Ultrafast laser and amplifier sources

Abstract

There has been remarkable progress in the development of high peak-power ultrafast lasers in recent years. Lasers capable of generating terawatt peak powers with unprecedented short pulse durations can now be built on a single optical table in a small laboratory. The rapid technological progress has made possible a host of new scientific advances in high-field science, such as the generation of coherent femtosecond X-ray pulses, and the generation of MeV-energy electron beams and high-energy ions. In this paper, we review progress in the development and design of ultrafast highpower lasers based on Ti:sapphire, including the ultrafast laser oscillators that are a very important enabling technology for high-power ultrafast systems, and ultrafast amplified laser systems that generate 20 fs duration pulses with several watts average power at kilohertz repetition-rates. Ultrafast waveform measurements of these pulses demonstrate that such short pulses can be generated with high fidelity. Finally, we discuss applications of ultrafast high-power pulses, including the generation of femtosecond to attosecond X-ray pulses. The 1990s have seen rapid progress in ultrafast laser technology [1, 2]. Ultrafast light sources today are “turn-key” devices, producing peak output powers on the order of a megawatt with pulse durations under 10 fs, directly from a simple laser [3–8]. Since an optical cycle period in the visible and near-infrared is 2–3 fs, this pulse duration approaches fundamental limits for devices operating in this wavelength range. Important new measurement techniques [9] have also been developed, which can extract the complete waveform of an optical pulse of only a few cycles in duration [10, 11]. Such precise measurement tools are essential to properly characterize and utilize extremely short optical pulses. A major application of ultrafast lasers is to study the interaction of atoms, molecules, and plasmas with intense light. These high-field science applications require peak power in the range of 1010–1015 W. Such high-power short pulses are generated using laser amplifier systems. The technology for generating high-power amplified ultrafast pulses has progressed very rapidly over the past decade [12–22]. Amplification of the energy in an ultrashort pulse by factors of 107 or more is now routinely achieved, resulting in a peak power of in the terawatt range from small-scale lasers, and of up to a petawatt from larger systems. By focusing these high-power optical pulses, light intensities of greater than 1020 W cm−2 can be generated, which correspond to an intensity greater than that which would be obtained by focusing the entire solar flux incident on the earth onto a pin-head. Because the size of a laser depends primarily on its pulse energy, by reducing the duration of the pulse being amplified, we can achieve extremely-high power densities using lasers of a realistic and compact scale. As a result, by reducing the pulse duration to 10–20 fs, even very small-scale laboratory lasers operating at kilohertz repetition rates [23, 24] are now easily capable of generating light intensities corresponding to an electric field in excess of that binding a valence electron to an atom. Remarkable progress in high-field optical science has followed as a direct result of this technological progress. It is now possible to experimentally investigate highly-nonlinear processes in atomic, molecular, plasma, and solid-state physics, and to access previously-unexplored states of matter. Ultrashort-pulses at ultraviolet and soft X-ray wavelengths can be generated through harmonic upconversion [25–28] and also at hard and soft X-ray wavelengths through the creation of an ultrafast laser-produced plasma [29] or by scattering off electron beams [30–32]. Such ultrafast softand hard-X-ray pulses can be used to directly probe both longand short-range atomic dynamics and to monitor the evolution of highly-excited systems [33]. In other work, the use of extremely short-pulse high-intensity lasers may make it possible to generate coherent X-rays with pulse duration as short as 10−16 seconds (or 100 attoseconds) [34–36]. 1 Ultrashort-pulse sources The technology of femtosecond lasers changed dramatically with the demonstration in 1990 of the self-modelocked Ti:sapphire laser by Sibbett and his group [1]. Titaniumdoped sapphire is a solid-state laser material with extreme-