continues to push the frontiers of laser science. Researchers have
taken the ultrashort pulses of the record-shattering Petawatt laser
and found new uses for them. They are also applying a novel technology
to create extremely short laser pulses with high average power and
high energy to access and investigate extreme-field conditions.
The technology improvements will benefit the Stockpile Stewardship
Program as well as national defense and manufacturing.
The Petawatt laser operated
for three years and routinely produced more than 500-joule laser
pulses lasting 500 femtosecondsless than a trillionth of a
second. Experiments with the Petawatt evaluated the fast ignitor
method of achieving inertial confinement fusion, generated powerful
electrons or x rays for radiography research, and produced short,
powerful gamma rays for nuclear physics experiments. (See S&TR,
March 2000, The
of the Petawatt.) In addition, the discovery of intense, high-energy,
collimated proton beams emitted from the rear surface of Petawatt
laser targets has opened the way to new applications such as proton
radiography. The Petawatt laser still holds the worlds record
for the highest peak power ever achieved by a laser.
The Petawatt operated on
one of the 10 beam lines of Livermores Nova laser. When the
Nova laser was decommissioned in 1999, the Petawatt went with it.
But work on short-pulse lasers by no means stopped, notes physicist
Mark Hermann, associate program leader for Livermores Short-Pulse
Lasers, Applications, and Technology program, known as SPLAT, which
is a part of the National Ignition Facility (NIF) Programs Directorate.
His team of about 30 people is advancing the science of short-pulse
lasers and applications, developing new laser components, fielding
advanced laser systems, and developing new optical components and
optical fabrication technologies. There is also an active program
in short-pulse technology in the Physics and Advanced Technology
(PAT) Directorate. This research stems from the need to develop
high-temperature plasma sources and accurate plasma probes for high-energy-density
In SPLAT, a diverse set of
challenging projects focused on developing high-average-power, short-pulse
lasers for a variety of customers is under way. Current SPLAT-developed
laser systems use conventional titanium-doped sapphire (Ti:sapphire)
amplifiers, but now the team is developing new chirped-pulse amplifier
technologies geared toward high average power. One is a direct,
diode-pumped, chirped-pulse amplifier laser crystal that promises
efficient, compact, and robust picosecond-pulse laser systems. Another
is an optical-parametric chirped-pulse amplification (OPCPA) technique,
described in more detail below.
The SPLAT team is using a
short-pulse laser to create unique nanocrystals and gain knowledge
about the novel properties of nanostructures. This knowledge affects
the basic sciences, from solid-state physics to biology. Being able
to synthesize nanocrystals of specific size and properties, at an
industrial rate, may revolutionize the field of nanotechnology and
enable a broad sector of manufacturing, from semiconductors to pharmacology.
The team is collaborating
on a project that integrates a short-pulse laser with a Livermore
linear accelerator for stockpile stewardship applications. Supporting
the teams efforts, Livermores Diffractive Optics Group
is developing new optical technologies and fabricating new optics
for petawatt-class lasers around the world, for the National Ignition
Facility, and for the National Aeronautics and Space Administration
to use in space-based telescopes. The group currently produces the
worlds largest diffraction gratings.
The100-megaelectronvolt linear accelerator and (b) the Falcon
ultrashort-pulse laser are being integrated to produce a short-pulse
x-ray source. Livermore scientists will use the x rays to probe
the dynamics of materials under shock conditions.
Path to a Short Pulse
An interesting new development
in short-pulse laser technology has been the emergence of OPCPA.
Laser-pumped nonlinear crystals made of beta-barium borate (BBO)
would replace the Ti:sapphire used in the Petawatt and other conventional
lasers as the preamplifier. In a Ti:sapphire regenerative power
amplifier, a pulse passes 10 to 100 times through a regenerative
cavity, increasing in energy with each pass. By the time it leaves
the amplifier, its energy has increased by 10 million, from about
1 nanojoule to approximately 10 millijoules. In contrast, a pair
of BBO crystals can produce the same energy gain with a single pass
of the light pulse.
With a regenerative amplifier,
a tiny bit of energy leaks out with each round trip of the laser
pulse. If this leak, or prepulse, is not attenuated, it may cause
a preplasma, which changes the coupling of the laser to the target.
Many stockpile stewardship experiments use lasers to probe materials
essential to nuclear weapons to learn more about their behavior.
Keep in mind that the prepulse causes changes that are miniscule
by most standards, but when powerful laser pulses of less than a
trillionth of a second are used to study detailed physics processes,
even minor changes can be significant.
Livermore did not invent
OPCPA. But, according to Hermann, Livermore is pushing the
frontier of OPCPA technology, combining Livermores unique
expertise in high-beam-quality, high-average-power lasers and nonlinear
A major application for OPCPA
will likely be in laser machining, which requires high average power,
high beam quality, and ultrashort pulses (about 20 to 1,000 femtoseconds).
Unlike other chirped-pulse amplification approaches, OPCPA produces
negligible thermal aberrations that in turn cause degradation of
the laser beam. Although not yet demonstrated at high average powers,
an OPCPA laser should be able to produce hundreds of watts or even
kilowatts of average power with high beam quality. In contrast,
most conventional Ti:sapphire chirped-pulse lasers, including Livermores
systems, have operated at 20 watts or less. Higher power should
translate into faster production and better process control during
Much of the SPLAT
programs work is related to stockpile stewardship and improving
Livermores ability to verify the safety and reliability of
the nations aging nuclear weapons stockpile. Before the arrival
of OPCPA technology, the team built the Falcon, a 3-terawatt, 35-femtosecond
laser facility with a moderate repetition rate, to use as a material
probe. A joint team of SPLAT and PAT personnel recently began to
integrate the output from Falcon with the electron beam generated
by Livermores 100-megaelectronvolt linear accelerator. Together,
the laser and the accelerator will be an advanced light source whose
ultrafast and ultrabright pulsed x rays will be used as probes for
dynamic studies of solid-state and chemical systems.
notes that the Falcon and other short-pulse lasers at Livermore
may benefit from being upgraded with the OPCPA system. Controlling
and in some cases eliminating the prepulse is desirable, says
Hermann. It means that researchers will be able to control
the experimental initial conditions of the laser material dynamics.
A laser with 500-picosecond pulses caused the high
explosive LX-16 to burn during cutting. (b) In Livermores
system, the 150-femtosecond laser pulses are so short and fast
that they deliver virtually no heat to the area being cut.
technologies for optical-parametric chirped-pulse amplification
and diffractive optics will soon find their way into several Livermore
lasers, from small high-average-power systems for manufacturing
to high-energy systems such as NIF, the 192-beam laser being built
to support stockpile stewardship science research. NIF and SPLAT
personnel are assessing the potential for adding these technologies
to produce short pulses on NIF. Combining ultrashort pulses with
the powerful, multimegajoule capacity of NIF would result in a unique
system, one that may be able to demonstrate fast ignition for the
production of fusion energy, increase NIFs stockpile stewardship
capabilities, and investigate new areas of extreme-field science.
diffraction gratings, Falcon laser, femtosecond laser machining
and cutting, high-average-power lasers, nanocrystals, optical-parametric
chirped-pulse amplification (OPCPA), Petawatt, ultrashort-pulse
information contact Mark Hermann (925) 423-8672 (firstname.lastname@example.org).