will absorb more light and heat up more quickly; examples include colored
glasses and absorptive filters. If non-steady state accumulation of
heat occurs in the optic, damage will quickly ensue, especially without
the addition of an effective cooling system. Even then, if optical components
are non-homogenous their ability to conduct heat is non-uniform
and hot spots in the material could quickly and more effectively cause
damage to the component. Similarly to the temperature coefficient of
refractive index, understanding thermal conductivity is important for
modeling high power laser systems and understanding what optical performance
effects to expect.
Section 8.5:
Scatter from Inclusions and Bubbles
Inclusions are foreign particles present in optical glass that are introduced
in several ways including contamination during melting, substrate
batches not melting completely, and wall materials featuring low solubility.
Bubbles are also formed by reactions occurring in the glass melt. The
bubbles are almost entirely removed during the refining step in glass
melting, but some residual bubbles may be left behind from non-perfect
refining. Sophisticated manufacturing processes ensure that optical
glass is nearly free of inclusions and bubbles, but small amounts are
unavoidable. In laser optics, inclusions reduce Laser Induced Damage
Threshold (LIDT) due to light being scattered off the inclusions. The
magnitude of the effect depends on the number, nature, and size of inclusions
in the glass.
The concentration of inclusions and bubbles of a glass is given by the total
cross section in mm2 of a 100cm3 volume of the material, calculated
as the sum of the detected cross sections of bubbles and inclusions. The
maximum allowable diameter and number of bubbles per 100cm3 volume
is defined for each cross section. Inclusions are treated as bubbles
of equivalent size. The three bubble classes include: standard, VB (increased
bubble selection), and EVB (extra increased bubble selection).
Section 8.6: Homogeneity
The homogeneity of an optical substrate characterizes refractive index
variations, leading to a deformation of the transmitted wavefront and
polarizing transmission effects,5 This is defined as:
Δs is the wavefront deviation, d is the substrate thickness, and Δn is
the P-V variation in refractive index. A high degree of homogeneity, or
rather, a low degree of variation, is important for applications with highpowered
lasers. Homogeneity variations develop from the processes in
which the materials are melted. Improper mixing and thermodynamic
imbalances induce density variations and the cooling and annealing processes
can lead to strain profiles. Inhomogeneity takes the form of either
global inhomogeneity, which is a refractive index variation across the
entire piece of glass, or striae, which are spatially short range variations
of the homogeneity in a glass covering a distance of about 0,1 mm up to
2 mm. Table 8.3 shows defines the maximum refractive index variations
of the common homogeneity classes.
Graded-Index (GRIN) lenses are just one example of a type of lens that
are intentionally non-homogeneous with a nonrandom and deterministic
refractive index profile used to nonlinearly bend light rays.
Non-homogeneity causes scattering, which degrades laser system performance
and could lead to laser induced damage from high-power
lasers. To prevent damage, as well as efficiently use energy, it is important
for transmissive laser optics to be highly homogenous and thereby
avoid deformation of the transmitted wavefront and polarizing transmission
effects.
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8.4
Standard VB EVB
Max. cross section in mm2 per
100cm3 of glass 0,03 0,02 0,006
Maximum quantity per 100 cm3 10 4 2
Standard VB EVB
Volume (cm3) Max. allowable diameter of a single bubble (mm)
800 0,55 0,45 0,25
500 0,44 0,36 0,20
300 0,34 0,28 0,15
200 0,28 0,23 0,12
100 0,20 0,16 0,09
50 0,14 0,11 0,06
Table 8.1: SCHOTT classes of bubbles and inclusions in optical media4
Corning inclusion class
0 ≤0,03 0,10
1 ≤0,10 0,28
2 ≤0,25 0,50
Table 8.2: Corning classes of inclusions in optical media
SCHOTT
homogeneity class
Max. cross section
in mm2 per 100 cm3
of glass
ISO 10110 part 4
homogeneity class
CORNING
homogeneity class
Max. inclusion size
in mm
Max. variation of
refractive index
according ISO 10110
part 4
S0 0 – ±50 × 10-6
S1 1 – ±20 × 10-6
H1 1 – ±20 × 10-6
H2 2 F ±5 × 10-6
H3 3 C ±2 × 10-6
H4 4 A ±1 × 10-6
H5 5 AA ±0,5 × 10-6
Table 8.3: Classes of homogeneity and their maximum refractive index
variation values6