Scientists have switched on the world’s most powerful laser, which for one-trillionth of a second is 2,000 times more powerful than all the power plants in the United States. The laser’s output tops a petawatt, which is a quadrillion (1,000,000,000,000,000) watts of power.
In the basement of the physics building at the University of Texas at Austin, the school’s High Intensity Laser Science Group built a petawatt laser in hopes of recreating astronomical phenomena like supernovae in miniature.
"We can put materials into states that you can’t access here on earth," said Mikael Martinez, the laser project’s manager. "You’d have to go out into space and hang out with an exploding star to observe what we plan to observe here in Texas."
When the scientists turned on the laser on March 31, it became the world’s most powerful operational laser, but it doesn’t yet hold the record for the most powerful laser ever built. That honor, at least for a few more months, belongs to the now mothballed Nova laser built at Lawrence Livermore National Lab. The Nova produced 1.25 petawatts of power when it was first switched on in 1996. Martinez said he expected his project to break that record within the year, reaching between 1.3 and 1.5 petawatts.
Below, we take a virtual walk through the tech — amplifiers, compressors and crystals — that make this Texas-size laser so powerful.
The power of a laser, its output in watts, is determined by the energy of the laser pulse, measured in joules, divided by its duration, measured in seconds (tiny fractions of a second in this case). So, to get high power, you can either turn up the energy or cram the same amount of energy into a shorter duration pulse — or do both. The problem is that turning up the energy makes it more difficult to get short pulses.
The solution to this problem require an almost Rube-Goldberg setup inside a 1,500-square-foot cleanroom. The most powerful laser in the world starts, poetically enough, with a "seed laser" that puts out a wimpy nanojoule of energy for a couple hundred femtoseconds (that’s 10-15 seconds). It must be run through a series of amplifiers, compressors, and stretchers before it can recreate the conditions inside the sun for a trillionth of a second.
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The seed laser is run through what is known as a stretcher. The device uses a diffraction grate that works like a prism does for standard light to separate the laser into its constitutent wavelengths. This, in effect, lengthens the pulse from the femtosecond range (10-15 second) to the nanosecond range (10-9 second). Doing so, however, reduces its energy even further from nanojoules to picojoules. Scientists go through this process because it makes the pulse easier to manipulate in the next step: amplification.
First, the newly elongated seed pulse is juiced by entirely different lasers using special crystals in a process called optical parametric amplification. This takes the power of the laser all the way to one joule. Then, it hits the rod amplifier, which is a 24-centimeter-long piece of glass that gets pumped with lights that the laser pulse can absorb. The scientists run the laser through these rods eight times to bring the energy of the laser to 20 joules. Below, we see what is called the "laser chain," with a green pump lamp running.
Finally, that pulse is fed through a disk amplifier, which can be seen below. Inside this amplifier, two disks of glass are juiced with pump lights that, after four passthroughs, bring the laser up to about 250 joules of energy.
The final step is to recompress the pulse, which the stretcher had previously elongated, for maximum power. Below, we see the compressor chamber.
Another diffraction grate inside this chamber, seen in the very top picture, recombines the spread-out wavelengths into a short pulse about 150 femtoseconds long, although with considerable energy loss. But even at 1 joule of energy, the setup yields that whopping petawatt of power.
Martinez summed up the whole process, saying, "We play a trick on the laser. We take the short pulse and stretch it wider. Then we go and amplify the pulse. Then the very last thing we do is recompress that pulse back."
The actual laser pulse will come out of the round hatch seen at the left of the chamber, where it will be directed at a target to mimic a nuclear explosion or an exotic dense star.
The work is sponsored by the National Nuclear Security Administration and totaled about $7 million in equipment costs.
Images courtesy Mikael Martinez and the Texas Petawatt Project, led by Todd Ditmire.
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