laser pulse generates plenty of heat. Some lasers, like Livermore’s
National Ignition Facility or the older Nova laser, produce a highly
intense burst of light and then must cool for hours before another
shot can be fired. Other laser systems are designed to operate
almost continuously. The challenge in designing this latter variety,
known as average-power lasers, is to maintain the beam quality
in the presence of heat. Heat can cause aberrations to the laser
beam, degrading its quality.
Livermore physicist Chris Ebbers and technician Keith Kanz, working
with visiting scientist Hitoshi Nakana from Japan’s Kinki
University, have helped to solve this problem by developing a Q-switch
that compensates internally for the heat from the laser beam. They
received an R&D 100 Award for their innovation.
which maintain the quality, or “Q,” of
the beam, stop unwanted laser oscillations—laser pulses before
and after the main laser pulse. An efficient Q-switch is essential
for a high-energy, multipass laser to function at all. In fact,
the thermally compensated Q-switch enables an entirely new type
of laser architecture—a higher-average-power system that
is more compact and more efficient than was previously possible.
The team designed the award-winning switch for Livermore’s
Mercury laser, which has an unusually large beam size, but the
switch can be used for any high-average-power laser.
thermally compensated Q-switch. (a) Light leaking from the
first crystal is propagated through the polarization rotator
and the second crystal. (b) Livermore’s Q-switches for
large aperture (3.25-by 6-centimeter) and smaller aperture
(1.5- by 3-centimeter) high-average-power laser systems.
The switches are made of carefully machined and aligned potassium
di-deuterium phosphate (KD2PO4).
electronically controlled shutter is required to stop unwanted
laser oscillations, and it must respond in a time frame as short
as the desired laser pulse. A Q-switch—also known as a Pockels
cell or an electro-optic switch—uses a crystal whose refractive
index is dependent on the voltage applied to it. It is one of the
few electrically driven devices that can respond in a nanosecond.
Because light travels approximately 1 foot (or 0.3 meter) in 1
nanosecond, a light switch must change the propagation direction
of the laser light on this nanosecond time scale.
problem with existing electro-optic cells is that heat absorbed
from the laser beam prevents the cell from functioning. At average
powers above 30 watts, the cell’s crystal heats up. The switch
then allows light to “leak” because the laser beam
is depolarized. That leaked light can be amplified as well, creating
spurious parasitic laser beams, degrading the quality of the initial
beam, reducing the average power output, and possibly damaging
the laser’s optics.
electro-optic Q-switches use a single electro-optic crystal made
of potassium di-deuterium phosphate, also known as
deuterated KDP or KD2PO4. All single-crystal
Q-switches exhibit a temperature-dependent loss of power above
15 to 30 watts, which allows light to leak. Livermore’s thermally
compensated Q-switch is made of the same material, but it incorporates
a quartz rotator and a second, identical crystal, as shown in the
figure on the next page. The leakage, or depolarization loss, exhibited
by the first crystal is canceled because the leaked light is propagated
through the polarization rotator and the second crystal.
critical to the device’s success is the care with which
it is fabricated. “The precision with which the parts have
been machined and aligned and the process of binding the crystal
to ceramic are unequaled,” says Ebbers.
new Q-switch shows less than a 1-percent loss up to 100 watts of
laser light, which was the testing limit in the laboratory. “Extrapolating
from these measurements, we expect the device to operate up to
and even beyond 300 watts,” says Ebbers. That range is 10
times higher than any equivalent commercially available electro-optic
Q-switch. A laser system using Livermore’s new switch could
thus generate an unprecedented 5 kilowatts of average power or
more without significant light leakage.
Chris Ebbers (left) and Keith Kanz
show the high-average-power electro-optic Q-switch.
Putting the Q-Switch to Work
Q-switch is one of several devices and systems developed in recent
years to enable the construction of the Mercury
laser, a large-aperture (large-beam-size), high-average-power laser
with a high repetition rate. The Mercury laser is a smaller version
of a potential prototype for an inertial fusion energy driver.
As such, it will be used to study how high-intensity light interacts
with matter. It will produce
100-joule pulses at a repetition rate of 10 hertz, for an average
power of 1 kilowatt. Funding from the Laboratory Directed
Research and Development Program has been key to developing the
laser and components such as the Q-switch that make this unique
Similar high-average-power lasers are also being considered for
defense and civilian applications. For example, a compact laser
in a helicopter would function as a very bright flashlight to detect
mines, look for bodies in murky water, or search for obstacles
on the floor of an ocean bay. The high repetition rate and short
pulse width made possible by the Q-switch would give this detection
tool excellent time resolution.
peening, a process developed at Livermore several years ago to
strengthen metals, also makes use of high-average-power lasers.
By inserting the Q-switch into this system, the laser’s pulse
could be tailored to any desired shape to match the needs of the
material being peened
Key Words: high-average-power lasers, laser peening, Mercury laser,
R&D 100 Award, thermally compensated Q-switch.
For further information contact Chris Ebbers (925) 423-9465
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