Section 15:
Laser-Induced
Damage Threshold (LIDT)
Laser-induced damage threshold (LIDT), or laser damage threshold
(LDT), is defi ned within ISO 21254 as the “highest quantity of laser radiation
incident upon the optical component for which the extrapolated
probability of damage is zero”.1 The purpose of LIDT is to specify the
maximum laser fl uence (for pulsed lasers, typically in J/cm2) or the maximum
laser intensity (for continuous wave lasers, typically in W/cm2)
that a laser optic can withstand before damage occurs. Because of the
statistical nature of laser damage testing, LIDT cannot be considered as
the fl uence below which damage will never occur, but rather the fl uence
below which the damage probability is less than the critical risk level.
The level of risk depends on several factors such as the beam diameter,
the number of test sites per sample, and the number of samples tested
in order to determine the specifi cation.
Laser-induced damage in optical components causes degradation in
system performance that can even result in catastrophic failure. An incorrect
understanding of LIDT may lead to signifi cantly higher costs or
to component failures. Especially when dealing with high-power lasers,
LIDT is an important specifi cation for all types of laser optics including
refl ective, transmissive, and absorptive components. The lack of an
industry consensus on how LIDT should be tested, how damage should
be detected, and how the test data should be interpreted makes LIDT
a complicated specifi cation. An LIDT value on its own does not convey
the diameter of the beam used for testing, how many shots per testing
site were administered, or the way the test data was analyzed.
Section 15.1: Introduction to LIDT
In order to determine whether a laser’s fl uence may cause damage to
an optic, the following specifi cations of the laser should be reviewed:
power, beam diameter, beam profi le, and whether the laser is continuous
wave or pulsed. For pulsed lasers, the pulse duration must also be
considered.
Laser Intensity: Not as Straightforward as it Seems
The intensity of a laser beam is the optical power per unit area, typically
measured in W/cm2. The distribution of the intensity of the laser across
a cross-section of the beam is the intensity profi le. Some of the most
common intensity profi les are fl at top beams and Gaussian beams. Flat
top beams, or top hat beams, have an intensity profi le that is constant
across a cross-section of the beam. Gaussian beams have an intensity
profi le that decreases as the distance from the center of the beam increases
following a Gaussian function. The peak fl uence of a Gaussian
beam is twice as large as that of a fl at top beam with the same optical
power (Figure 15.1).
The eff ective beam diameter of a Gaussian beam also scales with fl uence.
As fl uence increases, a larger portion of the beam’s width has suffi
cient fl uence to initiate laser-induced damage (Figure 15.2). This can be
avoided by using a fl at top beam instead of a Gaussian beam (see Section
2: Gaussian Beam Propagation pages 8-11 for more information).
The intensity of a laser plays an important role in determining the required
LIDT for optics used with it. Some lasers also contain unintentional
regions of higher intensity called hot spots, which can contribute
to laser-induced damage.
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Gaussian and Flat Top Beam Profiles
Gaussian
1
0.7
0.5
0.4
0.3
0.2
0.1
-200 -100 0 100
Flat Top
Relative Intensity
Radial Pos.
(m)
0.9
0.8
0.6
0
Figure 15.1: Comparison of Gaussian and fl at top beam profi les with
the same optical power2
6
5
4
3
2
1
0
Effect of Fluence on Effective Diameter
Fluence (J/cm2)
Position
Defect Threshold
Figure 15.2: The eff ective diameter of a Gaussian beam increases as
fl uence increases, leading to a higher probability of laser-induced damage
as indicated by more damage sites falling under the width of the
curves with the highest fl uence