The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.

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Ultrafast lasers—reliable tools for advanced materials processing

Nanotubes and graphene have emerged as promising materials for use in ultrafast fibre lasers. Their unique electrical and optical properties enable them to be used as saturable absorbers that have fast responses and broadband operation and that can be easily integrated in fibre lasers.

Nanotube and graphene saturable absorbers for fibre lasers

Hollow-core photonic crystal fibres are attractive because they exhibit pressure-adjustable normal or anomalous dispersion, low-loss guidance, very low nonlinearity and high damage threshold. This Review overviews nonlinear optical phenomena in gas-filled, hollow-core photonic crystal fibres that may lead to a new generation of versatile and efficient pulse-compression devices and gas-based light sources.

Hollow-core photonic crystal fibres for gas-based nonlinear optics

We demonstrate a compact diode-pumped Yb:KGW femtosecond oscillator-Yb:YAG Innoslab amplifier master oscillator power amplifier (MOPA) with nearly transform-limited 636fs pulses at 620W average output power, 20MHz repetition rate, and beam quality of Mx
2=1.43 and My
2=1.35. By cascading two amplifiers, we attain an average output power of 1.1kW, a peak power of 80MW, and a 615fs pulse width in a single linearly polarized beam. The power-scalable MOPA is operated at room temperature, and no chirped-pulse amplification technique is used.

Compact diode-pumped 1.1 kW Yb:YAG Innoslab femtosecond amplifier

A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

We present an ultrafast thin disk laser that generates an average output power of 275 W, which is higher than any other modelocked laser oscillator. It is based on the gain material Yb:YAG and operates at a pulse duration of 583 fs and a repetition rate of 16.3 MHz resulting in a pulse energy of 16.9 μJ and a peak power of 25.6 MW. A SESAM designed for high damage threshold initiated and stabilized soliton modelocking. We reduced the nonlinearity of the atmosphere inside the cavity by several orders of magnitude by operating the oscillator in a vacuum environment. Thus soliton modelocking was achieved at moderate amounts of self-phase modulation and negative group delay dispersion. Our approach opens a new avenue for power scaling femtosecond oscillators to the kW level.

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275 W average output power from a femtosecond thin disk oscillator operated in a vacuum environment.

Ultrafast laser oscillators have become ubiquitous in science and technology. For many years, however, their pulse energy has been limited to the nanojoule regime. Applications requiring more intense pulses relied on complex amplifier systems, which typically operate at low pulse repetition rates of the order of kilohertz. Recently, the pulse energy of femtosecond laser oscillators has greatly increased, such that some of these experiments can now be driven at multimegahertz repetition rates, which opens promising new avenues for many applications. We review the current state of the art of high-energy femtosecond laser oscillators, in particular mode-locked thin-disk lasers, and discuss their potential to drive high-field science experiments at multimegahertz repetition rates. Diode-pumped thin-disk lasers are now capable of generating femtosecond light pulses with a pulse energy in the microjoule regime at multi-megahertz repetition rates. This review describes the progress that has been made in scaling the performance of such lasers and the applications that may benefit as a result.

Femtosecond laser oscillators for high-field science

Ultrafast thin disk laser oscillators achieve the highest average output powers and pulse energies of any mode-locked laser oscillator technology. The thin disk con- cept avoids thermal problems occurring in conventional high-power rod or slab lasers and enables high-power TEM00 operation with broadband gain materials. Stable and self-starting passive pulse formation is achieved with semi- conductor saturable absorber mirrors (SESAMs). The key components of ultrafast thin disk lasers, such as gain mater- ial, SESAM, and dispersive cavity mirrors, are all used in re- flection. This is an advantage for the generation of ultrashort pulses with excellent temporal, spectral, and spatial proper- ties because the pulses are not affected by large nonlineari- ties in the oscillator. Output powers close to 100 W and pulse energies above 10 µJ are directly obtained without any ad- ditional amplification, which makes these lasers interesting for a growing number of industrial and scientific applica- tions such as material processing or driving experiments in high-field science. Ultrafast thin disk lasers are based on a power-scalable concept, and substantially higher power lev- els appear feasible. However, both the highest power levels and pulse energies are currently only achieved with Yb:YAG as the gain material, which limits the gain bandwidth and therefore the achievable pulse duration to 700 to 800 fs in

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High-power ultrafast thin disk laser oscillators and their potential for sub-100-femtosecond pulse generation

We present a semiconductor saturable absorber mirror mode-locked thin disk laser based on Yb:Lu(2)O(3) with an average power of 141 W and an optical-to-optical efficiency of more than 40%. The ideal soliton pulses have an FWHM duration of 738 fs, an energy of 2.4 microJ, and a corresponding peak power of 2.8 MW. The repetition rate was 60 MHz and the beam was close to the diffraction limit with a measured M(2) below 1.2.

Femtosecond thin-disk laser with 141 W of average power. Opt. Lett. 35, 2302-2304

We present a semiconductor saturable absorber mirror mode-locked thin disk laser based on Yb:Lu2O3 with an average power of 141W and an optical-to-optical efficiency of more than 40%. The ideal soliton pulses have an FWHM duration of 738fs, an energy of 2.4μJ, and a corresponding peak power of 2.8MW. The repetition rate was 60MHz and the beam was close to the diffraction limit with a measured M2 below 1.2.

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Cyrill R. E. Baer

Papers

275 W average output power from a femtosecond thin disk oscillator operated in a vacuum environment.

Femtosecond laser oscillators for high-field science

High-power ultrafast thin disk laser oscillators and their potential for sub-100-femtosecond pulse generation

Femtosecond thin-disk laser with 141 W of average power. Opt. Lett. 35, 2302-2304

Femtosecond thin-disk laser with 141 W of average power