Tech Briefs Magazine - September 2022 - PIT-20

Challenges of Fabricating and Coating Laser Crystals
L
aser crystals can be considered the
" engines " of solid-state lasers. They are
used for gain media, for frequency conversion,
and to manage laser characteristics
and performance. Like the engine of
a car, if laser crystals are clean and working
properly, they allow the larger system
to operate at a higher level. In the case of
a laser system, operating at a high level
means creating a stable beam and reaching
high optical powers. Some advantages
of laser crystals over other solid-state
gain media are that they typically offer
less absorption, a narrower emission
bandwidth, higher transition cross-sections,
and higher thermal conductivity.
Laser crystals are critical for enabling a
wide variety of applications including
laser materials processing, laser surgery,
sensing, defense applications like rangefinding,
and more.
Because laser crystals are sensitive optical
components and are often used with
high-power lasers, depositing the correct
coatings onto them without introducing
any defects is essential. While the complex
geometries and high laser-damage threshold
(LDT) requirements make the fabrication
of laser crystals challenging, keeping
several key considerations in mind helps
ensure that the crystal and its coating behave
as intended.
Fabricating Laser Crystals
Laser crystals are optical crystals typically
doped with transition metals or rare
earth ions. There are many different
crystal types and shapes and each crystal
has its own unique set of attributes that
must be considered. Some common crystal
shapes include rods, cubes, and zigzag
slabs used to reduce thermal lensing
and stress-induced birefringence.
Raw boules, or synthetically grown ingots,
of crystals are cut, ground, and polished
to the tightly-toleranced specifications
needed for the application. The
parallelism and perpendicularity of the
different faces of the crystal must be
tightly controlled since the alignment of
the crystal inside a laser cavity is crucial
to proper functioning. Protecting previously
polished surfaces while polishing
the other surfaces is critical to maintaining
surface quality. Polishing is carefully
monitored to minimize subsurface damage,
which could lead to light loss and
even complete failure if high-power laser
light scatters off of defects or is absorbed.
In-process metrology ensures that
requirements for surface figure, paralFigure
1. Laser crystals are optical components
often doped with rare earth ions or transition
metals. They can be thought of as the " engines "
of solid-state lasers.
20
Figure 2. Ultrasonic cleaning uses high-frequency sound waves in a liquid to knock off any contaminants
from the optics being cleaned.
Photonics & Imaging Technology, September 2022

Tech Briefs Magazine - September 2022

Table of Contents for the Digital Edition of Tech Briefs Magazine - September 2022

Tech Briefs Magazine - September 2022 - Intro
Tech Briefs Magazine - September 2022 - Sponsor
Tech Briefs Magazine - September 2022 - Cov1
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