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

Enabling broad tunability, high peak and average power, ultrashort pulse duration, and all known modes of laser operation—transition-metal (TM)-doped II–VI chalcogenides are the materials of choice for direct lasing in the mid-IR. The host materials feature broad infrared transparency, high thermal conductivity, low phonon cutoff, low optical losses, and are available as either single crystals or polycrystalline ceramics. Doped with TM ions, these media exhibit a four-level energy structure, the absence of excited state absorption, as well as broad absorption and emission bands. Doped single-crystals of high optical quality are difficult to grow; however, the advent of postgrowth diffusion doped ceramics has resulted in significant progress in laser development. Here, we summarize recent experimental laser results on Cr and Fe doped II–VI chalcogenides providing access to the 1.8–6 μm spectral range with a high (>60%) efficiency, multi-Watt-level [140 W in continuous wave (CW)] output powers, tunability of >1000 nm, short-pulse (<16 fs) multi-Watt oscillation, and multi-Joule output energies in free running and gain-switched regimes. We also review recent results on hybrid fiber-bulk (Er-fiber/Er:YAG, Tm-fiber: Ho:YAG/YLF) systems combining high efficiency of CW fiber lasers with high pulse energies of bulk materials and serving as pump sources of gain-switched Cr:II-VI lasers.

Frontiers of Mid-IR Lasers Based on Transition Metal Doped Chalcogenides

Ultrafast lasers continue to be at the forefront of many scientific breakthroughs and technological achievements and progress in the performance of these systems continue to open doors in many new and exciting interdisciplinary fields. In particular, in the last decade, the average power of ultrafast lasers has seen a significant increase, opening up exciting new perspectives. Among the different technologies that have shaped these advances, thin-disk lasers have generated particularly spectacular breakthroughs. We review here the latest state-of-the-art of the technology and highlight new application fields of these cutting-edge laser systems.

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The amazing progress of high-power ultrafast thin-disk lasers

The main attention of this study is focused on the interaction dynamics of breather, and hybrid solitons and breather for an extended generalization of Vakhnenko equation. The general form of the N-order auxiliary function is first derived by the Hirota bilinear method. Then, the N-order solutions can be obtained. When 
$$N\ge 2$$

, the loop-like breather may emerge by taking the dispersion coefficients as conjugate complex number. The breather is like a magical spring with good elasticity, changeable period and thickness. Furthermore, novel interaction features between breathers, between soliton/solitons and breather/breathers, are observed by visualizing method. The results show that there are abundant dynamics during the interactions, such as elastic collision, phase shift, amplitude amplification, collapse and bulge effect.

Interaction dynamics of hybrid solitons and breathers for extended generalization of Vakhnenko equation

We report a semiconductor saturable absorber mirror (SESAM)-modelocked thin-disk laser oscillator delivering a record 350-W average output power with 940-fs, 39-µJ pulses at 8.88-MHz repetition rate and 37-MW peak power. This oscillator is based on the Yb:YAG gain material and has a large pump spot on the disk. The cavity design includes an imaging scheme, which results in multiple reflections on the disk gain medium to enable a larger output coupling rate compared to those used in thin-disk oscillators with a single reflection on the disk. This reduces the intracavity power for a given output power, thus decreasing the stress on the intracavity components. We operate the laser in a low-pressure environment in order to limit the disk's thermal lensing and drastically reduce the nonlinearity picked up in the intracavity air medium. The combination of the imaging scheme and low-pressure operation paves the way to further power scaling of ultrafast thin-disk oscillators toward the kW milestone.

Power scaling of ultrafast oscillators: 350-W average-power sub-picosecond thin-disk laser.

Lasers based on Cr2+-doped II-VI material, often known as the Ti:Sapphire of the mid-infrared, can directly provide few-cycle pulses with octave-spanning spectra, and serve as efficient drivers for generating broadband mid-infrared radiation. It is expected that the wider adoption of this technology benefits from more compact and cost-effective embodiments. Here, we report the first directly diode-pumped, Kerr-lens mode-locked Cr2+-doped II-VI oscillator pumped by a single InP diode, providing average powers over 500 mW and pulse durations of 45 fs - shorter than six optical cycles at 2.4 µm. These correspond to a sixty-fold increase in peak power compared to the previous diode-pumped record, and are at similar levels with respect to more mature fiber-pumped oscillators. The diode-pumped femtosecond oscillator presented here constitutes a key step toward a more accessible alternative to synchrotron-like infrared radiation and is expected to accelerate research in laser spectroscopy and ultrafast infrared optics.

Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator.

We present directly oscillator-driven self-compression inside an all-bulk Herriott-type multi-pass cell in the near-infrared spectral range. By utilizing precise dispersion management of the multi-pass cell mirrors, we achieve pulse compression from 300 fs down to 31 fs at 11 µJ pulse energy and 119 W average power with a total efficiency exceeding 85%. This corresponds to an increase in peak power by more than a factor of three and a temporal compression by almost a factor of ten in a single broadening stage without necessitating subsequent dispersive optics for temporal compression. The concept is scalable towards millijoule pulse energies and can be implemented in visible, near-infrared and infrared spectral ranges. Importantly, it paves a way towards exploiting Raman soliton self-frequency shifting, supercontinuum generation and other highly nonlinear effects at unprecedented high peak power and pulse energy levels.

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Self-compression at 1 µm wavelength in all-bulk multi-pass geometry

Carrier‐Envelope‐Offset Frequency Stable 100 W‐Level Femtosecond Thin‐Disk Oscillator

We demonstrate an efficient hybrid-scheme for nonlinear pulse compression of high-power thin-disk oscillator pulses to the sub-10 fs regime. The output of a home-built, 16 MHz, 84 W, 220 fs Yb:YAG thin-disk oscillator at 1030 nm is first compressed to 17 fs in two nonlinear multipass cells. In a third stage, based on multiple thin sapphire plates, further compression to 8.5 fs with 55 W output power and an overall optical efficiency of 65% is achieved. Ultrabroadband mid-infrared pulses covering the spectral range 2.4-8µm were generated from these compressed pulses by intra-pulse difference frequency generation.

Efficient nonlinear compression of a thin-disk oscillator to 8.5 fs at 55 W average power.

Several approaches to power scaling of mode-locked thin-disk oscillators exist. One of these approaches is based on the increased gain provided by multiple passes through the thin-disk laser medium. For the first time, to the best of our knowledge, we applied this approach to a Kerr-lens mode-locked thin-disk oscillator. The so obtained additional gain allowed mode-locked operation with up to 50% output coupling rate. This first demonstration is of particular importance for gain media with inherently low-emission cross sections and paves the way to even more powerful Kerr-lens mode-locked thin-disk oscillators. Moreover, the experimental results indicate an increased self-amplitude modulation related to an overall increase in the soft-aperture Kerr-lens effect.

Sebastian Gröbmeyer

Papers

Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator.

Self-compression at 1 µm wavelength in all-bulk multi-pass geometry

Carrier‐Envelope‐Offset Frequency Stable 100 W‐Level Femtosecond Thin‐Disk Oscillator

Efficient nonlinear compression of a thin-disk oscillator to 8.5 fs at 55 W average power.

Kerr-lens mode-locked thin-disk oscillator with 50% output coupling rate.