National Institute for Laser, Plasma, and Radiation Physics ... - IESL

Răzvan DABU
Aurel STRATAN
Constantin FENIC
Cătălin LUCULESCU
Constantin UNGUREANU
Laurenţiu RUSEN
Liviu NEAGU
Marian ZAMFIRESCU
Constantin BLANARU
Alexandru POPA
National Institute for
Laser, Plasma, and Radiation Physics
ATOMISTILOR 409, PO Box MG-36, 077125
Bucharest, Romania
TEL /FAX : +4021-4575066; 4574467
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National Institute for Lasers, Plasma and Radiation Physics
Bucharest, Romania
Main role of NILPRP in the LASERACT project:
-To develop the laser system with the specially required
characteristics for the specific measuring techniques:
shearography and holographic interferometry in digital
version.
-To provide expertise in custom laser devices.
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NILPRP activity in the workpackages (till MID-TERM REVIEW)
Work description:
W.P.1.1. Implementation of techniques.
1.1.4. Definition of the operational parameters of the system.
Deliverables:
D2. Parameters of suitable laser-sensor design.
Milestones:
M1. Successful implementation of diagnostic procedure.
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WP 2.1. Laboratory prototype instrumentation
Work description:
Deliverables:
Milestones:
2.1.1 Laboratory prototype laser.
2.1.2. Laboratory prototype multitask sensor.
2.1.3. Laboratory prototype software.
D9. Report of fundamental studies of system operation
D10. Specification report on multitask recording with sensor/laser/ software laboratory
prototype.
D11.Technology implementation
M3. Successful operation of the laboratory prototype.
M4. Successful operation of the laboratory prototype to generate diagnostic information
M5. MID-TERM REVIEW
-Successful development of a laser diagnostic technique
-Successful implementation of a laboratory prototype.
-Committed interest and strategic plans of partnership
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NILPRP participation in the technical meetings:
1. Kick –off meeting, Iraklion, 27-30 March 2003.
2. Technical Meeting, Ancona, 4-6 June 2003.
3. Technical/review Meeting, Bucharest, 12-16 January 2004.
4. Common experiments of shearography with BIAS, Bremen,
30 June-6 July 2004.
5. Common experiments (digital holography) with FORTH-IESL
and ProOptica), Iraklion, 26 July-1 August 2004.
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Required parameters of the laser source.
Laser specifications according to the project proposal:
1. Laser wavelength: 532 nm.
2. Laser pulse duration: 2mJ.
≥
4. Laser pulse repetition rate: 1Hz.
5. Coherence length: > 1m.
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Output beam parameters of the laser system presented in the
technical/review Meeting Bucharest, January 2004, and used for common
experiments in Bremen, June-July, and Iraklion, July 2004.
1. Laser wavelength – 532 nm.
2. Laser pulse duration - 8 nsec.
3. Pulse energy - 2.3mJ.
4. Laser pulse repetition rate - 0.5 – 5Hz , adjustable.
5. Coherence length >1m.
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Laser system setup
Fig.1.M 4 –M 5 , high reflecting mirrors at 1064 nm wavelength; YAG, Nd:YAG rod amplifier; Beam expander x 2.4; µ-Laser:
ultra-compact diode pumped Q-switched microalser,1064 nm wavelength, 10 ns pulsewidth, 50µJ pulse energy,
0-2 kHz adjustable pulse repetition rate, external trigger, coherence length >2m; KTP, doubling crystal 4 x 4 x 10 mm;
M 6 – 7 , dichroic mirrors HR @ 532 nm, HT @ 1064 nm; α, return-angle of the amplifier path; D, dumper; *, front-panel connector;
**, rear-panel connector; ATT, optical attenuators and green filter; PA, photocell preamplifier.
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A
B
Fig. 2.The intensity beam
profile of the microlaser
beam measured at the
amplifier input
Fig. 3. Temporal beam
profile
A: sample profile;
B: averaged profile
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Fig. 4. Energy bar-graph of the microlaser output pulses.
Standard deviation ~ 0.71 %.
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Fig. 5. Histogram of the microlaser pulse energy.
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Fig.6. Energy bar-graph of the microlaser pulses measured at the non-pumped amplifier
output. Standard deviation ~ 2.7%.
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Fig.7. Histogram of the microlaser pulses measured at the non-pumped amplifier
output.
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Fig.8.Bar-graph of the laser pulse energy at the amplifier output (23 J pump-
energy) (a 3-dB- atenuation before energy detector). Standard deviation ~ 3.7 %.
- Pump energy restricted to 23 J in order to avoid parasitic feedback effects between
the microlaser resonator and two pass amplifier (due to the lack of the optical
isolation between microlaser and amplifier)
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Fig.9. Histogram of the pulse energy at the amplifier output (1064nm) (a 3-dBattenuation
before energy detector).
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Fig.10. The bar-graph of the green pulse (532nm) energy.
Standard deviation ~ 25.1%.
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Fig.11. Histogram of the green pulse energy.
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Fig.12. Temporal profile of the green laser pulses.
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Fig.13. Near-field intensity profile (3D) of the green output pulse.
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Coherence length of the green laser
We measured the coherence length of the green laser pulses using a Michelson interferometer set-up shown.
As coherence length, we considered the path length difference for which fringe visibility V in
the Michelson interferometer is reduced to ½:
where I max , I min are related to the intensity of the interference fringe pattern.
V>0.5 for path length difference of 176 cm
max min ≥
Fig. 14. Experimental set-up for coherence length evaluation
µL, microlaser; M1, flat mirrors, HR @ 1064nm; KTP, doubling crystal, 4x4x10 mm;
A, attenuator; M2-4, dichroic mirrors, HR @ 532 nm, HT @ 1064 nm ; BS,
beam splitter (50,50) 532 nm; CCD, beam analyser camera.
V
=
I
I
max
− I
+ I
min
0.
5
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Fig.15. Interference fringe patterns acquired in 14-176 cm range of
the path length difference.
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Drawbacks of the pulsed laser system used for common experiments in Bremen
(July 2004) and Iraklion (July 2004):
- Large fluctuations of the green output pulse energy (for both shearography
and digital holography experiment).
- Too small pulse energy for shearography experiments.
- Under these conditions, software compensation of the pulse energy is required
for both shearography and digital holography measurements.
Targets:
1. To increase the green pulse energy.
2. To improve the stability of the green pulse energy.
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1. Reasons of small pulse energy:
- Lack of optical isolation between microlaser and amplifier.
Solution :
- Faraday optical isolator between microlaser and amplifier.
2. Reasons of green pulse energy fluctuations:
- Intrinsic power instability of the microlaser output pulses, amplified during
SHG process.
- Diffraction effects that modulate the transverse intensity profile of the
fundamental beam during the amplification process.
- Parasitic feedback between the microlaser resonator, the amplifier stage and
KTP frequency doubler crystal.
- The main cause:
Intrinsic fluctuations of the polarization state of the microlaser.
SOLUTION:
- Saturation of the amplifier stage.
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Experimental set-up for measuring the fluctuation of the Polarization
state of the microlaser.
µ Laser
Glan Polarizer
Energymeter
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Fig.16. Energy bar-graph of the microlaser output pulses without Glan polarizer.
Standard deviation ~ 4.3%.
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Fig.17. Energy bar-graph of the microlaser output pulses after Glan polarizer.
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Experimental setup for comparative measuring SHG pulse energy of a passively
Q-switched microlaser and single-longitudinal mode acousto-optically
Q-switched Crystalaser microlaser.
L
µ Laser KTP F Photocell
Fig .18. Experimental setup for comparative study.
L - plane convex lens, f = 100 mm
KTP – 4x4x10 mm frequency doubler crystal
F – filter (cut 1064nm)
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Comparative SHG pulse energy.
Fig.19.A – Passively Cr:YAG Q-switched Nd:YAG microlaser.
A
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Comparative SHG pulse energy.
Fig.20. B –Single longitudinal mode, acoustoptically Q-switched Nd:YAG microlaser
(Crystalaser).
B
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Laser head set-up including Faraday Rotator
Fig.21. Experimental setup including Faraday Rotator.
M1 – M 5 , high reflecting mirrors at 1064 nm wavelength; YAG, Nd:YAG rod amplifier;
KTP, doubling crystal; M 6 – 7 , dichroic mirrors HR @ 532 nm, HT @ 1064 nm;
α, return-angle of the amplifier path; DL, divergent lens; PA, photocell preamplifier.
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Fig.22. Bar-graph of the microlaser pulse energy after Faraday Rotator.
Standard deviation ~ 23.6%.
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Fig.23. Average 1064 nm amplified pulse energy versus pump energy
of the amplifier flash-lamp.
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Fig.24. Bar-graph of the amplified 1064 nm pulses for pulsed pump energy of 21 J,
(10 dB attenuation before energy detector). Standard deviation ~ 6.2 %.
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Fig.25. Bar-graph of the amplified 1064 nm pulses for pulsed pump energy of 25 J,
(10 dB attenuation before energy detector). Standard deviation ~ 4.4 %.
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Fig.26. Bar-graph of the amplified 1064 nm pulses for pulsed pump energy of 30 J,
(10 dB attenuation before energy detector). Standard deviation ~ 1.9 %.
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Fig.27. Bar-graph of the amplified 532 nm pulses for pulsed pump energy of 25 J,
(Divergent lens was used).
- Average energy (532 nm pulses) – 7.82 mJ
- Std.Dev - 420µJ, ~ 5.4%.
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Fig.28. Histogram of the green pulses (532nm) for 25J pulsed pump
energy. (Divergent lens was used). Standard deviation ~ 5.4%.
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Fig.29. Bar-graph of the amplified 532 nm pulses for pulsed pump energy of 30 J,
(Divergent lens was used).
- Average energy (532 nm pulses) – 8.84 mJ
- Std.Dev – 415 µJ, ~ 4.7%.
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Fig. 30. Histogram of the green pulses (532nm) for 30J pulsed pump
energy (divergent lens was used). Standard deviation ~ 4.7%.
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Fig.31. The output intensity beam profile of the 532 nm beam.
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Conclusions:
Output parameter of the actual system:
1. Wavelength: 532 nm
2. Max. pulse energy: 9 mJ
3. Standard deviation: < 5 %
4. Pulse duration : 8 nsec
5. Repetition rate: Adjustable, max. 5Hz
6. Coherence length : > 1.7 m
Mode of operation:
Green pulse energy adjustable by:
- Pump energy (25-30J);
- Optical attenuators at 532 nm (glass attenuators; λ/2 waveplate @ 532 nm
and Glan polarizer);
- Laser intensity modulator (crossed polarizers and electrooptical modulator)
with transversal electric field, applied voltage controlled by computer.
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