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Frequency doubling of a Q-switched Yb-doped rod-type 4 × 4 multicore fiber (MCF) laser system is reported. A second harmonic generation (SHG) efficiency of up to 52% was achieved with type I non-critically phase-matched lithium triborate (LBO), with a total SHG pulse energy of up to 17 mJ obtained at 1 kHz repetition rate. The dense parallel arrangement of amplifying cores into a shared pump cladding enables a significant increase in the energy capacity of active fibers. The frequency-doubled MCF architecture is compatible with high-repetition-rate and high-average-power operation and may provide an efficient alternative to bulk solid-state systems as pump sources for high-energy titanium-doped sapphire lasers.

The state of the art of science and technology in fiber lasers and amplifiers is reviewed. Prospects for future power scaling using advanced fiber designs in combination with modern laser and amplifier architectures are discussed.

Coherent beam combining refers to the process of generating a bright output beam by merging independent input beams with locked relative phases. We report the first quantum mechanical noise limit calculations for coherent beam combining and compare our results to quantum-limited amplification. We highlight a favourable noise scaling for both pure and noisy input states.

We present nonlinear self-compression of ultrashort pulses from a powerful thulium- doped fiber laser using a gas-filled antiresonant hollow-core fiber. 104 sub-2 cycle pulses have been generated around 1.85 µm at 98 kHz repetition rate.

We present a thulium-doped fiber chirped pulse amplifier delivering 790 W average power, 250 fs pulses with diffraction-limited beam quality and a relative intensity noise of 0.23% (RF-band 10 Hz to 1 MHz).

We demonstrate a first fiber-laser driven two-color air-plasma based THz source operating at 30 kHz repetition rate. The generated THz spectrum covers 0.1 - 5 THz and has an average power in the mW-range.

We present efficient, ultrabroadband intrapulse difference-frequency generation driven by a Tm-doped fiber laser resulting in an average mid-infrared output power of 450 mW spanning more than one octave (−10 dB width) at a central wavelength of 12 µm.

We present the nonlinear self-compression of pulses from a high repetition rate thulium-doped fiber laser system using a gas-filled antiresonant hollow-core fiber. Sub-3-cycle pulses with several GW peak power at 21.4 W of average power have been generated.

The term coherent Raman scattering (CRS) summarizes imaging methods that do not only poses inherent 3D sectioning capabilities but also enables chemical sensitive detection of living tissue. Instead of relying on external markers that are potentially changing its behavior, CRS measures the vibration frequencies of the molecular bonds by the means of inelastic scattering known as Raman scattering. Among these techniques, stimulated Raman Scattering (SRS) has the advantage to be background-free but requires an extremely quiet, widely tunable dual-wavelength laser source. Up to now, such features could only be provided by large and sensitive laser systems that limited the used of this imaging modality to specialized laser laboratories. We present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber. We employ an all-fiber frequency- and repetition-rate tunable laser in order to enable wideband conversion in the linear OPO cavity arrangement. The signal and idler radiation can be tuned between 764 and 960 nm and 1164 and 1552 nm at 9.5 MHz. Thus, all biochemically relevant Raman shifts between 922 and 3322 cm−1 may be addressed in combination with a secondary output, which is tunable between 1024 and 1052 nm. This ultra-low noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging. We measure the relative intensity noise of this output beam at 9.5 MHz over the tuning range to be between −145 and −148 dBc, which is low enough to enable direct high-speed SRS imaging with a good signal-to-noise ratio without the use of balanced detection. The laser system is computer controlled to access a certain energy differences within one second. Combining FWM based conversion, with all-fiber Yb-based fiber lasers enables the construction of the first automated, turn-key and widely tunable fiber laser. This laser concept could be the missing piece to establish CRS imaging as a reliable guiding tool for clinical diagnostics and surgical guidance.

Tm-doped fused silica fibers allow for the efficient, linear amplification of broad spectra in the 1.9 µm wavelength region. The single-channel fiber chirped pulse amplification system presented herein delivers a pulse-peak power of >5 GW and a nearly transform-limited pulse duration of 90 fs at 10 kHz repetition rate. An increase of the repetition rate to 100 kHz allowed for unprecedented 45 W average power and >4 GW peak power in this wavelength regime.

Thulium-fiber CPA with 200 fs pulses and >1 GW peak power at 50kHz repetition-rate enables nonlinear-compression to sub-30 fs with 140 µJ pulse energy at 2 µm wavelength - a unique laser for high photon-energy HHG.

Thulium-doped fibers with very large mode-field areas offer new opportunities for the power scaling of mid-IR ultrashort-pulse laser sources. The current single-channel laser system presented herein delivers a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. A tenfold improvement in peak power has been achieved by optimizing the pulse shape and by mitigating atmospheric water absorption. Ongoing efforts include the full exploitation of the thulium-gain bandwidth and the coherent combination of multiple amplifier channels aiming for peak powers of 20 GW at MHz repetition rate in the near future.

In this contribution we present the first study on the behavior of the mode instability threshold in bidirectional fiber amplifiers and lasers taking into account the impact of photodarkening.

We present nonlinear pulse compression in the wavelength region around 2 µm using two different approaches. Pulse-shortening to sub-30 fs at 26.5 W of average power or 200 MW peak power is presented, respectively.

Thulium-doped photonic crystal fibers exhibit cross relaxation with slope efficiencies of up to 55% and enabled a high power fiber CPA system emitting a record compressed average output power of 152 W and 4 MW peak power.

The scaling of the average output power of a single fiber laser emitting diffraction-limited beam-quality is limited by the onset of mode instabilities. The combination of several emitters working below their mode instability threshold is an efficient way around this challenge. In this contribution a robust and compact setup is presented, which employs a multicore fiber with four signal cores. An average output power of 536 W has been extracted, which constitutes a fourfold higher power than possible from a single-core emission of the same fiber, hence, the mode instability threshold of the system scales linearly with the number of cores.

We report on an ultrafast thulium fiber oscillator delivering pulses with 0.7 nJ pulse energy at 22.9 MHz repetition rate. The oscillator output could be externally compressed to sub-200 fs pulse duration.

We have achieved a record pulse peak power of ~200 MW at 24 W average power and 120 μJ pulse energy in a state-of-the-art thulium-doped fiber CPA system operating at 2 μm wavelength. The achieved peak power is 50 times higher than any other result based on thulium doped fiber amplifiers presented to date. Such performance was enabled by a Tm-doped large-pitch fiber with a mode field diameter of 65 μm, highly efficient dielectric gratings and a Tm-based fiber oscillator operating in the positively stretched-pulse regime.

In this contribution we describe the impact of photodarkening on mode instabilities. It is shown that even very small amounts of photodarkening can lead to a very significant increase of the heat load in high-power fiber laser systems.

The threshold-like onset of mode instabilities in high power fiber laser and amplifiers has quickly become the most limiting effect for the further average power scaling. We will review the current understanding and mitigation strategies.

In this paper it is shown that pumping at the excited state of Thulium high-power fibers leads to a significant cooling and efficiency improvement. Nearly a factor 2 higher efficiency is expected with this technique.

More than 100W have been extracted from a 36cm-long Ytterbium-doped large-pitch fiber. Increasing the refractive index of the cladding in this fiber allows for a higher Yb-concentration without altering the fiber characteristics.

We present the generation of 9Watt of coherent pulsed radiation at 650nm with excellent beam quality by exploiting degenerate Four-Wave-Mixing in an endlessly single-mode photonic crystal fiber. The pulses had a maximum duration of ~90ps.

Mode instabilities occurring in fiber laser systems during high power operation are currently the most limiting effect for average power scaling. We experimentally and theoretically investigate this effect. A high-resolution high-speed camera has been used to temporally resolve the dynamics of mode-instabilities. We show that periodic fluctuations of the beam are observable right above the mode instability threshold. Additionally, the temporal fluctuations become slower with larger mode-field diameters. These results support the thesis of a thermally induced long-period grating as the origin of mode-instabilities.

We present the development and experimental demonstration of a parametric all-fiber laser source for CARS microscopy. Since the pump and Stokes wavelengths are generated by four-wave-mixing, both pulses are emitted from a single fiber end with intrinsic synchronization. The generated wavelength pair around 780 nm and 1030 nm was used to probe the aromatic CH-stretch vibrational resonance of toluene at 3050 cm−1. Reproduction of the CARS resonance bandshape by spectral tuning as well as microscopic imaging within the aromatic CH-stretching region demonstrates the CARS capabilities of the laser.

Mode-interference in few-mode fiber laser systems induces an oscillating temperature profile which gives rise to an index grating via the thermo-optic effect. Such gratings most likely cause the observed mode instabilities.

Thermal waveguide changes in high power fiber lasers can have a significant effect on the waveguide properties of large mode area fibers. We demonstrate this by overcoming avoided crossings and measuring the fundamental mode shrinking.

We demonstrate a Q-switched fiber laser system delivering 20ns pulses with 22mJ pulse energy at 110W average power combined with near-diffraction limited beam quality out of a Large-Pitch fiber with 101µm mode field diameter.

Inhibition of SRS in doublecladd fiber amplifiers using LPGs is reported. Three LPGs couple the Stokes wavelength from core to cladding and double the extractable Raman free output power of a test pulse amplifier.

Ytterbium-doped Large Pitch Fibers with different design parameters are investigated in a high power fiber amplifier configuration. An average output power of 294 W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62 µm. Compared to previous Yb-doped very large mode area designs the threshold of mode instabilities is increased by a factor of about 3.

We report on the realization of a mode-locked fiber laser emitting 27 W of average power. Pulses are compressed to sub-100 fs (80% compressor efficiency) corresponding to 3.2 MW of peak power.

Modal interference along a LMA fiber creates an inversion grating with the right period to provide energy transfer between the interfering modes. This effect can lead to a substantial degradation of the beam quality.

The analysis of the influence of a core index depression on the higher-order mode discrimination and the beam quality of the fundamental mode in different designs of active large pitch photonic crystal fibers is discussed.

We show that the classic Raman threshold formula is unsuitable to accurately predict the onset of Raman scattering in high-power CW fiber amplifiers. We propose a new analytical formula which accuracy is tested with simulations.

We report on a detailed study on 980nm emission in Yb-fiberlasers highlighting the appropriateness of a short-length LMA PCF as gain medium. An experimental realization resulted in 94W output power in a close-to-diffraction-limited beam.

We experimentally compare two distributed fiber Raman amplified bus networks for the wavelength-division multiplexing of optical sensors. The main difference between them is the type of fiber used in the pre-amplifier in each case.

We describe an interferometric technique for the demodulation of serial fiber Bragg grating sensor arrays, yielding absolute measurement of the individual grating mean wavelengths. We compare two methods for decoding the Bragg wavelengths of gratings.