Ultrafast fibre lasers are an important optical system with industrial, medical and purely scientific applications. Essential components and the operation regimes of ultrafast fibre laser systems are reviewed, as are their use in various applications.

Ultrafast fibre lasers

The rapid development of the femtosecond laser has revolutionized materials processing due to its unique characteristics of ultrashort pulse width and extremely high peak intensity. The short pulse width suppresses the formation of a heat-affected zone, which is vital for ultrahigh precision fabrication, whereas the high peak intensity allows nonlinear interactions such as multiphoton absorption and tunneling ionization to be induced in transparent materials, which provides versatility in terms of the materials that can be processed. More interestingly, irradiation with tightly focused femtosecond laser pulses inside transparent materials makes three-dimensional (3D) micro- and nanofabrication available due to efficient confinement of the nonlinear interactions within the focal volume. Additive manufacturing (stereolithography) based on multiphoton absorption (two-photon polymerization) enables the fabrication of 3D polymer micro- and nanostructures for photonic devices, micro- and nanomachines, and microfluidic devices, and has applications for biomedical and tissue engineering. Subtractive manufacturing based on internal modification and fabrication can realize the direct fabrication of 3D microfluidics, micromechanics, microelectronics, and photonic microcomponents in glass. These microcomponents can be easily integrated in a single glass microchip by a simple procedure using a femtosecond laser to realize more functional microdevices, such as optofluidics and integrated photonic microdevices. The highly localized multiphoton absorption of a tightly focused femtosecond laser in glass can also induce strong absorption only at the interface of two closely stacked glass substrates. Consequently, glass bonding can be performed based on fusion welding with femtosecond laser irradiation, which provides the potential for applications in electronics, optics, microelectromechanical systems, medical devices, microfluidic devices, and small satellites. This review paper describes the concepts and principles of femtosecond laser 3D micro- and nanofabrication and presents a comprehensive review on the state-of-the-art, applications, and the future prospects of this technology.

Femtosecond laser three-dimensional micro- and nanofabrication

Rare earth-doped fibres are a diode-pumped, solid-state laser architecture that is highly scalable in average power. The performance of pulsed fibre laser systems is restricted due to nonlinear effects. Hence, fibre designs that allow for very large mode areas at high average powers with diffraction-limited beam quality are of enormous interest. Ytterbium-doped, rod-type, large-pitch fibres (LPF) enable extreme fibre dimensions, i.e., effective single-mode fibres with mode sizes exceeding 100 times the wavelength of the guided radiation, by exploiting the novel concept of delocalisation of higher-order transverse modes. The non-resonant nature of the operating principle makes LPF suitable for high power extraction. This design allows for an unparalleled level of performance in pulsed fibre lasers. A new design of optical fibre could allow fibre lasers to reach unprecedented output powers while maintaining excellent beam quality. The design, developed by Jens Limpert and co-workers from Friedrich-Schiller Universitat, Helmholtz Institute Jena and Fraunhofer Institute for Applied Optics and Precision Engineering in Jena, Germany, uses a large-pitch photonic-crystal fibre doped with ytterbium to provide gain. The key to the fibre's performance is the delocalisation of higher order modes due to the transversal arrangement of air-holes. The concept ensures that the fibre operates with a large fundamental mode that has a high-quality beam profile and good power handling characteristics, while suppressing unwanted higher order modes. A pulsed fibre laser based on this design emitted diffraction-limited pulses containing 26 mJ of energy with an average power of 130 W.

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Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation

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.

The Innoslab design, already established for neodymium doped laser crystals, was applied to ytterbium doped laser materials. Recent progresses in brightness of high power diode lasers facilitate efficient pumping of quasi-three-level laser materials. Innoslab amplifiers are compared to competing thin-disk and fiber fs-amplifiers. A compact diode-pumped Yb:YAG Innoslab fs-oscillator-amplifier system, scalable to the kilowatt range, was realized. Numerical simulations result in conditions for high efficiency and beam quality. Nearly transform and diffraction limited 680 fs pulses at 400 W average output power and 76 MHz repetition rate without using CPA technology have been achieved at room temperature so far.

400W Yb:YAG Innoslab fs-Amplifier.

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

We report on a Yb:YAG Innoslab laser amplifier system for generation of subpicsecond high energy pump pulses for optical parametric chirped pulse amplification (OPCPA) at high repetition rates. Pulse energies of up to 20 mJ (at 12.5 kHz) and repetition rates of up to 100 kHz were attained with pulse durations of 830 fs and average power in excess of 200 W. We further investigate the possibility to use subpicosecond pulses to derive a stable continuum in a YAG crystal for OPCPA seeding.

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Yb:YAG Innoslab amplifier: efficient high repetition rate subpicosecond pumping system for optical parametric chirped pulse amplification

Optical parametric chirped-pulse amplifiers with high average power are possible with novel high-power Yb:YAG amplifiers with kW-level output powers. We demonstrate a compact wavelength-tunable sub-30-fs amplifier with 11.4 W average power with 20.7% pump-to-signal conversion efficiency. For parametric amplification, a beta-barium borate crystal is pumped by a 140 W, 1 ps Yb:YAG InnoSlab amplifier at 3.25 MHz repetition rate. The broadband seed is generated via supercontinuum generation in a YAG crystal.

Power scaling of supercontinuum seeded megahertz-repetition rate optical parametric chirped pulse amplifiers.

The InnoSlab concept was developed at Fraunhofer ILT more than 15 years ago using Nd-based laser crystals. InnoSlab is a highly efficient amplification setup that allows for an extremely broad range of laser parameters due to its inherently simple setup. Today InnoSlab using Yb-based laser crystals is the leading amplifier technology for achieving highest output power in the ultrashort pulse regime.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/latj.201400031

High Power Femtosecond Lasers

Laser sources of high average power are essential to transfer femtosecond technology to industrial applications. We demonstrate a compact diode-pumped Yb:KGW femtosecond oscillator-Yb:YAG Innoslab amplifier MOPA with nearly transform and diffraction limited 636 fs pulses at 620 W average output power and 20 MHz repetition rate. By cascading two amplifiers an average output power of 1.1 kW and peak power of 80 MW is achieved in a single, linearly polarized beam. The MOPA is operated at room temperature and no CPA technique is used. The specific properties of Innoslab MOPAs are compared with fibers and thin-disks.

Torsten Mans

Papers

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

Yb:YAG Innoslab amplifier: efficient high repetition rate subpicosecond pumping system for optical parametric chirped pulse amplification

Power scaling of supercontinuum seeded megahertz-repetition rate optical parametric chirped pulse amplifiers.

High Power Femtosecond Lasers

1100 W Yb:YAG femtosecond Innoslab amplifier