Ultrashort lasers provide an important tool to probe the dynamics of physical systems at very short timescales, allowing for improved understanding of the performance of many devices and phenomena used in science, technology, and medicine. In addition ultrashort pulses also provide a high peak intensity and a broad optical spectrum, which opens even more applications such as material processing, nonlinear optics, attosecond science, and metrology. There has been a long-standing, ongoing effort in the field to reduce the pulse duration and increase the power of these lasers to continue to empower existing and new applications. After 1990, new techniques such as semiconductor saturable absorber mirrors (SESAMs) and Kerrlens mode locking (KLM) allowed for the generation of stable pulse trains from diode-pumped solid-state lasers for the first time, and enabled the performance of such lasers to improve by several orders of magnitude with regards to pulse duration, pulse energy and pulse repetition rates. This invited review article gives a broad overview and includes some personal accounts of the key events during the last 20 years, which made ultrafast solid-state lasers a success story. Ultrafast Ti:sapphire, diode-pumped solid-state, and novel semiconductor laser oscillators will be reviewed. The perspective for the near future indicates continued significant progress in the field.
Current status and introductionAs we look back from today in 2010 for the last 20 years, we see that ultrafast solid-state lasers have become the key U. Keller ( ) Institute of Quantum Electronics, Physics Department, ETH Zurich, Zurich, Switzerland e-mail: keller@phys.ethz.ch enabling technology for many new applications, and have established themselves successfully in at least several industrial areas. The situation was substantially different 20 years ago, when ultrafast lasers were predominantly based on dye laser technology or active modelocked solid-state lasers, and it was assumed that passive modelocking of diode-pumped solid-state lasers was very difficult, if not impossible. Key breakthroughs came with the invention of the semiconductor saturable absorber mirror (SESAM) [1,2] and Kerr-lens modelocking (KLM) [3]. Previously, Q-switching instabilities prevented stable passive modelocking of diode-pumped solid-state lasers for more than 25 years. This breakthrough was enabled through the major progress in solid-state laser technology: first with the discovery of the Ti:sapphire laser material [4] and second with the rapid progress in highpower semiconductor diode-laser arrays to efficiently pump solid-state lasers.For this invited review paper for the special celebration of Volume 100 in Applied Physics B, I will provide some more details describing the events that led to the rapid progress in ultrafast solid-state lasers. I have been actively involved at the frontier of this field for more than 20 years and can provide some more personal insight into how the field evolved. Furthermore, many important results have been ...