Tech Briefs Magazine - May 2022 - PIT-20

Challenges of Coating Aspheric Lenses
A
spheric lenses let optical system designers
improve resolution, reduce
the number of lens elements required,
and increase system light throughput.
Their complex, non-spherical surfaces
correct for more spherical and higher-order
aberrations than the spherical
surfaces of conventional lenses. However,
aspheres often have steeper surface
angles than spherical lenses, which introduces
unique challenges while depositing
optical coatings.
The coating effects caused by steep
aspheric surfaces may increase unwanted
reflections and shift the spectral performance
of the lenses. However, careful
attention to coating materials and
deposition parameters can minimize
these effects. Understanding the impact
of lens steepness on coating performance
can help optical system designers
choose the proper coatings for their
application.
Why Would Aspheres be Steeper
than Other Lenses?
The answer is quite simple: because
you can while still maintaining high perOBJ:
 
formance. There is a direct relationship
between a lens' f/# and its diffraction-limited
spot size, meaning that
smaller focused spot sizes can be theoretically
achieved if the lens has a smaller
f/# or larger numerical aperture. However,
in practice, decreasing f/# to improve
diffraction-limited resolution introduces
greater spherical aberrations,
which end up decreasing resolution.
Therefore, traditional spherical lenses
have to make a tradeoff between minimizing
diffraction-limited resolution
and minimizing aberrations.
The complex surfaces of aspheric
lenses correct for spherical aberrations,
allowing them to reduce f/# while still
focusing light to a diffraction-limited
spot. Figure 1 illustrates this in an example.
The images on the left show spherical
lenses moving to smaller f/#'s. While
their diffraction-limited spot size is decreasing,
the actual spot size achieved increases
because of greater spherical aberrations
being introduced. However, the
images on the right show that aspheres at
the same f/#'s are able to maintain diffraction-limited
performance, leading to
even smaller spot sizes. This improves
system performance, but the steepness
of the actual lens surfaces increases in
order to focus light at sharper angles.
This is why aspheres often have steeper
surfaces than spherical lenses. One
thing to note is that Figure 1 shows
some spot sizes smaller than the diffraction
limit because this simulation is
based on geometrical ray tracing and is
not using physical optics propagation.
The actual focused spot size will, of
course, not be smaller than the diffraction
limit.
Impact of Surface Slope on
Coating
Two main challenges are introduced
because of steep aspheric surfaces: the
deposition effect and the angle-of-incidence
(AOI) effect.
The deposition effect is an issue that
occurs inside the coating chamber that
has to do with the optic's geometry. It
causes the thickness of the deposited
coating to vary with lens diameter. As the
vapor stream of depositing atoms arrives
on an optic, if the optic were flat the
OBJ:  
f/10
IMA:  mm
IMA:  mm
OBJ:  
OBJ:  
f/5
IMA:  mm
IMA:  mm
OBJ:  
OBJ:  
f/2.5
IMA:  mm
IMA:  mm
Figure 1. The spherical aberration correction of aspheric lenses allows them to reduce f/# and achieve a smaller diffraction-limited focus without spherical
aberrations reducing performance. However, steep lens surfaces are needed to reach these low f/#'s. All spot diagrams shown have the same scale and are
200µm X 200µm.
20
Photonics & Imaging Technology, May 2022
PIT Application Briefs 0522_1.indd 20
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Tech Briefs Magazine - May 2022

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