Tech Briefs Magazine - November 2021 - PIT-12

Lenses and Optics
Figure 1. The mirror face of an SiC optic.
Figure 2. The back of an SiC optic.
It is also non-toxic, unlike a key material
with similar attributes - beryllium.
The Anatomy of a Silicon Carbide
Optic
The mirror face of the SiC optic is
made up of the face sheet with a cladding
material deposited on top prior to polishing
(Figure 1). Bare SiC usually doesn't
meet the surface quality or roughness
requirement needed for a laser quality
surface, which is why the cladding is
added.
Cladding should uniformly extend to
the edge of the substrate, so during the
design phase it should be considered
whether an incidental over-cladding on
the edge is permissible. The implications
of this in terms of manufacturability, cost,
and lead time should also be considered.
Cladding thickness must also be taken
into account from the perspective of
potential reworking and the finishing
processes required.
The Substrate. When choosing the substrate
material, it is important to realize
not all SiC is the same. There are many
different " flavors, " as there are many different
ways of manufacturing SiC substrates,
each having different parameters
in terms of strength, stress, and other
design considerations. Different flavors
have different cost structures and lead
times, and there are several general
processes used to create SiC substrates
with proprietary technology used by individual
substrate manufacturers.
Primary importance to the end-user is
the specific properties of a particular SiC,
and whether the manufacturing technique
is compatible with the type of optic
required. This is where the expertise of
the chosen optics partner is of vital
importance. The pre-clad surface quality
of the chosen substrate is also a huge consideration.
Substrate flatness concerns
and subsurface damage of the pre-clad
12
Cov
ToC
mirror face can lead to extra optical fabrication
down the line.
Cladding. The two most common
claddings are Chemical Vapor Deposited
(CVD) SiC and silicon. Silicon cladding
produces a lower surface roughness and
better surface quality, but it introduces a
CTE mismatch. Silicon cladding also
tends to be thin - less than 30 microns
- so it conforms to the base substrate
very well. However, this doesn't leave
much room for potential rework in the
event that a defect is picked up during
the manufacturing process.
CVD SiC can yield a good surface figure,
but it is limited on roughness and
surface quality due to the porosity of
the cladding. It does tend to be thicker
(greater than 100 microns) so it conforms
a little less to the shape of the
base substrate, but it may give some leeway
in the event that there is some
rework needed. Voids, pits, and inclusions
in CVD SiC cladding can also
affect quality.
Mechanical Features & Mirror
Geometry
For an optical manufacturing partner
to quote realistically, required mechanical
features and mirror geometry must be
clear. As such, it is always best to include
up front not just drawings showing optical
and mechanical requirements, but
also the 3D CAD solid model which helps
in evaluating the manufacturability of the
specific application. When evaluating the
manufacturability, the face sheet thickness
and the lightweighted rib structure
need to be assessed.
Designers tend to always want to keep
face sheet thicknesses as thin as possible
in order to keep the mirror as light as
possible, but there is a fine line here
because a very thin face sheet can cause
lightweighting ribs from the backside to
print through to the optical surface.
In terms of the lightweighting rib
structure (Figure 2), the wall thickness
of the ribs is limited by certain substrate
manufacturers, so if there is a tolerance,
that is a " must-have " specification. It
might steer the designer towards one
manufacturer over another. Different
suppliers have different parameters for
the cross-section thickness, volume etc.
Other factors to consider are center of
gravity and mass moment of inertia; how
will the mirror be held - threaded holes
or inserts; will additional machining be
necessary after the substrate is formed?
Finally, with dimensional tolerances it is
better to try not to over specify non-critical
features when working with SiC.
Sometimes you can get away with this
when working with other ceramics, but
with SiC, it is better to keep it simple.
Optical & Coating Requirements
It is important to work with an optical
manufacturing partner
that has
the
expertise to translate a customer's system
level requirements into an optical tolerance
for the mirror. As an example, the
closer the aperture is to the edge of the
optic the higher the cost, and there are
several other considerations such as slope
or surface roughness that need to be evaluated
based on beam size and laser
power. A good optics partner can walk
you through this process.
When looking at thin film coatings,
the basic coating considerations include
required spectral performance, critical
wavebands, angle(s) of incidence, polarization,
laser damage, durability, quantity
of deliverable witness samples, and
environmental factors. It goes without
saying that it requires experience to
marry all these considerations together.
Also, HEL applications typically
require low loss coatings which can be
stressful on an optic with CTE differences
between the substrate and thin
Photonics & Imaging Technology, November 2021
http://info.hotims.com/79419-902
Tech Briefs Magazine - November 2021

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