Section 8:
Characteristics of Laser
Grade Substrates
Optical components used in laser systems require additional attention to
the characteristics of the component’s substrate, especially when using
high-power lasers. A wide range of optical glass and crystalline materials
are used as substrates for laser optic components. Understanding the
properties of these materials and the defects that can be introduced in
the manufacturing process will ensure you choose the correct laser optics
for your application. A number of key characteristics of laser grade
substrates including dispersion, absorption, thermal properties, homogeneity,
and subsurface damage will signifi cantly aff ect the substrate
chosen for laser optics components.
Section 8.1: Absorption
Laser light can be absorbed inside an optical substrate through several
diff erent methods. Electrons in discrete energy levels of the atoms that
make up the optical medium absorb radiative photons and are pushed to
semi-stable, higher energy levels. These atoms then fl uoresce and emit radiation
(photons) through spontaneous emission when electrons fall back
to a lower energy level. Unintentional fl uorescence causes loss of energy
and interference with signal detection, which can be detrimental in laser
optics applications. Fluorescence is often nearly isotropic and radiates in
all directions, which makes things worse. Fluorescence is typically caused
by impurities in the substrate such as rare earth ions. For example, UV
grade fused silica demonstrates high transmittance in the UV and visible
spectra, but experiences dips in transmittance centered at 1,4 μm, 2,2
μm, and 2,7 μm due to absorption from hydroxide (OH-) ion impurities.
Meanwhile, IR grade fused silica contains a reduced amount of OH- ions,
resulting in higher transmission throughout the NIR spectrum (Figure 8.1).
Optical media may also absorb radiation in the form of thermal energy
or heat. Hotspots are local excesses of heat caused by material inhomogeneity
or subsurface damage and cause optics to degrade much more
quickly. Exposure to high-energy radiation, such as UV or X-rays, solarize
a material, changing its color and increasing absorption by forming
color centers that absorb specifi c wavebands. Therefore, it is important
to understand how, and in what ways diff erent types of radiation, including
laser radiation, are absorbed by diff erent glass types in order to
mitigate damage.
Section 8.2:
Coeffi cient of Thermal Expansion
For applications prone to temperature fl uctuations, an athermal optical
system should be developed. Athermal optical systems are insensitive
to an environment’s thermal change and the resulting system defocus.
Developing an athermal design, which is dependent on the coeffi cient
of thermal expansion (CTE) of the materials and the change in index
with temperature (dn/dT), is especially critical in the infrared. CTE is a
measure of the fractional change of a material's size due to a change in
temperature. This thermal expansion is defi ned as:
L is the original length, ΔL is the change in length, αL is the linear CTE,
and ΔT is the change in temperature (Figure 8.2). Typically, as an object
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8.1
T (%)
80
60
40
20
UV Grade Fused Silica Transmission
0
0 200 400 600 800 1000 1200 1400 1600
(nm)
OH absorption peak
around 1400nm
Figure 8.1: The absorption of OH content in UV grade fused silica
causes a dip in transmission around 1400 nm
T
T + T
L
L L
Figure 8.2: Changes in temperature (ΔT) lead to a change in the
length of a material (ΔL) based on the material’s coefficient of
thermal expansion (CTE)