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

Ultrafast lasers play an increasingly important role in many applications. Nanotubes and graphene have emerged as promising novel saturable absorbers for passive mode-locking. Here, we review recent progress on the exploitation of these two carbon nanomaterials in ultrafast photonics.

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Ultrafast lasers mode-locked by nanotubes and graphene

Few-layer molybdenum disulfide (MoS2) is emerging as a promising quasi-two-dimensional material for photonics and optoelectronics, further extending the library of suitable layered nanomaterials with exceptional optical properties for use in saturable absorber devices that enable short-pulse generation in laser systems. In this work, we catalog and review the nonlinear optical properties of few-layer MoS2, summarize recent progress in processing and integration into saturable absorber devices, and comment on the current status and future perspectives of MoS2-based pulsed lasers.

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Few-layer MoS 2 saturable absorbers for short-pulse laser technology: current status and future perspectives [Invited]

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.

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

The principal ideas of the thin-disk laser design will be illustrated and the advantages for operating different laser materials will be explained. The results for continuous-wave (CW) and Q-switched operation as well as for amplification of short (nanosecond) and ultrashort (picosecond, femtosecond) pulses demonstrate the potential of the thin-disk laser design. The scaling laws for this laser design show that the power limit for CW operation is far beyond 40 kW for one single disk and the energy limit is higher than 3 J from one disk in pulsed operation. Also, the applicability of the thin-disk laser concept to optically pumped semiconductor structures will be discussed. When pumping directly into the quantum wells, the energy defect between the pump photon and the laser photon can be smaller than 5%, thus reducing the waste heat generated inside the semiconductor structure. First results demonstrate the potential of this new concept. Finally, a short overview of the industrial realization of the thin-disk laser technology will be given.

Fifteen Years of Work on Thin-Disk Lasers: Results and Scaling Laws

Space debris laser tracking is a versatile tool for mitigation of collision risks in low earth orbit. Thin Disk lasers are highly suitable to provide the required laser parameters. A transportable kW-class pulsed laser system was developed and tested. New Thin Disk concepts promise a more compact future system. Beyond monitoring, the Thin Disk can also be part of a ground-based laser debris removal system.

J. Speiser

Papers

Fifteen Years of Work on Thin-Disk Lasers: Results and Scaling Laws

Thin Disk Laser Development for Space Debris Monitoring and Mitigation

Development of a compact and agile 400 mJ, 100 Hz amplifier for spaceborne applications