Glass
Type
LIDT (J/cm2)
1064 nm @ 12 ns 532 nm @ 10 ns 1064 nm @ 74 ps 532 nm @ 74 ps
N-BK7 2017 74,4 31,8 8,2
N-FK5 1574 226 35,2 9,7
F2 690 7,7 16,7 3,5
N-LASF44 720 18,5 13,8 3,7
N-LAF21 933 15,0 12,6 4,7
SF6 185 Surface Damage 6,4 Surface Damage
Fused Silica 1866 280 39,2 11
Table 15.2: Comparison of LIDT for common optical glasses used in
high-power laser applications,10
Single Shot Test Results
www.edmundoptics.eu/LO 47
For these new optical elements, the LIDT now approaches the bulk damage
threshold of the glass. Because of the absence of surface coatings,
contamination, and defects, the surface breaks down by the same mechanism
as in the bulk of the glass,9 Therefore, it is important to explore the
underlying causes of bulk damage in optical glass to better understand
why the LIDT is so much higher for optics with Nebular™ Technology.
In a 2013 press release, SCHOTT released a rather comprehensive
analysis of the bulk damage threshold of common optical glasses using
several laser types,10 Table 15.2 shows the final results for the multi-shot
(>1800 pulses) bulk LIDT of each glass, as reported by SCHOTT,10, 11
Section 15.3:
Different Types of LIDT Specifications
Not all optical components are tested for LIDT and not all tests are performed
the same way. This results in different types of specifications.
Edmund Optics (EO) specifies LIDT using the following three methods
based on the performance requirements of the given optical component:
1) Laser-Induced Damage Threshold (LIDT), Certified
A large enough sample size of the optics has been tested following
ISO 21254 to guarantee performance to the best of our ability.
2) Laser-Induced Damage Threshold (LIDT), Reference
At least one representative optic has been tested, but not enough to
guarantee adherence to the specification.
3) Laser-Induced Damage Threshold (LIDT), By Design
The optics are untested by EO, but our engineers or third-party suppliers
have confidence that parts should adhere to the specification.
This specification could be used for optics similar to others with certified
or reference LIDT specifications.
Our in-house laser damage testing lab allows us to perform our own metrology,
ensuring the testing conditions mimic those of the intended end
application. We have confidence in our LIDT, Certified specifications
because we control the entire development process, from in-house design,
to manufacturing, to testing. Having oversight of the whole process
also ensures that changes in manufacturing processes do not impact the
LIDT of optics intended for use with high-power lasers.
Section 15.4:
Testing Laser Damage Threshold
Laser damage testing is inherently destructive. The optic undergoing
testing is exposed to a level of laser fluence and is then examined, typically
with Nomarski-type differential interference contrast (DIC) microscopy.
The fluence is then increased, and the exposure and examination
steps are repeated. This process continues until damage is observed on
the optic. While this is conceptually a simple process, there are several
levels of complexity.
According to ISO 21254, any detectable change in an optic under test is
considered “damage”. Different LIDT values may be produced depending
on how the damage was evaluated, as not all tests use the same
damage detection schemes and different operators might choose different
signal-to-noise thresholds. It is important to realize that what ISO
defines as “damage” does not necessarily imply performance degradation
because it is application-dependent.
LIDT testing is specified by either a single or multi-shot test. A singleshot
test, also known as a 1-on-1 test, involves one shot of laser radiation
on at least 10 different sample sites across an optical component with
varying laser fluence. The number of damaged sites over the total number
of tested sites at that fluence determines the damage probability at
a particular fluence. The damage probability is plotted as a function of
fluence and the data is linearly extrapolated to find where the damage
probability is 0%, which gives the LIDT value (Figure 15.7).
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0 10 20 30 40 50
Damage Prob.
Fluence
(J/cm2)
Figure 15.7: Sample data from a single shot test
/LO