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Among other modern imaging techniques, stimulated Raman Scattering (SRS) requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber laser systems. 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 to be between -145 and -148 dBc, which is low enough to enable high-speed SRS imaging with a good signal-to-noise ratio. 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.

Here, we present a passive 30-m long enhancement cavity that supports a steady-state enhancement of 198, which is the highest enhancement that has ever been reached in such a long cavity. Furthermore, we demonstrate the extraction of a short burst with a total energy of 53.6 μJ employing an acousto-optic modulator (AOM) as a switching device. The cavity was seeded with pulses of 1.49 μJ energy at 10 MHz repetition rate. The individual output coupled pulses showed an energy enhancement of up to 8.5 while the whole burst contained the entire energy of 36 input pulses. In the last section theoretical considerations for the single pulse extraction are presented and briefly discussed.

We demonstrate for the first time both spatial and temporal multiplexing in a scalable amplification scheme of ultrashort pulses. Using a division into two amplification channels and four pulse replicas high recombination efficiencies have been achieved at output energies far beyond the single-emitter damage threshold.

Over the last decade, the performance of femtosecond fiber laser systems has been rapidly improved. However, further improvements might be held back due to different physical limitations such as nonlinearities or optically induced damage. We demonstrate that with the coherent combination of four parallel fiber amplifiers record pulse energies and peak-powers of 5.7 mJ and 22 GW, respectively, could be achieved. These values could be realized with a chirped-pulse-amplification (CPA) laser system running at a repetition rate of 40 kHz and delivering a compressed average power of 230 W. A high combination efficiency of 89% was achieved demonstrating the scalability of the combining approach to a larger number of channels.

The active phase stabilization of spatially and temporally combined ultrashort pulses is investigated theoretically and experimentally. Particularly, considering a combining scheme applying 2 amplifier channels and 4 divided-pulse replicas a bistable behavior is observed. The reason is mutual influence of the optical error signals that is intrinsic to temporal polarization beam combining. A successful mitigation strategy is proposed and is analyzed theoretically and experimentally.

Large mode area rod-type fibers have enabled amplification of ultra-short pulses to mJ pulse energy and MW peak powers. For very large mode field areas, fibers have to be designed as rigid rods with typical fiber lengths of around 1 m for efficient operation. A shorter fiber length can be desirable to reduce the packaging size of commercial systems and to decrease the impact of parasitic nonlinear effects for peakpower scaling. The fiber design presented here is based on a modified large-pitch fiber with an effectively higher ytterbium concentration in the fiber core. To achieve index matching the cladding index needs to be changed. In this contribution we propose to co-dope the passive host material with germanium to match both indices and to obtain a higher Yb-concentration within the active core. Compared to standard LPF, where the core index is reduced by co-doping the core with Flourine, the ytterbium doping concentration of this novel germanium-pedestal LPF is doubled. A detailed numerical and experimental investigation shows that with short fiber lengths

Novel laser applications such as laser-wake-field acceleration of particles require extreme parameters from ultra-short-pulse systems. We propose a concept capable to realize simultaneously multi-TW peak powers and multi-kW average powers by employing spatially and temporally separated amplification of chirped laser pulses delivered by fiber-amplifiers. As a combining element for the temporally 100 ns separated pulses (10 MHz repetition rate) we suggest a non-steady-state enhancement cavity using a fast switching-element to dump out the enhanced pulses at 15 kHz.

The coherent combining of ultrashort pulses is a concept for scaling the pulse energy and average power of laser systems emitting ultrashort pulses. This concept has already been demonstrated for two-channel systems. In this contribution we report on a four-channel system that delivers 2.1 mJ pulse energies at a repetition rate of 100 kHz and 210 W of average power after compression. A combination efficiency of 88% was achieved and this demonstrates the scalability of the combining approach to a larger number of channels.

Coherent combination applied to state-of-the-art femtosecond fiber CPA systems has recently opened the route towards high energy and average power laser systems. The ~2 mJ, 340 fs, 196 W (100 kHz) pulses of such a system are coupled to an argon-filled hollow-core fiber for spectral broadening via self-phase modulation. Subsequent compression in a chirped mirror compressor leads to ~1 mJ, 45 fs, 96 W pulses. Under different conditions 580 μJ, 26 fs, 11.6 GW, 135 W (250 kHz) pulses are achieved. This is an unprecedented combination of average power and pulse energy.

Divided-pulse amplification employing passive coherent beam combining implementations causes a strong degradation in efficiency. In this contribution typical implementations are analyzed and a solution using an active stabilization system is presented. With this 380 fs pulses at 1.25 mJ corresponding to a peak power of 2.9 GW have been achieved demonstrating the potential of this approach.

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.

In the last year there have been reports from various research groups around the globe about the onset of modal instabilities (or dramatic degradations of the beam quality) in high average power laser systems. Many of these reports describe how the usual output Gaussian beam of the fiber laser system transforms into a higher order mode (most typically a LP 11 -like mode). This effect is commonly attributed to tr ansversal hole burning. However, this theory alone cannot really explain reports of this effect being observed in fibers in which only the central part of the fiber core has been doped (such as the Rod-Type fibers). As far as we know, up to date no theoretical work has been published in the subject to investigate the true physical origin of this effect. In this paper we present such study and the conclusions obtained from it. It has been found that conventional transversally-resolved rate equation models, able to take transversal hole burning into account, cannot explain this eff ect when preferential ga in designs are investigated. Acco rding to our investigations the inclusion of modal interference along the fibe r is crucial to explain this effect in all type of fibers. Unfortunately current BPM models (able to account for modal interference along the fiber) are not able to take into account transversal hole burning. Thus, we have developed an advanced active fiber model that combines BPM and the transversally-resolved rate equations. This model reveals, for the first time, the im portant role played by modal interference along the fiber. Keywords: Mode Instabilities, Beam Propagation, Fiber Laser

We present second harmonic generation of a high average power, high energy femtosecond Yb doped fiber chirped pulse amplifier. This system is operated at various repetition rates at a central wavelength of 1040 nm. After two pre-amplification stages a main amplifier is used to achieve the required pulse energy for efficient second harmonic generation. It is comprised of a 1.2 m long photonic crystal fiber with a mode field diameter of 45 μm. A dielectric grating based compressor is used for compression to a pulse duration of about 406 fs. Second harmonic generation is then achieved in a 500 μm thick BBO crystal. The conversion efficiency of the second harmonic generation remained almost constant at >60 % for all repetition rates and average power levels. At 5.25 MHz the highest average power of 135 W at 520 nm was achieved. In addition this comes with an excellent beam quality which is validated by a measured M2 < 1.2.

An Yb-based 78-MHz repetition rate fiber-amplified frequency comb is used to investigate the power scaling limitations of a standard-design bow tie high-finesse enhancement cavity for XUV generation. With a Xenon gas jet in the 22-μm-radius focus, the 200-fs intra-cavity circulating pulse reaches a maximum of 20 kW of time-averaged power. A novel cavity design is presented, conceived to overcome the observed enhancement limitations and offering the prospect of few-nm high-power high-harmonic generation. Several applications which come into reach for the first time are discussed.

We experimentally demonstrate that circular polarization state is beneficial if the Kerr-nonlinearity has to be lowered during the amplification of laser pulses. It can be shown that in a fiber-based chirped pulse amplification (CPA) system, the use of circularly and linearly polarized light result in different B-integrals, which are measured using phase-only pulse-shaping. The theoretical value of 2/3 for the ratio of the B-integrals of circularly and linearly polarized light is experimentally confirmed. Circularly polarized light facilitates peak-power scaling, moreover, the self-focussing threshold can be enhanced.

We experimentally demonstrate phase-shaping in fiber CPA-systems, providing pulse-energies at the mJ-level. The applied method is based on an analytical model describing the impact of SPM in CPA-systems. Using this phase-shaping technique nearly transform limited pulses are produced at B-integrals up to 10 rad. Compared to a nonlinear CPAsystem with the best performance being achieved by adjusting the compressor, operation of the same system using the phase-shaping method permits peak-power enhancement by a factor better than 2.

To further scale the peak-power of state-of-the-art fiber CPA-systems, a careful optimization of the spectral as well as temporal dynamics is required. The wavelength dependence of the small-signal gain, as well as the saturation of the amplifier, strongly affect the signal bandwidth. For unsaturated amplifiers only a spectral optimization is required. It can be shown that both the spectral center and the width of the input spectrum strongly affect the output bandwidth. An optimization regarding these two parameters will be given. Design guidelines are presented. We develop a simple yet efficient model to simulate the impact of saturation in broadband Ytterbium-doped fiber CPA-systems. Using this model, we reveal that significant peak-power scaling up to 10 GW of current fiber CPA-systems is possible.

We report on the generation of 830 W compressed average power at 78 MHz pulse repetition frequency and 640 fs pulse duration. We discuss further power scaling including the issue of transversal spatial hole burning. Therefore, we describe a low-nonlinearity fiber design capable of producing fundamental mode radiation at ultra high average powers from short length (range of 1m) and large mode field diameter (>50μm) fibers. In conventional large mode area fiber most of the core is typically uniformly doped. As a consequence gain factors for the fundamental mode and the next higher order modes are comparable. Furthermore, the fundamental mode extracts inversion only in the central part of the core according to its intensity profile, leading at high pump and signal power levels to high and unused inversion density with a strong overlap with higher order transversal modes. In experiments this leads to a threshold-like onset of mode instability, originating from mode competition. Finally, this effect avoids further power scaling. The presented fiber features an optimized doping profile to prefer the amplification of the fundamental mode. In addition non-extracted inversion is minimized avoiding the issue of transversal spatial hole burning. As a consequence ultrafast fiber laser systems with novel performance are in reach, i.e. systems delivering simultaneously >1GW peak power and >1kW average power. In a first iteration a ROD-type fiber with 60μm MFD and 1.7m length was used in a CPA system to produce pump power limited 355 W of average power at 1 MHz.

We report on an ytterbium-doped fiber CPA system delivering 325 W of average power at 40 MHz repetition rate corresponding to 8.2 μJ pulse energy. The pulse duration is as short as 375 fs resulting in 22 MW of peak power.

Pulsed lasers with high average output power in the green spectral range are of interest for laser annealing applications. In this paper an efficient pulsed diode-pumped Yb:YAG thin disk laser with intracavity frequency doubling is presented. The Yb:YAG laser crystal disk has a thickness of 180 mm and a diameter of 10 mm and is pumped by a laser diode stack at a wavelength of 938 nm. The disk is soldered to a water-cooled Cu-W heat sink and exhibits a nearly perfect spherical surface. The folded resonator is dynamically stable with a beam factor of M 2 = 5 and which is matched to the requirements of the application. Acousto-optical and electro-optical switches are investigated to operate the laser in the cavity-dumping mode. An average output power of 150 W at 515 nm is achieved. The diode-to-green efficiency is about 28%. A critically phase matched LBO crystal is used for intracavity second harmonic generation. We show that stable pulsing is obtained from 10 kHz to 150 kHz. The pulse-width can be varied from 200 ns to 700 ns by control of the low-loss period of the switching element. The experimental results are compared with theoretical modeling of the system and first application results are discussed.