![]() Terawatt lasers such as Nova at Lawrence Livermore National Laboratory (Livermore, CA), for example, can be converted to petawatt lasers with subpicosecond pulses. In addition, pulse stretching and compression are added before and after the main amplification stage, so the technique can be back-fitted to existing high-power lasers with relatively small expense. Bench-top terawatt lasers are thus feasible. Not only are far-higher powers achieved with chirped pulse amplification, but lasers can be thousands of times more compact for the same energy output, because component materials have energy densities thousands of times greater than that of dye lasers. Recompression allows a pulse to reach a length of a few femtoseconds with peak power in the petawatt range (10 15 W). This limit, however, is far surpassed when the amplified pulse is recompressed by a pair of parallel gratings. The pulse, now with perhaps less than a watt of peak power, is then fed into a medium of high energy density such as Ti:sapphire, which amplifies the pulse significantly-up to the limit of several gigawatts imposed by nonlinearities. The process essentially stretches a pulse as short as 5 fs by up to 10 5 times to as much as a nanosecond. The longer the wavelength of light, the shorter the travel path and the sooner pulses arrive at the next stage. The pulse is then put through a stretcher-a telescope of unity magnification connecting two antiparallel diffraction gratings. With amplification, a laser produces a very short pulse with very little energy (in the range of nanojoules) and peak power of less than a megawatt. Originally, chirped pulse amplification was developed for radar, but it translated well into the optical regime. This in turn meant that lasers with any respectable energy output had to be large and expensive. Because this limits intensity, short-pulse production requires using materials with low energy density such as dyes and excimers with energy densities of only millijoules per square centimeter. At intensities approaching gigawatts per square centimeter, the index of refraction of the gain material varies linearly with intensity, so that a beam that is more intense in the center produces a higher index of refraction at the center, creating a lensing effect and destroying beam quality. Prior to that, materials appeared to limit the intensity that laser pulses could reach. The key to this rapid advance was the development in the late 1980s of chirped pulsed amplification. Experiments that previously might have been contemplated only at large facilities with millions of dollars can now be performed with small teams at dozens of facilities. Some tabletop lasers offer extremely high performance at a cost that is within the reach of some smaller laboratories at universities and hospitals. ![]() What is perhaps equally exciting is that, like computer circuits, high-power lasers have shrunk dramatically in the last decade in both cost and size. Such pulsing capability may even be the key to providing very compact particle accelerators or generating fusion energy. In these extreme conditions, researchers can probe the behavior of matter at high energy and test theories ranging from astrophysics and general relativity to quantum mechanics. Such intense radiation can accelerate electrons to nearly the speed of light, generate pressures hundreds of times those at the center of the Earth, and create quite powerful magnetic fields. Peak powers have exceeded 1300 TW, with intensity on targets of 10 21 W/cm 2. Ultrafast lasers now can deliver pulses shorter than 5 fs with only a couple of oscillations of light. Since 1987, when the current methods of compressing laser pulses were first developed, the maximum intensity of laser pulses-the concentration of power per unit area-has doubled every few months, with no limit yet in sight. Relatively inexpensive compact tabletop laser systems are now generating shorter, more-intense pulses than those previously possible with equipment only at the largest government laboratories. The production of femtosecond laser pulses-often with only a few oscillations and extremely high intensity-has become almost a common-place activity in hundreds of small and midsize research laboratories around the world.
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