A fluence value in kilojoules is incredibly high and will almost certainly
damage an optic, making factoring the beam diameter and not only laser
energy in the calculations crucial.
Damage Mechanisms:
In addition to thermal buildup and dielectric breakdown, laser-induced
damage can be triggered by the interaction of the laser with some type
of defect. Defects include subsurface damage left behind from grinding
and polishing processes, microscopic particles of polishing abrasive left
on the optic, or clusters of metallic elements left behind from coating.
Each of these defect sources exhibits distinct absorption characteristics,
as the nature and size of any given defect determines the laser fluence
the optic can withstand without causing damage.
As previously mentioned, pulse duration has a large impact on which
mechanisms lead to laser-induced damage (Figure 15.5). Pulse durations
on the order of femtoseconds to picoseconds may excite charge
carriers from the valence band of a material to the conduction band,
leading to nonlinear effects including multiphoton absorption, multiphoton
ionization, tunnel ionization, and avalanche ionization (Table
15.1). Pulse durations on the order of picoseconds to nanoseconds may
lead to damage by relaxing charge carriers from the conduction band
back down to the valence band through carrier-carrier scattering and
carrier-phonon scattering.
Varying root causes of damage create different morphologies of laserinduced
damage (Figure 15.6). Understanding these morphologies is
important for coating and process development, but for laser optics
applications, the morphology is only important in determining whether
the damage significantly degrades the laser system’s performance. The
amount of performance degradation a system can handle is application
dependent. For example, in some situations a 10% reduction in transmission
may be tolerable, while another system may fail if more than
1% of the incident light is scattered. According to the ISO 21254:2011
standard, any detectable change in an optic after exposure to a laser is
considered damage.
Scaling LIDT:
It is important to keep in mind that damage threshold is dependent on
wavelength and pulse duration. If the specified LIDT of an optic is at
a different wavelength or pulse duration than that of the application,
the LIDT must be evaluated at the application conditions. LIDT scaling
should be avoided when possible; providing firm rules on scaling that
are applicable in all situations is difficult, but general rules exist for scaling
a LIDT value from the initial wavelength (λ₁) and pulse duration (τ₁)
to a new wavelength (λ₂) and pulse duration (τ₂),7
This scaling should not be applied over large wavelength or pulse duration
ranges. For example, Equation 15.5 would be adequate for a wavelength
shift from 1064 nm to 1030 nm, but should not be applied for
scaling an LIDT value at 1064 nm to a drastically different wavelength,
such as 355 nm.
Section 15.2:
Bulk Laser Damage in Glass
Most of the literature on LIDT focuses exclusively on issues related to
surface damage, which is justified because, historically, the LIDT of optical
components was limited by dielectric coatings and surface quality,
rather than bulk damage,8 However, with the recent development of
nano-structured anti-reflective surfaces, called Nebular™ Technology by
Edmund Optics, dielectric coatings are not always necessary. More information
on Nebular™ Technology can be found in Section 12 on pages
38-39.
46 +44 (0) 1904 788600 | Edmund Optics®
Carrier exitation Nonlinear effects
Carrier relaxation
Structural and
thermal events
Carrier - carrier scattering
Carrier - phonon scattering
Dielectric breakdown
Thermal effects
10-14 10-12 10-10 10-8 10-6 10-4 10-2
Pulse Duration (s)
Figure 15.5: Temporal dependence of various laser induced-damage
mechanisms6
Damage
Mechanism Description
Multiphoton
Absorption
Absorption process where two or more photons with energies lower than the material’s
bandgap energy are absorbed simultaneously, making absorption no longer
linearly proportional to intensity.
Multiphoton
Ionization
Absorption of two or more photons whose combined energy leads to the photoionization
of atoms in the material.
Tunnel
Ionization
The strong electric field generated by ultra-short laser pulses allows electrons to
“tunnel” through the potential barrier keeping the electrons bound to atoms,
allowing them to escape.
Avalanche
Ionization
The strong electric field generated by ultra-short laser pulses causes electrons to accelerate
and collide with other atoms. This ionizes them and releases more electrons
that continue to ionize other atoms.
Carrier-Carrier
Scattering
Electrons accelerated by the electric field collide with other electrons, scattering
them and causing them to collide with more electrons.
Carrier-Phonon
Scattering
Electrons accelerated by the electric field excite phonons, or vibrations in the lattice
of the material.
Dielectric
Breakdown
A current flowing through an electrical insulator due to the applied voltage exceeding
the material’s breakdown voltage.
Thermal
Effects
Heat diffusion resulting from distortions and vibrations in the material caused by
the energy of the laser pulses.
Table 15.1: Descriptions of different damage mechanisms
15.5
Figure 15.6: Various morphologies of laser-induced damage resulting
from different root causes