Centre Spatial de Liège Université de Liège © Centre Spatial de Liège 2002 1 High Resolution Dynamic Holography with Photorefractive Crystals : Principles

$Page 1: Centre Spatial de Liège Université de Liège © Centre Spatial de Liège 2002 1 High Resolution Dynamic Holography with Photorefractive Crystals : Principles$
1

Centre Spatial de LiègeUniversité de Liège

© Centre Spatial de Liège 2002

High Resolution Dynamic HolographyHigh Resolution Dynamic Holographywith Photorefractive Crystals :with Photorefractive Crystals :

Principles and Applications to Vibrations Principles and Applications to Vibrations MeasurementMeasurement

Marc GEORGES, Centre Spatial de LiègeMarc GEORGES, Centre Spatial de Liège

2



Holographic InterferometryHolographic Interferometry

Full-field, non-contact techniqueFull-field, non-contact technique Displacements measurement : 10 nm - 25 microns (one shot)Displacements measurement : 10 nm - 25 microns (one shot) Higher resolution compared to Speckle-basedHigher resolution compared to Speckle-based Needs of potential userNeeds of potential user

Easy to set upEasy to set up Quantified data, easy to interpreteQuantified data, easy to interprete Transportable/portable, compact, robust, flexible, …Transportable/portable, compact, robust, flexible, … Configuration adaptableConfiguration adaptable Cheap, low consumptionCheap, low consumption

3



Real-time Holographic InterferometryReal-time Holographic Interferometry

I(x,y)=I0(x,y) [1+m(x,y) cos((x,y))]InterferogramInterferogram


4



Critical segment for applicability : Holographic mediumCritical segment for applicability : Holographic medium– FastFast

– Homogeneous (optical quality)Homogeneous (optical quality)

– Processable in-situProcessable in-situ

– Erasable, reversibleErasable, reversible

– Low diffusion noise (high signal-noise ratio)Low diffusion noise (high signal-noise ratio)

– No or fewest operations possible for obtaining informationNo or fewest operations possible for obtaining information


5



Photorefractive crystalsPhotorefractive crystals

Local space charge fieldLocal space charge field

created between dark created between dark

and illuminated areaand illuminated area

1.1. Fringe pattern created by Fringe pattern created by interference between 2 wavesinterference between 2 waves

2.2. Charges generated byphoto-excitation in illuminated area,Charges generated byphoto-excitation in illuminated area,

migrate and are trapped in dark areamigrate and are trapped in dark area

6



3. Electro-optic effect (Pockels)3. Electro-optic effect (Pockels)

Refractive index Refractive index nn is modulated by space-charge field is modulated by space-charge field

Recording of a volumic refractive index grating Recording of a volumic refractive index grating (thick (thick hologram)hologram)

4. Processus is dynamic and reversible4. Processus is dynamic and reversible

In-situ recordingIn-situ recording

Erasure possible = Re-recordingErasure possible = Re-recording


7



Crystal familiesCrystal families– Sillenites : BiSillenites : Bi1212SiOSiO2020 (BSO), Bi (BSO), Bi1212GeOGeO2020 (BGO), Bi (BGO), Bi1212TiOTiO2020 (BTO) (BTO)

– Ferroelectrics : LiNbOFerroelectrics : LiNbO33, BaTiO, BaTiO33, KNbO, KNbO33, KTN, SBN,…, KTN, SBN,…

– Semiconductors : CdTe, ZnTe, AsGa, InP,…Semiconductors : CdTe, ZnTe, AsGa, InP,…

Figures of meritFigures of merit

n = nsat (1-exp(-t/))

– Recording energy at saturation : ERecording energy at saturation : Ess = = .I.I

– Diffraction efficiency : Diffraction efficiency : = I= Idiffdiff/I/Irefref ~ ( ~ (n)n)22


8



Particular properties : depend on crystal cutParticular properties : depend on crystal cut

θ θ


Γl =4πΔnlλ

≈1

a

P2P1PdiffPt

Anisotropic diffraction Isotropic diffraction

Interferogram contrast depends on the analyser orientation Interferogram contrast depends on the product :-coupling constant-crystal thickness

9



Sillenites : BSO - BGO - BTOSillenites : BSO - BGO - BTOSensitive in blue-green, red with dopantsSensitive in blue-green, red with dopants

EESS ~ 1-10 mJ/cm ~ 1-10 mJ/cm22, , ~ 0.1 %, ~ 0.1 %, ~ 0.5 cm~ 0.5 cm-1-1

Ferroelectrics : LiNbOFerroelectrics : LiNbO33 - KNbO - KNbO33 - BaTiO - BaTiO33 - SBN ... - SBN ...Sensitive blue-green, red-near IR with dopantsSensitive blue-green, red-near IR with dopants

EESS ~ 1-10 J/cm ~ 1-10 J/cm22, , ~ 100 %, ~ 100 %, ~ 1 - 40 cm~ 1 - 40 cm-1-1

Semiconductors : CdTe - ZnTe - CdZnTe .…Semiconductors : CdTe - ZnTe - CdZnTe .…Sensitive in near IRSensitive in near IR

EESS ~ 0.1-1 mJ/cm ~ 0.1-1 mJ/cm22, , ~ 1 %, ~ 1 %, ~ 0.5 cm~ 0.5 cm-1-1


10



Developed by CSL : 1993-1998Developed by CSL : 1993-1998

Cw Holographic CameraCw Holographic Camera

– Optical head : L=25 cm, diam=8 cmOptical head : L=25 cm, diam=8 cm

1 kg1 kg

– Laser : DPSS, VERDI 5WLaser : DPSS, VERDI 5W

– Laser light brought by optical fiberLaser light brought by optical fiber

– Specialty fiber developedSpecialty fiber developed

(5 m, Transmission 80%, 5W injected)(5 m, Transmission 80%, 5W injected)

– Mobile rack includingMobile rack including• laser + power supplylaser + power supply

• camera, piezo,.. electronic controlscamera, piezo,.. electronic controls

11



Applications : static measurementsApplications : static measurements– NDT (defect detection) : impacts-delamination in CFRPNDT (defect detection) : impacts-delamination in CFRP

Interferogram obtained after Interferogram obtained after thermal stimulation (40X55 thermal stimulation (40X55 cmcm22))

Calculated phase imageCalculated phase image Unwrapped image with vertical Unwrapped image with vertical differentiationdifferentiation


– NDT (defect detection) : lack of soldering in flat cables (NDT (defect detection) : lack of soldering in flat cables (10 x 5 cm10 x 5 cm22))

12



a

aluminium plate(back side)piezosheetspoint where the force is appliedclamping points of the plate

– Displacement metrology :Displacement metrology :• calibration of piezoelectric sheets (40x25 cmcalibration of piezoelectric sheets (40x25 cm22))

• sensor-actuators for smart structure controlsensor-actuators for smart structure control

• High fringe densityHigh fringe density


13



Cw Holographic CameraCw Holographic Camera– Displacement metrology :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 After T : Measure difference of displacements betw.T : Measure difference of displacements betw.– top of object : piston effecttop of object : piston effect

– baseplate : piston effectbaseplate : piston effect

a

samplebaseradiator

holographiccameraoptical fiber

object illuminationbeamwindow

14



Applications : Stroboscopic Real-TimeApplications : Stroboscopic Real-Time

– Acousto-optic shutter synchronizedAcousto-optic shutter synchronized

with sinusoidal excitationwith sinusoidal excitation

– Open at maximum object displacementOpen at maximum object displacement

– Displacement btw. average & maximum positionsDisplacement btw. average & maximum positions

– Duty cycle : 0.15 - 0.2Duty cycle : 0.15 - 0.2

– Compromise between fringe contrast - image intensityCompromise between fringe contrast - image intensity


a

t0-T+Tholographicrecordingstroboscopicreadout

a

tstroboscopic readout

StroboscopicshutterObjectdisplacementHologr.RecordingVisualisationFrequency scan

a

tstroboscopic readout

StroboscopicshutterHologr.RecordingImagecapturePhase-shiftingObjectdisplacement Acquisition

15



ApplicationsApplications– Academic demonstration : Metallic plate excited with loudspeakerAcademic demonstration : Metallic plate excited with loudspeaker

(M. Georges, Ph. Lemaire, Optics Comm. 98)(M. Georges, Ph. Lemaire, Optics Comm. 98)

– Recent tests (by Optrion) : Compressor blades for new aircraft engineRecent 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 systemStroboscopic system

16



17



PositivePositive– High quality resultsHigh quality results

– Convenient for mode shape visualizationConvenient for mode shape visualization

– Convenient for comparison with predicted frequencies / mode Convenient for comparison with predicted frequencies / mode shapesshapes

– Userfriendly device, indefinitely reusableUserfriendly device, indefinitely reusable

Limits :Limits :– Displacements : from 15-20 nm to 30 micronsDisplacements : from 15-20 nm to 30 microns

– StroboscopeStroboscope• loss of light (80 % with 0.2 duty cycle)loss of light (80 % with 0.2 duty cycle)

• small objects (25x25 cmsmall objects (25x25 cm22) with 500 mW laser) with 500 mW laser

18



MotivationsMotivations– Luminous Energy concentrated over a few nanosecondsLuminous 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 delay2 pulses with variable delay• High vibration amplitudesHigh vibration amplitudes

• Fast transient eventsFast transient events

Pulsed systemPulsed system

19



First worksFirst works– LCFIO (group of G. Roosen-G. Pauliat)LCFIO (group of G. Roosen-G. Pauliat)

• Labrunie Labrunie et alet al., Opt. Lett. 20 (1995)., Opt. Lett. 20 (1995)

• Labrunie Labrunie et alet al., PR ’95., PR ’95

• Labrunie Labrunie et alet al., Opt. Comm. 140 (1997)., Opt. Comm. 140 (1997)

PR crystal weak sensitivityPR crystal weak sensitivity at 694 nmat 694 nm

New crystal BGO:CuNew crystal BGO:Cu

BSO - BGOBSO - BGO 488 - 514 - 532 nm488 - 514 - 532 nm

(J-C. Launay, ICMCB Bordeaux)(J-C. Launay, ICMCB Bordeaux)

– Ruby LaserRuby Laser

– Quality of results (vibration mode of turbine blade) was average, Quality of results (vibration mode of turbine blade) was average, tough acceptabletough acceptable


20



New developments since 1998 (CSL and LCFIO)New developments since 1998 (CSL and LCFIO)

– Use Q-switch YAG laser (COHERENT Infinity)Use Q-switch YAG laser (COHERENT Infinity)

frequency doubled : 532 nm (adapted to sillenite crystals)frequency doubled : 532 nm (adapted to sillenite crystals)

pulses : 3 nspulses : 3 ns

energies : 0 to 400 mJ/pulseenergies : 0 to 400 mJ/pulse

repetition rate : 0,1 to 30 Hzrepetition rate : 0,1 to 30 Hz

– Additional equipment for energy balance between pulsesAdditional equipment for energy balance between pulses

– Application in vibration measurementApplication in vibration measurement


21




– Pulse 1 : all energy used for the recording Pulse 1 : all energy used for the recording

– Pulse 2 : readoutPulse 2 : readout• decrease Edecrease Eobjobj to avoid CCD blooming to avoid CCD blooming

• decrease Edecrease Erefref to not erase the hologram to not erase the hologram

a

Pockels 1

a

Pockels 3

aa

delaylineM4M5

M1M2M3

PRcrystalO1O2PBC3O3O3’Cam 1Cam 2PBC2HWP1

DLCLDLM6 object

a

CSP2Pockels 2

a

lighttrap– Phase Phase measurement : measurement :

• Cam 1 : I = ICam 1 : I = I0101 (1+m sin (1+m sin ))

• Cam 2 : I = ICam 2 : I = I0202 (1+m cos (1+m cos ))

22



VibrationsVibrations 4 pulses technique4 pulses technique

Rt = A f cos Φ f + ωt

D1 = A f cos Φ f+ ωτ + ωt0 – cos Φ f+ ωt0

D2 = A f cos Φ f+ ωτ + ωt0 + π

2– cos Φ f+ ωτ + π

2

a

0 t

A f=D1

2 + D22

2 1 – cos ωτ

a

W1t0

a

R1

a

W2T/4

a

R2

23



In practiceIn practice– Laser :Laser : 1 pulse1 pulse

30 Hz max30 Hz max

– High frequencies : Use several cycles at a given frequency High frequencies : Use several cycles at a given frequency

Results :Results :– Object : Aluminium plate clamped on one edgeObject : Aluminium plate clamped on one edge

– Excitation : LoudspeakerExcitation : Loudspeaker

Frequency range : 20-380 HzFrequency range : 20-380 Hz Interferograms serie exampleInterferograms serie example

359-365 Hz359-365 Hz

24



Amplitude of the frequency response in 2 pointsAmplitude of the frequency response in 2 points

0

1

2

3

4

5

0 50 100 150 200 250 300 350 400

( )fréquence Hz

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0 50 100 150 200 250 300 350 400

( )fréquence Hz

25



Conclusion - Future prospectsConclusion - Future prospects Present : PHIFE « Pulsed Holographic Interferometer for Present : PHIFE « Pulsed Holographic Interferometer for

analysis of Fast Events »analysis of Fast Events » Development of holographic headsDevelopment of holographic heads

– Improvement of existing ones (new crystal configuration/properties)Improvement of existing ones (new crystal configuration/properties)

– different wavelengthsdifferent wavelengths

Development of double-pulse laser (INNOLAS)Development of double-pulse laser (INNOLAS)– YAG Q-switchedYAG Q-switched

– 25 Hz, 8 nsec, 800 mJ (1064 nm)25 Hz, 8 nsec, 800 mJ (1064 nm)

– delay : up to 0.1 delay : up to 0.1 ss

Applications in industrial cases (vibrations, transient events, Applications in industrial cases (vibrations, transient events,

aerodynamics)aerodynamics)

Documents

Centre Spatial de Liège Université de Liège © Centre Spatial de Liège 2002 1 High Resolution Dynamic Holography with Photorefractive Crystals : Principles