Using chirped pulse amplification and the Nova laser, researchers have generated kilojoule-class pulses with peak powers exceeding 1.5 PW.
By: Kristin Lewotsky
Researchers at Lawrence Livermore National Laboratory recently claimed the title of king of the hill in the high-power laser world by generating 1.5 PW of output from a titanium-doped sapphire (Ti:sapphire) based laser system. Based on the Nova laser, the system uses 94-cm diffraction gratings to produce 660 J of power in a 440-fs pulse.
Seed laser and front end
The front end of the system incorporates a pair of Ti:sapphire-based regenerative amplifiers. "We wanted to do as much amplification in Ti:sapphire as possible to maintain bandwidth," said project member Brent Stuart. A Kerr-lens modelocked Ti:sapphire oscillator operating at 1054 nm produces the seed pulse, which is temporally expanded to 3 ns by a grating-based pulse stretcher. Amplification begins with a linear-configuration Ti:sapphire regenerative amplifier. Pumped at both ends by 150 mJ from a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 532 nm, the amplifier boosts the seed pulse up to 6.7 mJ at output, for a net gain of 108 (see Figure 1).
The second power boost comes from a ring cavity regenerative amplifier, which incorporates a pair of 35° mirrors, a pair of thin-film polarizers, and a 10-m-focal-length lens to produce a TEM00 beam. The pulse makes 12 passes through a Ti:sapphire crystal pumped from both ends by 300 mJ from another frequency-doubled Nd:YAG laser; output powers for the 1.4-ns pulses are approximately 50 mJ.
Following the Ti:sapphire amplifiers, the beam passes into a series of 19 mm x 240 mm and 45 mm x 240 mm neodymium-doped phosphate glass (Nd:glass) rod amplifiers. The resultant 38-mm-dia. beam achieves energies as high as 15 J. Pulse bandwidth is 5.3 nm, with a 900 ps duration.
A trio of relay telescopes pass the pulse into the Nova laser, where a series of disk amplifiers increase the energy to 1120 J for a 56.3-cm beam. Limiting the beam diameter to the central 80% of the Nd:glass amplifier disks improves beam quality, and gain-narrowing reduces pulse bandwidth to 3.8 nm, with an 800 ps duration.
Following amplification, a large-diameter grating-based compressor squeezes the pulses down to between 0.43 and 20 ps. The compressor incorporates a pair of 94-cm-dia., 1480 lines/in. diffraction gratings that operate with efficiencies on the order of 94%. Optical damage considerations force the team to operate the system at 75% of damage level, limiting incident beam energy to 880 J.
The system is being used for materials science experiments such as fusion ignition.
Michael Perry, et. al., "Petawatt laser pulses," Optics Letters 24, pp. 160-162, (1999).