Centre Spatial de Liège Université de Liège © Centre Spatial de Liège High Resolution Dynamic Holography with Photorefractive Crystals : Principles and Applications to Vibrations Measurement Marc GEORGES, Centre Spatial de Liège

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège 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

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Real-time Holographic Interferometry I(x,y)=I 0 (x,y) [1+m(x,y) cos(  (x,y))]Interferogram Holographic Interferometry

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  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 Holographic Interferometry

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

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège 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 Photorefractive crystals

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Crystal families –Sillenites : Bi 12 SiO 20 (BSO), Bi 12 GeO 20 (BGO), Bi 12 TiO 20 (BTO) –Ferroelectrics : LiNbO 3, BaTiO 3, KNbO 3, KTN, SBN,… –Semiconductors : CdTe, ZnTe, AsGa, InP,…  Figures of merit  n =  n sat (1-exp(-t/  )) –Recording energy at saturation : E s = .I –Diffraction efficiency :  = I diff /I ref ~ (  n) 2 Photorefractive crystals

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Particular properties : depend on crystal cut Photorefractive crystals Anisotropic diffractionIsotropic diffraction Interferogram contrast depends on the analyser orientation Interferogram contrast depends on the product : -coupling constant -crystal thickness

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Sillenites : BSO - BGO - BTO Sensitive in blue-green, red with dopants E S ~ 1-10 mJ/cm 2,  ~ 0.1 %,  ~ 0.5 cm -1  Ferroelectrics : LiNbO 3 - KNbO 3 - BaTiO 3 - SBN... Sensitive blue-green, red-near IR with dopants E S ~ 1-10 J/cm 2,  ~ 100 %,  ~ cm -1  Semiconductors : CdTe - ZnTe - CdZnTe.… Sensitive in near IR E S ~ mJ/cm 2,  ~ 1 %,  ~ 0.5 cm -1 Photorefractive crystals

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Developed by CSL : Cw Holographic Camera –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 supplylaser + power supply camera, piezo,.. electronic controlscamera, piezo,.. electronic controls

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Applications : static measurements –NDT (defect detection) : impacts-delamination in CFRP Interferogram obtained after thermal stimulation (40X55 cm 2 ) Calculated phase image Unwrapped image with vertical differentiation Cw Holographic Camera –NDT (defect detection) : lack of soldering in flat cables (10 x 5 cm 2 )

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège –Displacement metrology : calibration of piezoelectric sheets (40x25 cm 2 )calibration of piezoelectric sheets (40x25 cm 2 ) sensor-actuators for smart structure controlsensor-actuators for smart structure control High fringe densityHigh fringe density Cw Holographic Camera

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège Cw Holographic Camera –Displacement metrology : Determination of CTE of carbon fiber rods or assembliesDetermination of CTE of carbon fiber rods or assemblies Observe top of object and baseplateObserve top of object and baseplate After  T : Measure difference of displacements betw.After  T : Measure difference of displacements betw. –top of object : piston effect –baseplate : piston effect

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Applications : Stroboscopic Real-Time –Acousto-optic shutter synchronized with sinusoidal excitation with sinusoidal excitation –Open at maximum object displacement –Displacement btw. average & maximum positions –Duty cycle : –Compromise between fringe contrast - image intensity Cw Holographic Camera Frequency scanAcquisition

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  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 shapesCertification predicted resonance frequencies and mode shapes Several modes found were not predictedSeveral modes found were not predicted Stroboscopic system

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Positive –High quality results –Convenient for mode shape visualization –Convenient for comparison with predicted frequencies / mode shapes –Userfriendly device, indefinitely reusable  Limits : –Displacements : from nm to 30 microns –Stroboscope loss of light (80 % with 0.2 duty cycle)loss of light (80 % with 0.2 duty cycle) small objects (25x25 cm 2 ) with 500 mW lasersmall objects (25x25 cm 2 ) with 500 mW laser

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Motivations –Luminous Energy concentrated over a few nanoseconds One can deal with perturbed environmentOne can deal with perturbed environment No more illumination constraints at the readout stepNo more illumination constraints at the readout step (like in the case of stroboscopic readout with cw laser) (like in the case of stroboscopic readout with cw laser) –2 pulses with variable delay High vibration amplitudesHigh vibration amplitudes Fast transient eventsFast transient events Pulsed system

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

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  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 Pulsed system

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège Pulsed system –Pulse 1 : all energy used for the recording –Pulse 2 : readout decrease E obj to avoid CCD bloomingdecrease E obj to avoid CCD blooming decrease E ref to not erase the hologramdecrease E ref to not erase the hologram –Phase  measurement : Cam 1 : I = I 01 (1+m sin  )Cam 1 : I = I 01 (1+m sin  ) Cam 2 : I = I 02 (1+m cos  )Cam 2 : I = I 02 (1+m cos  )

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège Vibrations  4 pulses technique

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  In practice –Laser :1 pulse 30 Hz max –High frequencies : Use several cycles at a given frequency   Results : –Object : Aluminium plate clamped on one edge –Excitation : Loudspeaker  Frequency range : Hz Interferograms serie example Interferograms serie example Hz Hz

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège  Amplitude of the frequency response in 2 points

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège 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  s  Applications in industrial cases (vibrations, transient events, aerodynamics)