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Thulium-based fiber lasers potentially provide for the demand of high average-power ultrafast laser systems operating at an emission wavelength around 2 μm. In this work we use a Tm-doped photonic-crystal fiber (PCF) with a mode field diameter of 36 μm enabling high peak powers without the onset of detrimental nonlinear effects. For the first time a Tmdoped PCF amplifier allows for a pump-power limited average output power of 241 W with a slope efficiency above 50%, good beam quality and linear polarization. A record compressed average power of 152 W and a pulse peak power of more than 4 MW at sub-700 fs pulse duration are enabled by dielectric gratings with diffraction efficiencies higher than 98% leading to a total compression efficiency of more than 70%. A further increase of pulse peak power towards the GW-level is planned by employing Tm-doped large-pitch fibers with mode field diameters well above 50 μm. The coherent combination of ultrafast pulses might eventually lead to kW-level average power and multi-GW peak power.

Tm-based fiber-laser systems are an attractive concept for the development of high-performance laser sources in the spectral region around 2 μm wavelength. Here we present a system delivering a pulse-peak power higher than 200 MW in combination with 24 W average power and 120 μJ pulse energy. Key components enabling this performance level are 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 stretched-pulse regime.

Large-pitch fibers (LPFs) have enabled the current records for average power, pulse energy and pulse peak power in ultra-fast fiber laser systems. In this paper the working principle of LPFs, which is based on higher-order mode delocalization, is numerically analyzed paying special attention to thermal effects and index mismatch. An enhanced design concept is proposed with a reduced symmetry to improve the delocalization of higher-order modes. This enhanced design has been obtained by transferring the most important characteristics of spiral geometries to a common hexagonal lattice.

We present a novel ytterbium (Yb)-doped large-pitch fiber design with significantly increased pump absorption and higher energy storage/gain per unit length, which enables high-peak-power fiber laser systems with smaller footprints. Up to now index matching between core and surrounding material in microstructured fibers was achieved by co-doping the active core region with fluorine. Here we carry out the index matching by passively doping the cladding with germanium, thus raising its index of refraction. Hence, the fluorine in the core can be omitted, which leads to an effective increase of the core doping concentration, while detrimental effects such as photo-darkening and lifetime quenching are avoided by maintaining the bulk Yb concentration. Experiments and simulations show that a gain higher than 50 dB/m and an output average power higher than 100 W with excellent beam quality are feasible even with a fiber length of only 40 cm.

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.

We present an enhanced large-pitch fiber design with lower symmetry which results in very high mode discrimination. This structure has been obtained by translating the non-symmetric characteristics of spiral geometries into a hexagonal lattice.

Summary form only given. Pulsed Ytterbium-doped fiber laser systems have increased significantly in average output power, pulse energy and peak power within the last two decades. This progress was enabled by the development of large mode area fiber designs that inhibit nonlinear effects. Recently, single-mode fiber lasers delivering 100 W of average power at a record mode-field diameter of 100 μm have been demonstrated. In very large mode area (mode field diameter > 50 μm) fibers operating at high average powers, even the small quantum defect of Ytterbium is able to produce sufficient thermal load to substantially affect the modal properties of the waveguide [1]. In this contribution we show that these thermal waveguide changes alone are strong enough to create guidance for a fundamental mode in an air-clad fiber with an index-antiguiding core.This thermally guiding fiber possesses an Yb-doped region with a diameter of 42 μm that has an index of refraction that is about 2·10-5 smaller than the surrounding cladding (see Fig. 1a). The air-clad has a diameter of 170 μm, the outer diameter amounts to 630 μm. This 1.3 m long straight fiber was used as a counter-propagating amplifier by cladding-pumping it around 976 nm. The near-field image of the output beam was monitored with a camera. In passive and low power (cold) operation, the fundamental mode was delocalized from the core (leftmost image in Fig. 2). However, when increasing the power, the thermal load caused by quantum defect heating starts to imprint a parabolic index profile onto the core that decays logarithmically in the cladding [2], as illustrated (Fig. 1c). This thermally-induced index profile localizes the energy in the fiber core. This process starts at the pump-coupling end of the fiber and propagates towards the seed-coupling end.For powers up to 129 W robust single-mode operation was observed. The mode-field diameter of the output mode decreased more strongly than in the case of fibers with a guiding structure. Exceeding 129 W of average output power led to the onset of mode instabilities predominantly with radially symmetric modes, making this the first demonstration of mode instabilities with this class of modes. Furthermore, these experiments emphasize the advantages of higher-order-mode delocalization and show the ultimate limit of mode-area scaling with standard step-index fibers. Furthermore, the relation to gain-guiding index-antiguiding fibers is discussed in detail.

Pulsed fiber lasers and amplifiers in the wavelength region around one micron have seen a dramatic increase in average output power, peak power and pulse energy over the last decade. Recently, these advances have been extended towards the two micron region by the first demonstrations of very large mode area Tm-doped photonic crystal fibers [1,2]. In this work we present the latest progress for Q-switched Tm-doped fiber oscillators.

We investigate high-power operation of a very-large-mode-area (VLMA) fiber concept based on an index-antiguiding, thermally guiding core in which an ytterbium-doped region is completely surrounded by silica with a slightly higher refractive index. Experimentally, regimes of antiguidance, single-mode operation, and mode instabilities predominantly with radially symmetric higher-order modes are observed. Fundamental limitations for conventional VLMA step-index fibers are discussed.

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.

Large-pitch photonic-crystal fibers have demonstrated their unique capability of combining very large mode areas, high output powers and robust single-mode operation at a wavelength of 1 μm. In this Letter, we present the experimental realization of thulium-doped very large mode-area fibers based on the large-pitch fibers with record mode-field diameters exceeding 60 μm and delivering more than 52 W of output power.

Mode instabilities, i.e. the rapid fluctuations of the output beam of an optical fiber that occur after a certain output power threshold is reached, have quickly become one of the most limiting effects for the further power scaling of fiber laser systems. Even though much work has been done over the last year, the exact origin of the temporal dynamics of this phenomenon is not fully understood yet. In this paper we show that the origin of mode instabilities can be explained by taking into account the interplay between the temporal evolution of the three-dimensional temperature profile inside of the active fiber and the related waveguide changes that it produces via the thermo-optical effect. In particular it is proposed that non-adiabatic waveguide changes play an important role in allowing energy transfer from the fundamental mode into the higher order mode. As it is discussed in the paper, this description of mode instabilities can explain many of the experimental observations reported to date.

We demonstrate a Q-switched fiber laser system emitting sub-60 ns pulses with 26 mJ pulse energy and near-diffraction-limited beam quality (M2

Thermally induced waveguide changes become significant for very large mode area fibers. This results in a reduction of the mode-field diameter, but simultaneously in an improvement of the beam quality. In this work the first systematic experimental characterization of the reduction of the mode-field diameter in various fibers during high-power operation is carried out. It is shown that the reduction of the mode-field diameter shows a characteristic behavior that scales with the core size but that is independent of the particular fiber design. Furthermore, the strength of the actual index change is experimentally estimated, and its use to overcome avoided crossings is discussed and experimentally demonstrated.

The temporal behavior of mode instabilities in active large mode area fibers is experimentally investigated in detail. Thus, apart from the onset threshold of mode instabilities, the output beam is characterized using both high-speed camera measurements with 20,000 frames per second and photodiode traces. Based on these measurements, an empiric definition of the power threshold of mode instabilities is introduced. Additionally, it is shown that the temporal dynamics show a transition zone between the stable and the unstable regimes where well-defined periodic temporal fluctuations on ms-timescale can be observed. Finally, it is experimentally shown that the larger the mode-field area, the slower the mode-instability fluctuation is. The observations support the thermal origin of mode instabilities.

Nonlinear optical pulse compression of high-energy and high-repetition rate (>500kHz) Q-switched pulses is obtained from a fiber-amplified microchip laser system. 92ps and 70ps pulses are SPM-broadened in a double-stage fiber amplifier extracting energies over 100μJ and subsequently compressed to 4.7ps and 2.7ps, respectively.

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.

The impact of avoided crossings (also known as anti-crossings) in single and double-clad large mode area Photonic Crystal Fibers (PCFs) suitable for high-power laser systems is evaluated numerically. It is pointed out that an inappropriate choice of pump core diameter, bending radius and/or index depression may lead to avoided crossings that manifest themselves in unwanted deformations of the output beam.

Ytterbium-doped photonic crystal fibers have been the major player in the extremely rapid increase of the output power of high power femtosecond fiber laser systems. However, rare earth doping of silica leads to an increased refractive index of the core, which is, in turn, compensated by other codopands. In the case of Very Large Mode Area (VLMA) Large Pitch photonic crystal Fibers (LPFs) for high power laser systems it is vital to obtain an ideal index-matching between the actively doped core region and the passive surrounding air hole structure. In [1] it was demonstrated that index-matching is most important for mode area scaling of core-pumped LPFs with effective single-mode operation through mode-discrimination. However, surrounding the core structure by a pump cladding leads to a completely new structure, where core and cladding modes can interact through anticrossings. In [2] it was shown that fundamental mode (FM) anticrossings can be avoided and higher order mode (HOM) anticrossings can be exploited by the proper choice of the airclad diameter. With growing airclad diameter, the effective index of a core guided mode basically remains constant while cladding modes posses growing effective indices and, thus, will cross the effective index of the core guided FM. However, some HOMs deform when their effective index approaches the FM index, the shape of both modes converge. Finally the former HOM takes over the role of the FM while the former FM is deformed.

Large Pitch photonic crystal Fibers (LPFs) have enabled the output power increase of fiber lasers with mode field diameters exceeding 50 µm [1,2]. However, these fibers need strict refractive index matching between the glass matrix and the doped material in the signal core. On the one hand a higher refractive index for the doped material will lead to a step-index guiding structure, on the other hand a strong depression results in huge deformations of the modes. This effect is even more critical in double clad structures, necessary for high power diode pumping. In this case, core guided modes can interact with the modes of the highly multimode double clad structure. An index depression of the doped area can lead to mode anticrossings, as the effective refractive indices of both modes converge and both modes deform strongly. In this contribution the refractive index change in high power fiber lasers is investigated. Therefore, a fiber with a mode anticrossing is experimentally studied. By pumping this fiber, the mode anticrossing can be passed and a quantitative estimate for the refractive index change can be given.

Recent advances concerning the performance of mode-locked fiber lasers have been made, working at all-normal cavity dispersion whilst exploiting strong dissipation for pulse shaping. The current limitation of mode-locked step-index single-mode fiber lasers arises mainly from high nonlinearity accumulated in the small fiber core. By scaling the mode area exceptional performances in terms of pulse energy and peak power have been demonstrated using large-mode-area (LMA) photonic crystal fibers [1,2]. Most of these systems use a high modulation depth semi-conductor based saturable absorber mirror (SAM) for mode-locking, which limits average power scaling due to high non-saturable losses of the absorber and low thermal conductivity of the substrate, leading to strong thermal load and degradation of the SAM.

Ytterbium-doped Large Pitch Fibers with very large mode areas 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 active large mode area designs the threshold of mode instabilities is increased by a factor of about 3.

Photonic-Crystal Fibers (PCF) are among the most promising concepts to achieve large mode field areas suitable for the reduction of nonlinearities in fibers. Differential mode propagation loss is the cornerstone of effective single-mode behavior in passive and core-pumped active PCFs. In this work, we explore non-hexagonal PCF designs with increased mode discrimination in comparison to the classical hexagonal PCF designs. It is shown that a pentagonal design can increase the mode discrimination and, simultaneously, also improve the beam quality of Large-Pitch Fibers with mode field diameters well beyond 100 µm.

We report on an ultrashort pulse fiber CPA system that delivers clean pulses with 2.2 mJ pulse energy, sub 500fs and 3.8GW peak power. The main amplifier of the system is a 108µm core diameter Large Pitch Fiber.

We report on the design and experimental investigation of a preferential gain photonic-crystal fiber with a mode-field diameter of 47 µm. This few-mode fiber design confines the doping of Ytterbium-ions just to the center of the core and, therefore, promotes fundamental mode operation. In a chirped-pulse amplification system we extracted up to 303 W of average power from this fiber with a measured M2 value of 1.4.

We report on the generation of high-average-power and high-peak-power ultrashort pulses from a mode-locked fiber laser operating in the all-normal-dispersion regime. As gain medium, a large-mode-area ytterbium-doped large-pitch photonic-crystal fiber is used. The self-starting fiber laser delivers 27 W of average power at 50:57 MHz repetition rate, resulting in 534 nJ of pulse energy. The laser produces positively chirped 2 ps output pulses, which are compressed down to sub-100 fs, leading to pulse peak powers as high as 3:2 MW. © 2011 Optical Society of America.

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.

We present efficient second harmonic generation of a femtosecond fiber chirped pulse amplification system. At a record average power of 135W and a repetition rate of 5.25MHz we measured a M^2 value

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.

Rare-earth doped photonic crystal fibers rely ideally on an index matching of the doped core to the surrounding glass to work properly. Obtaining a perfect index matching is technologically very challenging, and fiber manufacturers opt for targeting an index depression instead, which still ensures the influence of the photonic structure on the light propagation. In this paper the analysis of the influence of this core index depression on the higher-order mode discrimination and on the beam quality of the fundamental mode of different designs of core-pumped active large pitch photonic crystal fibers is discussed. The most promising design is evaluated in terms of mode area scaling with a view to mode field diameters above 100 µm. Detailed requirements on the accuracy of the core index matching are deduced.

We report on the simulation of stimulated Raman scattering inhibition by lumped spectral filters both in passive optical transport fibers and in fiber amplifiers. The paper includes a detailed theoretical study that reveals the parameters that have the strongest influence on the suppression of the Raman scattering, such as the filter distribution and the insertion losses at the signal wavelength. This study provides guidelines for the use of spectral filtering elements, such as long period gratings, for Raman scattering inhibition in real-world high power fiber amplifiers.

We report for the first time, to the best of our knowledge, on the fabrication and characterization of CO2-laser written long-period gratings in a large-mode area photonic crystal fiber with a core diameter of 25 mu m. The gratings have low insertion losses (< 1 dB) and high attenuation (> 10 dB) at the resonant wavelengths, making them particularly interesting for high power applications.

Fiber lasers are a highly regarded solid-state laser concept due to their high efficiency, beam quality, and easy thermal management. Unfortunately, the performance of high-power fiber-laser systems is challenged by the onset of detrimental nonlinear effects. Their impact can be reduced dramatically by employing fibers with larger mode-field areas. Even though this is an efficient way to mitigate nonlinear effects, maintaining effective single-mode operation, and with it high beam quality, becomes increasingly difficult as the core is enlarged. In this paper the demands and challenges for the design of a very-large-mode-area (VLMA) fiber are discussed. The benefits of using higher-order mode delocalization as the working principle of active double-clad VLMA fibers are described. Finally, a new low-symmetry large-pitch fiber, which is expected to improve the performance of state-of-the-art fiber-laser systems by increasing higher-order mode delocalization, is proposed and thoroughly analyzed.

A high-power thulium (Tm)-doped fiber chirped-pulse amplification system emitting a record compressed average output power of 152 W and 4 MW peak power is demonstrated. This result is enabled by utilizing Tm-doped photonic crystal fibers with mode-field diameters of 35 μm, which mitigate detrimental nonlinearities, exhibit slope efficiencies of more than 50%, and allow for reaching a pump-power-limited average output power of 241 W. The high-compression efficiency has been achieved by using multilayer dielectric gratings with diffraction efficiencies higher than 98%.

We report on a high pulse energy and high average power Q-switched Tm-doped fiber oscillator. The oscillator produces 2.4 mJ pulses with 33 W average power (at a repetition rate of 13.9 kHz) and nearly diffraction-limited beam quality. This record performance is enabled by a Tm-doped large-pitch fiber, which allows for large core diameters in combination with effective single-mode operation.

We report on high-energy picosecond pulse generation from a passively Q-switched and fiber-amplified microchip laser system. Initially, the utilized microchip lasers produce pulses with durations of around 100 ps at 1064 nm central wavelength. These pulses are amplified to energies exceeding 100 μJ, simultaneously chirped and spectrally broadened by self-phase modulation using a double stage amplifier based on single-mode LMA photonic crystal fibers at repetition rates of up to 1 MHz. Subsequently, the pulse duration of chirped pulses is reduced by means of nonlinear pulse compression to durations of 2.7 ps employing a conventional grating compressor and 4.7 ps using a compact compressor based on a chirped volume Bragg grating.