Laser-induced Damage Threshold
Measurements according to ISO 11254
Task:
Measuring the laser-induced damage threshold (LIDT) and lifetime of
optics from the VUV to MIR spectral range from ultrashort pulses to cw radiation
Laser
Microscope
& Camera
Energy
Detector
Sample
Power meter
Lens
Attenuator
Shutter
x-y Stage Scatter
Detector
Lens
Photo diode
Caustic measurement
Knife method
Figure 1 Experimental set-up for LIDT measurement
Specifications:
Test
standards
ISO 11254-1:2000
1-on-1
ISO 11254-2:2001
S-on-1
ISO/FDIS 11254-3:2005 Certification
Wavelengths 157nm, 193nm, 355nm, 532nm,
780nm, 1064nm and 10.6µm
Pulse
150fs, 13ps, 8-400ns and cw
durations
Angle of
0° - 60 ° (s and p-polarization)
Incidence
Measurement principle:
The laser-induced damage threshold of
optical components is strongly influenced by the special operation conditions of
the laser system. Therefore, the test parameters should be as close as possible
to the intended operation. Considering single/cw or multiple pulses irradiation
conditions, the standard ISO 11254 distinguish between the 1-on-1 and S-on-1
test procedure. Whereas the 1-on-1 test delivers rough estimations for the LIDT,
the S-on-1 method can be employed to assess the long-term durability. The
third part of the standard contains the non-destructive proof of power handling
capability in respect of a certain laser intensity level.
Applying the 1-on-1 method, every test site of the sample is irradiated by only
one shot of a certain pulse energy. By varying the pulse energy from site to site,
the threshold for laser-induced damage (LIDT) can be extrapolated from the socalled survival curve, i.e. the relative frequency for damage depending on the
applied energy density.
For the S-on-1 procedure, a train of equal pulses impinges on one test site
unlike a single pulse for the 1-on-1 test. An automated damage detection
system allows for test interruption, if the site is damaged during irradiation.
Furthermore, the energy density and pulse number responsible for the damage
are recorded. In the evaluation procedure, the data set is examined for a
specific pulse number N between 1 and the maximum value S. Regarding the
damage status for this pulse number N, the damage frequencies form a survival
curve as known from the 1-on-1 test. The characteristic damage curve, i.e. the
dependency of the LIDT on the evaluation pulse number N, can be used for an
extrapolation of the damage behaviour for a large number of pulses.
For lifetime certification, the optics surface is irradiated with pulses of well
defined energy density. A high confidence in the power handling capability is
achieved, if most of the optical surface is probed by the test laser spot.
The LIDT is strongly dependent on the application parameters. Whereas defect
induced damage is the dominating mechanism in the ns-regime, the damage
process is completely based on photon-electron interaction mechanisms in the
fs-regime. In the DUV/VUV range, conditioning effects especially including the
creation of colour centres play a major role.
f = 500 mm, Deff = 141 um
60
55
energy density [J/cm²]
50
45
H0
H1
Delta
50%-LIDT
12,3
± 2,3
57,0
± 1,9
0,49
± 0,11
H0
H1
Delta
0%-LIDT
4,0
± 0,9
40,5
± 1,0
0,33
± 0,04
40
35
30
25
20
15
10
5
0
1
Figure 2: Morphology of damaged site of a
mirror HR1064 (Ta2O5/SiO2) measured in
the 1on1 procedure (deff=184µm, τeff=11ns)
10
100
1000
number of pulses
Figure 3: Characteristic damage plot of a
mirror HR1064 (Ta2O5/SiO2)
Measurement with special demands:
The test components
should be measured under environmental conditions comparable to the
intended application. Therefore, the damage investigations at the LZH can be
performed also in specific chambers for simulating tailored vacuum or purge gas
environments. This strategy is crucial especially for optics for space applications
and for coatings in the DUV/VUV spectral range.
Contact:
Dr. Kai Starke
Tel:
++ 49 511 2788 244
Fax:
++ 49 511 2788 100
E-Mail:
se@lzh.de
Laser Zentrum Hannover e.V.
Hollerithallee 8
30419 Hannover
www.lzh.de