avoid CCD blooming • decrease Eref to not erase the hologram • Phase f measurement : • Cam 1 : I = I01 (1+m sin f) • Cam 2 : I = I02 (1+m cos f)

  21. Vibrations • 4 pulses technique

  22. In practice • Laser : 1 pulse 30 Hz max • High frequencies : Use several cycles at a given frequency w • Results : • Object : Aluminium plate clamped on one edge • Excitation : Loudspeaker • Frequency range : 20-380 Hz Interferograms serie example 359-365 Hz

  23. Amplitude of the frequency response in 2 points

  24. Conclusion - Future prospects • Present : PHIFE « Pulsed Holographic Interferometer for analysis of Fast Events » • Development of holographic heads • Improvement of existing ones (new crystal configuration/properties) • different wavelengths • Development of double-pulse laser (INNOLAS) • YAG Q-switched • 25 Hz, 8 nsec, 800 mJ (1064 nm) • delay : up to 0.1 ms • Applications in industrial cases (vibrations, transient events, aerodynamics)

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