Defect Layer
Deformed Layer
UV Grade Fused Silica Refractive Index vs Wavelength
n
N-BK7
Fused Silica
www.edmundoptics.eu/LO 27
0.1 - 1μm
1 - 100μm
100 - 200μm
Defect-Free Bulk
Subsurface
Damage
Polishing Redeposition Layer
Figure 8.5: Subsurface damage left behind from manufacturing processes9
Section 8.7: Subsurface Damage
Every optic, no matter how meticulously manufactured, will have some
level of subsurface damage below the top surface of the optic such as
cracks, residual stresses, contaminants, and voids,7 These defects may
be caused in the manufacturing process or inherent to the type or quality
of the material used. When illuminated with a laser, subsurface damage
can increase absorption and scatter, generating heat and leading to decreased
throughput. These performance irregularities potentially lead to
system failure when using high power lasers or when a system is under
significant mechanical stress.
The grinding and polishing process leaves subsurface damage about 0,1
μm to tens of microns below the polishing redepostition layer, or the
Beilby layer. The redisposition layer is a top layer of the optic that is
reflowed over fine surface scratches due to a chemical reaction during
the polishing8. The defect layer below the redeposition layer contains the
majority of subsurface cracks and other defects and typically extends
down 1 - 100 μm beneath the optic’s surface. A deformed layer then
separates this defect layer from the defect-free bulk material (Figure 8.5).
Impurities may become trapped within the redeposition layer during
polishing unless special laser-grade polishing and cleaning processes are
used. Polishing with finer and finer grits can further reduce the amount
of subsurface damage, but this damage cannot be removed completely.
Polishing with finer grits improves the quality of the optic, but increases
the amount of time required for polishing, which in turn increases cost.
An effective polishing process for laser optics ensures removal of deep
subsurface damage, while an ineffective process simply hides the damage
under the Beilby layer.
Section 8.8: Dispersion
Dispersion is the dependence of light’s phase velocity or phase delay as
it transmits through an optical medium on another parameter, such as
optical frequency, or wavelength. Several different types of dispersion
can occur inside a laser optic substrate: chromatic (Figure 8.6) , intermodal,
and polarization mode dispersion1.
The refractive index is the ratio between the speed of light in vacuum
and a light wave’s phase velocity while travelling through a medium,
such as air or glass. In pulsed laser applications light is commonly described
using frequency because time is generally more critical and the
frequency of light is a fixed value, while its wavelength is dependent on
the refractive index it is travelling within. Wavelength (λ) is related to
angular frequency (ω), refractive index (n), and the speed of light (c) by:
The refractive index of a material is often described using the Selmeier
formula and the material constants B1, B2, B3, C1, C2, and C3:
Chromatic dispersion is a dependence of light’s phase velocity νp in a
medium on its wavelength, resulting mostly from the interaction of light
with electrons of the medium. Chromatic dispersion is described by the
Abbe number (Figure 8.7), which corresponds to the reciprocal of the
first partial derivative of refractive index with respect to λ, and partial
dispersion, which corresponds to the second derivative of refractive index
with respect to wavelength. The Abbe number is given by:
1.5
1.4
1.3
1.2
1.1
1
0 1 2 3 4 5 6
(m)
Figure 8.6: Refractive index of UV Grade fused silica as a function of
wavelength
1.8
1.7
1.6
1.5
1.4
0.65
0.64
0.63
0.62
0.61
0.60
0.59
0.58
0.57
0.55
0.54
0.53
0.52
100 90 80 70 60 50 40 30 20 10
Refractive Index (nd)
Abbe Number (Vd)
CTE
5
10
15 N-SF5
N-SF11
Calcium Fluoride
Pg,f
0.56
Figure 8.7: Abbe diagram plot showing the refractive index of common
glass types vs their Abbe number. CTE (coefficient of thermal expansion)
is defined in Section 8.2
8.5
8.6
8.7
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