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Ultrafast thulium-doped fiber lasers emitting in the 2 μm wavelength region have recently attracted a significant amount of attention regarding applications in material processing, life sciences, as well as in fundamental science, e.g. high harmonic generation, particle acceleration, mid-IR generation [1,2]. In addition to the application-relevant emission wavelength, Tm-doped silica fibers possess many interesting characteristics, such as a broad gain bandwidth spanning from 1700 nm to 2100 nm and more than 70% slope efficiency by exploiting cross-relaxation processes [3]. Furthermore, the longer operation wavelength reduces the impact of detrimental nonlinear effects in fiber lasers, and allows for a larger mode field area with diffraction-limited beam quality as compared to the well-established ytterbium-doped fiber laser systems [4].

The effect of mode instabilities has become a strong limitation for the average power scaling of fiber lasers and amplifiers with nearly diffraction-limited beam quality. In this contribution it is shown that the output average power of rod-type fiber amplifiers can be efficiently increased well beyond the threshold of mode-instabilities. This way a new average output power record of 2 kW, with an M2 of 3, emitted from a 1 meter long rod-type fiber amplifier has been achieved.

High-average-power ultrashort-pulse laser systems in the mid-infrared (MIR) molecular fingerprint region (2–25 μm) are of great interest for environmental applications, defense and fundamental research, in particular for studies of molecular dynamics on the femtosecond scale, chemical reactions and dynamics of photo-excited charge carriers in semiconductors [1-3]. High-sensitivity spectroscopic applications require excellent laser stability as well as high average powers enabling a high dynamic range and good signal to noise ratio. In addition, few-cycle pulse duration and the control over the carrier envelope phase (CEP) opens the possibility for MIR dual-comb spectroscopy, which allows for significantly shorter measurement times and improved stability [3].

We present an active Yb-doped Large Pitch Fiber design with 105 µm mode field diameter. The fiber was tested in a fiber CPA system delivering sub 500 fs, 2 mJ pulses with 3.8 GW peak power. The limiting influence of anticrossings on beam quality and average output power is discussed.

We report on a fiber based mode filter for radially and azimuthally polarized modes. The proof of principle is shown for the azimuthally polarized mode filtered with a step-index fiber and an fs-written Fiber-Bragg-Grating.

The phenomenon of Transverse Mode Instabilities (TMI) [I] has become one of the effects more discussed about in the field of high-power fiber laser systems. The reason is that TMI results in the sudden degradation of the quality and stability of the beam emitted by a fiber laser system once that a certain average power threshold has been reached, which seriously threatens the future applicability of this technology.

The output average power of fiber laser systems is currently limited by the effect of transverse mode instability (TMI) [1]. This effect is characterized by the sudden onset of beam fluctuations once that a certain average power threshold is reached [2], rendering the beam unfitting for most applications. Therefore, there is an urge in the fiber laser community to find suitable mitigation strategies for TMI. In order to do this, it is necessary to understand the underlying physics of this effect. In this context, the most important aspects of TMI are two: first TMI is the manifestation of energy coupling between different transverse modes in a fiber caused by a thermally-induced refractive index grating (RIG); second, for this energy transfer to take place, there must be a phase shift between the RIG and the modal interference pattern (MIP) in the fiber. Most of the mitigation strategies for TMI proposed up to now have focused on reducing the strength of the RIG (or of the MIP). However, a promising new path is to develop mitigation strategies that try to minimize the phase shift between the MIP and the RIG.

A method for the real-time monitoring of the modal content of a monolithic LMA fiber laser is reported for the first time. It allows for the direct observation of the dynamics of transversal mode competition.

Transverse mode instability (TMI) is still the most limiting effect for the further average-power scaling of fiber lasers and amplifiers with excellent beam quality [1]. For the modal energy transfer to happen, a phase shift between the thermally-induced refractive index grating and the modal interference pattern in the fiber is essential [2]. In former experimental and theoretical studies it has been shown that such a phase shift can be induced by changes of the pump power [3,4]. Thus, the investigation of the influence of pump-power noise on the TMI threshold is of high relevance since this noise could act as the main trigger for TMI.

A simplification strategy for Segmented Mirror Splitters used as beam combiners is presented. Numerical methods are used to optimize design parameters and maintain a high theoretical combining efficiency for several tens of beams.

In the last decades, ultrafast lasers and amplifiers have achieved an extraordinary power increase and have enabled a plethora of scientific, medical or industrial applications. However, especially in recent years, it has become more and more challenging to keep up with this pace since intrinsic physical limitations are becoming difficult to avoid. A promising way to get around this problem is the technique of spatially and/or temporally separated amplification and subsequent coherent addition of ultrashort pulses. It turns out that fiber amplifiers are perfect candidates for this concept due to their outstanding average-power capability and their simple single-pass setups, which can be easily parallelized. Herein we provide an overview of the most important experimental implementations of this concept and recent results. We discuss the ability of these approaches to generate laser parameters that, only a few years ago, seemed impossible to achieve.

The exponential growth of the average output power of fiber laser systems seen over the last decade has recently come to an abrupt halt due to the onset of a new phenomenon: mode instabilities [1]. In this effect, the stable Gaussian output beam of a fiber amplifier starts to fluctuate suddenly once that a certain average power threshold has been reached. This effect is particularly damaging because it undermines one of the most valued characteristics of fiber laser systems: their power-independent beam quality. Thus, in the fiber laser community there is a consensus that solutions are urgently needed. This explains the amount of attention that this effect has attracted over the last two years. In this period of time, the theoretical basis for the understanding of the effect has been set. Thus, the physical origin of mode instabilities has been identified to be a thermally induced long-period grating caused by the interference between two fiber modes [2]. It has been demonstrated that this thermally-induced grating is able to transfer energy between the otherwise orthogonal fiber modes that ultimately give rise to it [2-4].

We present HHG results obtained with thulium-doped fiber lasers. It is the first time that a photon energy cut-off close to 400 eV has been demonstrated using this highly scalable laser technology.

Intense, ultrafast laser systems with an emission wavelength at around 2 μm have become popular tools for many scientific research activities such as nonlinear frequency conversion into the long-wave mid-infrared [1] as well as the soft X-ray spectral region [2], in order to address numerous subsequent applications. There is, however, a strong application-driven demand for an increase in photon-flux, i.e. repetition rate and average power, of the driving laser systems. Thulium-doped fiber lasers are an average-power scalable laser concept at around 2 μm emission wavelength. They have been demonstrated with >100 W average power and show great promise to deliver peak powers of several tens of GW [3, 4]. Another advantage is the large gain bandwidth offered by Tm-doped fused silica, spanning from 1.8–2 μm [5].

The unique properties of a 10µm endlessly-single-mode LMA PCF allowed for the efficient conversion of a 8W @ 1MHz fiber amplified 200ps, 1064nm microchip laser to 3W @ 673nm and 450mW @ 2.539µm by degenerated FWM. Scaling potential as well as tuneability options are supported by numerical simulations and will be discussed in detail.

The most recent advances on ultra-large mode area fibers for high-power operation will be presented. Moreover, an approach to synthetize ultra-large mode area fibers that circumvents technical limitations by using multi-core fibers will be discussed.

We present the nonlinear compression of pulses from a high repetition rate thulium-doped fiber laser system in the 2 µm wavelength region using a nitrogen-gas-filled hollow-core capillary. Pulses shorter than 50 fs with more than 3 GW of pulse peak power and 15.4 W of average power have been achieved. In order to demonstrate the capabilities of this laser source, a HHG experiment generating photon energies beyond 150 eV has been performed in an Ar gas jet.

In the past, generation of high power, pulsed, red light was performed using dye lasers, OPOs or second harmonic generation of the 1.3µm transitions from neodymium based solid state lasers [1]. Using FWM in photonic-crystal fibers (PCF) in combination with pulsed 1µm laser sources has so far only accessed the spectral region around 730nm [2]. We present a simple approach to generate picosecond or nanosecond pulses in the visible by efficient FWM in an endlessly single-mode large mode area (LMA) PCF fiber. At a moderate pump peak power of 40kW the conversion efficiency can reach 35%. When pumping the fiber at 1030/1064nm, the resulting wavelength in the visible is 626/673nm, making it interesting for many applications like photodynamic therapy and fluorescence spectroscopy [3]. The experimental setup used to demonstrate the effect consists of the laser source emitting 50ps pulses at 1064nm at a repetition rate of 100kHz with variable peak power. This light is coupled into an endlessly single mode LMA PCF fiber with a length of ∼1m and a core diameter of 10µm. The spectrum at the output of the fiber shows two narrow peaks, the pump wavelength at 1064nm and the FWM generated idler wavelength at 673nm. No supercontinuum trace or Raman scattering can be observed as can be seen in figure 1. At an input peak power of 30kW, the conversion efficiency starts to saturate at ∼35% as shown in figure 2. This high conversion efficiency can be explained by the excellent modal overlap provided by the endlessly single mode nature of the fiber at all wavelengths. Our simulations show that the contribution of the waveguide dispersion for phase matching is almost negligible when employing fused silica PCF fibers with core diameters ≥10µm. This means that with bigger cores even higher peak powers could be handled while still fulfilling the phase matching conditions. In our institute, a master oscillator fiber amplifier system employing a LMA PCF fiber amplifier with a core diameter of 40µm delivered up to 700mW at 632nm wavelength, 100ps pulse duration and a repetition rate of 100kHz. The conversion efficiency from 1064nm to 632nm was ∼18%. The phase match diagram for the LMA PCF fiber with a 10µm core used in our experiment is presented in figure 3. It indicates a broad tuning range from ∼600 to ∼750nm when using a tuneable pump source covering the spectrum between 1020 and 1100nm, e.g. a tuneable pulsed fiber laser.

In this contribution, experimental approaches to control mode instabilities (MI) at average output powers higher than 330 W, which corresponds to three times the typical power threshold of the fiber laser system in use, is presented. Hereby, an acousto-optic deflector is used to change rapidly the beam position of the seed signal on the fiber core at the input side. Dynamic excitation is combined with an electronic feedback loop to actively stabilize the beam profile.

We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 µm, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.