High Resolution Dynamic Holography with Photorefractive Crystals : Principles and Applications to Vibrations Measurement

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High Resolution Dynamic Holographywith
Photorefractive Crystals Principles and
Applications to Vibrations Measurement

• Marc GEORGES, Centre Spatial de Liège

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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

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Holographic Interferometry

• Real-time Holographic Interferometry

Interferogram
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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

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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
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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

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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

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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
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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

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Cw Holographic Camera

• Developed by CSL 1993-1998

• Optical head L25 cm, diam8 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

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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)

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Cw Holographic Camera

• Displacement metrology
• calibration of piezoelectric sheets (40x25 cm2)
• sensor-actuators for smart structure control
• High fringe density

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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

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Cw Holographic Camera

• Applications Stroboscopic Real-Time

• 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

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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

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• 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

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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

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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 BGOCu
(J-C. Launay, ICMCB Bordeaux)

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

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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

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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 (1m sin f)
• Cam 2 I I02 (1m cos f)

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Vibrations

• 4 pulses technique

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• 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

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• Amplitude of the frequency response in 2 points

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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)