Although strong-field physics with conventional lasers had already started in the 1960s,2 the scientific revolution started about 10 years ago. At that time, we demonstrated that ultrashort pulses at the 100-fs level could be amplified, without damaging the amplifying media, to the terawatt level, using a technique that we called Chirped Pulse Amplification (CPA).3 Since their inception in 1960, the peak power of lasers has evolved by a succession of leaps of three orders of magnitude. They were produced each time by decreasing the pulse duration accordingly. First the lasers were free runnin g, with a duration in the 10-ps and peak power in the kilowatt range. In 1962, modulation of the laser-cavity quality factor allowed the same ... . energy to be released in a nanosecond time scale, a thousand times shorter, to produce pulses in the megawatt range. In 1964, locking the longitudinal modes of the laser (mode locking) allowed the' laser pulse duration to be reduced by another factor of a thousand, down to the picosecond level, pushing the peak power a thousand times higher, to the gigawatt level. At this point, the intensities associated with the ultrashort pulses were becoming prohibitively high-i. e., in the GW/cm2 range. At these intensities, the index of refraction becomes linearly dependent on the intensity to vary like n = YZo + n21, where n is the index of refraction, no the index of refraction at low intensity, 722the nonlinear index of refraction, and I the intensity. The result is that, for a beam with a Gaussian radial intensity distribution, the center of the beam sees a larger index of refraction than its sides. The material becomes a positive lens and alters the beam wave-front quality to an unacceptable level, to the point where it can create filaments and irreversible damage to the laser system. The only way to increase the peak power was to increase the beam diameter at the expense of size, low ,repetition rate, and cost. Although the pulse duration kept decreasing, for about 20 years the intensity limitation in laser systems kept the peak power about constant at the gigawatt level for a cm2 beam, until 1985-87, when the technique of CPA was demonstrated. The CPA concept is illustrated in Fig. 2. The short pulse is first produced by an oscillator. After generation it is not amplified directly, but stretched by a large amount, 103 to 105, from femtosecond to nanosecond, reducing the intensity accordingly. The pulse intensity is now low enough that the stored energy can be safely extracted out of the amplifier, without fear of beam distortions and damage. Once the stored energy is fully extracted, the pulse is recompressed, ideally to its initial value. The key point of the CPA technique is that it decouples pulse fluence (energy/cm2) and pulse intensity (power/cm2). So it reconciles two apparently conflicting needs: to have (a) the highest fluence for efficient energy extraction and (b) minimum intensity to avoid the undesired nonlinear effects. CPA had a dramatic impact. First, we could for the ...
