Tech Briefs Magazine - March 2022 - PIT-3

ferent types of solid-state lasers currently
in use. It's not possible to describe them
all here, and even categorizing them is
difficult, since existing solid-state lasers
cover an extremely broad space of output
characteristics. But, for the purposes
of this discussion, it's useful to break
them down into three broad categories:
continuous wave (CW) and nanosecond
pulse width, ultra-short pulse, and ultrafast
lasers.
Fiber
Delivery
Dichroic
Mirror
High Reflector
CW and Nanosecond Lasers
The most common solid-state lasers of
this category are based on neodymium
crystals, usually doped with either yttrium
aluminum garnet (Nd:YAG), yttrium
orthovanadate (Nd:YVO4
),
or
yttrium
lithium fluoride (Nd:YLF). The strongest
laser fundamental output for all these
crystals is in the infrared at around 1 µm.
These crystals are all in use because
each produces somewhat different operating
characteristics. For example, Nd:YVO4
is
best suited for high peak power,
high repetition rate pulsed lasers. In
contrast, Nd:YAG typically delivers higher
total pulse energy at lower repetition
rates. Nd:YLF provides even higher pulse
energies, usually at even lower repetition
rates.
There are also several laser crystals that
utilize holmium, thulium, ytterbium or
erbium dopants instead of Nd. The crystals
Er:YAG, Tm:YAG, Ho:YAG all lase at
around 2 µm. This wavelength is strongly
absorbed by living tissue containing water,
making these laser types useful for a
variety of medical applications.
Most of these crystals can be operated
continuous wave (CW). But, the majority
of materials processing and other
industrial solid-state lasers are operated
pulsed. Pulsing increases the peak power,
which is critical for getting above the
ablation threshold (minimum power
needed to melt or vaporize) for many
materials, particularly metals, or to produce
a surface color change for marking.
The most widely used pulsing method
is q-switching, typically implemented using
an acousto-optic deflector that acts as
Photonics & Imaging Technology, March 2022
Laser Crystal
Pump
Light
Output
Coupler
Laser Output
Laser
Output
Pump
Laser
Diode
Figure 2. Basic elements of a diode-pumped, solid-state laser cavity where pump light is supplied by
diode lasers using fiber optics. Their light passes through dichroic filters and is focused into the laser
crystal through its ends. The emitted laser light is reflected by the dichroic filters.
Pump Light In
Q-Switch
Closed
High
Reflected
Laser Crystal
Q-switch closed
Output
Coupler
High
Reflected
Laser Crystal
Q-switch opened
Output
Coupler
Pump Light In
Q-Switch
Closed
Laser
Output
Repetition Rate
1
Time
Figure 3. Schematic showing q-switch operation. Q-switching produces a series of high peak power
pulses of light, rather than continuous output.
3
PIT Laser Technology 0322_1.indd 3
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ToC
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Output Power
Stored Energy
Cavity Loss

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Tech Briefs Magazine - March 2022

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