Section 15.7:
Importance of Beam Diameter on LIDT
The diameter of a laser highly aff ects an optic’s LIDT as beam diameter
directly infl uences the probability of laser-induced damage.11 When the
beam size of a laser used for LIDT testing is signifi cantly larger than
the density of defects on the optic, the likelihood of triggering scarce
damage mechanisms is high - these unlikely events are detectable. If
the beam size is too small, low defect densities are not always detectable
and parts appear more resistant to damage than they actually are
(Figure 15.12).
The smallest beam diameter permitted for LIDT testing in ISO 21254 is
0.2mm. Many laser optics suppliers prefer to use as small of a beam as
possible because it is easier to achieve a high fl uence, although this may
lead to an “under-sampling” of the surface. Figure 15.13 demonstrates
how laser damage scales with the diameter of a beam. In the scenario
shown, a large number of the defects have a threshold fl uence of 10J
and a small number have a threshold fl uence of 1J. This simplifi ed model
provides insight for real world usage, as laser optics typically contain
various types of defects with diff erent densities and individual damage
thresholds. Scaling the beam diameter from 0.2mm to 10mm will drastically
change the damage probability function and therefore change the
LIDT value concluded from the test. With a 0.2mm beam, the chance
of detecting one of the 1J threshold defects is small. For this reason,
the damage probability will remain very low until a fl uence of 10J is
reached. Increasing the beam size from 0.2mm to 2mm makes detecting
1J threshold defects more likely, thereby causing a sharp increase in
damage probability at a fl uence of 1J. When the beam diameter is scaled
to 10mm, the damage probability at 1J increases to an almost certain
probability of damage.
While LIDT scales with changes in wavelength and pulse duration, it
also scales with beam diameter. For small changes in beam diameter,
this scaling can be approximated by multiplying the original LIDT value
by the square of the ratio of the original diameter to the new diameter.15
Section 15.8:
LIDT for Ultrafast Lasers – ADVANCED
Ultrafast lasers are pulsed, mode-locked lasers that emit pulses with extremely
short durations (on the order of femtoseconds or picoseconds)
and high peak powers. Due to the Fourier limit, or energy-time uncertainty,
shorter temporal pulse lengths corresponds to wide wavelength
spectrum spreads, therefore ultrafast pulses have a broader wavelength
bandwidth compared to longer pulses (Figure 15.14). Ultrafast lasers
are benefi cial for a wide variety of applications including high intensity
physics, femtosecond materials processing, and laser spectroscopy.5
In recent years, ultrafast laser-induced damage has been an active topic
of research because the extremely short pulse duration of ultrafast
lasers causes them to interact with optical coatings and components
diff erently than other lasers. In general, the heating of thin fi lm coatings
after ultrafast laser exposure arises from non-equilibrium energy
transport. Energy from incident photons is absorbed by ground state
electrons, which leads to an occupation of excited energy states within a
few femtoseconds. These “hot” electrons then relax back to their ground
state through picosecond-timescale phonon-electron and phonon-phonon
scattering, allowing for energy redistribution.16 Phonon-electron
scattering describes distortions in electron wave functions caused by
lattice vibrations, and phonon-phonon scattering describes lattice vibrations
inducing other vibrations in the lattice (Figure 15.15).
More information and equations for modeling ultrafast laser-induced damage
can be found online at www.edmundoptics.co.uk/ultrafast-lidt.
50 +44 (0) 1904 788600 | Edmund Optics®
Figure 15.13: In this example with 2 diff erent defect types, LIDT drops
by a factor of 10 when scaling the beam size from 0.2mm to 10mm
Time Wavelength
= phonon
= electron
Time
Phonon-Electron Scattering
Phonon-Phonon Scattering
Figure 15.15: Phonon-electron scattering is the energy transfer between
lattice vibrations and electrons, redirecting electrons inside the
lattice. Phonon-phonon scattering, on the other hand, is the interaction
of multiple lattice vibrations to make new phonons
15.6
Time
Wavelength
Wavelength
E (t) E (t) E (t)
Intensity Intensity Intensity
Figure 15.14: The wavelength bandwidth of ultrafast laser pulses is inversely
related to the amount of time per pulse
/ultrafast-lidt