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We review the main challenges and give design guidelines for high-peak-power high-average-power fiber-based chirped-pulse amplification (CPA) systems. It is clearly pointed out that the lowest order fiber nonlinearity (NL), namely the self-phase modulation, limits the scalability of high-energy ultrashort pulse fiber amplifiers. Therefore, a distinguished difference arises between the consequences of accumulated nonlinear phase originating from the pulse envelope and initial weak modulations, resulting in a strong recommendation to operate an amplification system as linearly as possible in order to generate high-contrast pulses. Low-NL rare-earth-doped fibers, such as the recently available designs of photonic crystal fibers, are the key element for successful peak power scaling in fiber laser systems. In this paper, we present a detailed analysis and optimization of the extraction characteristics in connection with the accumulated nonlinear phase in such extreme fiber dimensions. Consequently, millijoule pulse energy femtosecond pulses at repetition rates in the 100 kHz range have already been demonstrated experimentally in a Yb-fiber-based CPA system that has even further scaling potential.

Coherent combining is a novel approach to scale the performance of laser amplifiers. The use of ultrashort pulses in a coherent combining setup results in new challenges compared to continuous wave operation or to pulses on the nanosecond timescale, because temporal and spectral effects such as self-phase modulation, dispersion and the optical path length difference between the pulses have to be considered. In this paper the impact of these effects on the combining process has been investigated and simple analytical equations for the evaluation of this impact have been obtained. These formulas provide design guidelines for laser systems using coherent combining.

The coherent combining of ultrashort pulses is a concept for scaling the pulse energy and average power of laser systems emitting ultrashort pulses. In this contribution the experimental results of a coherently combined femtosecond fiber CPA laser system consisting of 2 amplifiers is presented. Combining efficiencies as high as 89% and compressed pulse energies of up to 3mJ were achieved. Additionally, the system showed excellent long-term stability.

The process of coherent combining of femtosecond pulses is influenced by different effects like dispersion, self-phase modulation or OPD fluctuations. Their impact on the combining process is investigated theoretically and experimentally.

We report on the coherent addition of two femtosecond fiber amplifiers. Combined and compressed pulse energies of up to 3mJ and average powers of up to 88W are achieved with high combining efficiencies.

We review basic considerations, current status, challenges and potential performance of coherently combined spatially separated ultrafast amplifiers.

Coherent combining is a novel approach to scale the performance of laser amplifiers. The use of ultrashort pulses in a coherent combining setup results in new challenges compared to continuous wave operation or to pulses on the nanosecond timescale, because temporal and spectral effects such as self-phase modulation, dispersion and the optical path length difference between the pulses have to be considered. In this paper the impact of these effects on the combining process has been investigated and simple analytical equations for the evaluation of this impact have been obtained. These formulas provide design guidelines for laser systems using coherent combining. The results show that, in spite of the temporal and spectral effects mentioned above, for a carefully adjusted and stabilized system an excellent efficiency of the combining process can still be achieved.

We present a fiber CPA system consisting of two coherently combined fiber amplifiers, which have been arranged in an actively stabilized Mach-Zehnder interferometer. Pulse durations as short as 470 fs and pulse energies of 3 mJ, corresponding to 5.4 GW of peak power, have been achieved at an average power of 30 W.

In this contribution, we report on the approach of coherently combined spatially separated amplifiers to increase the output power and pulse energy compared to a single amplifier emission. For continuous-wave and Q-switched laser systems this is a widely known concept [1–3]. Recently, the concept of coherent combining was adapted to femtosecond CPA systems in a low power proof of principle experiment [4]. Compared to the CW and Q-switched case, additional aspects have to be considered when combining femtosecond pulses, like dispersion management and nonlinear effects.

Coherent combining of ultrashort pulses from spatially separated amplifiers is a novel approach to scale the average power and peak power of laser systems. For continuous-wave and Q-switched laser systems this is a widely known concept. Recently, this approach was adapted to femtosecond CPA systems in a low power proof of principle experiment [1]. Compared to the CW and Q-switched case, additional aspects have to be considered when combining femtosecond pulses, such as dispersion management and the impact of nonlinear effects. This is especially true for fiber based systems.

We report on the coherent combination of two chirped pulsed fiber lasers. The beams coming from two 100 μm core diameter ytterbium-doped rod-type fibers were coupled in a Mach-Zehnder-type interferometer by means of a polarization beam splitter. Active stabilization of the interferometer was achieved by using a piezo-mounted mirror driven by a Hansch-Couillaud polarization detection system. Pulses with 120 μJ energy and a compressed duration of 800 fs were obtained. This, compared with the 66 μJ coming from each single amplifier, results in a combining efficiency as high as 91%.

We report on a novel concept to scale the performance of ultra-fast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. The splitting and combination process is based on the polarization combining technique using polarization cubes. A Hansch-Couillaud detector measures the polarization state of the combined beam. The error signal (deviation from linear polarization) is used to stabilize the optical path lengths in the different channels with a piezo mounted mirror. In a proof-of-principle experiment the combination of two femtosecond fiber-based amplifiers in a CPA systems is presented. A combining efficiency as high as 97% has been achieved. Additional measurements were carried out to investigate the stability of the system. The concept offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.

We present the coherent combination of high-energy ultrashort pulses from two fiber CPA channels. We achieved a combining efficiency of 91% with a compressed pulse duration of 800 fs and 120 µJ pulse energy.

We report on a Yb:YAG Innoslab laser amplifier system for generation of subpicsecond high energy pump pulses for optical parametric chirped pulse amplification (OPCPA) at high repetition rates. Pulse energies of up to 20 mJ (at 12.5 kHz) and repetition rates of up to 100 kHz were attained with pulse durations of 830 fs and average power in excess of 200 W. We further investigate the possibility to use subpicosecond pulses to derive a stable continuum in a YAG crystal for OPCPA seeding.

We report on the coherent combination of two chirped pulsed fiber amplifiers using a polarization cube. Femtosecond pulses were combined with an efficiency of 91% obtaining a combined pulse energy of 120 μJ.

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.

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.

In this work we present a new method for peak-power scaling in nonlinear CPA-systems. By clipping the tails of the spectrum we demonstrate pulse quality enhancement and an increase of peak-power at the output of the CPA-system. A theoretical model allows us to determine an optimal ratio between the spectral clipping bandwidth and the pulse bandwidth at a certain B-integral. Additionally, a simple redesign of a grating based stretcher unit, applying our new spectral clipping technique, would significantly increase the output peak-power of such nonlinear CPA-systems by a factor up to six due to the higher stretching ratio.

In this Letter we report on the generation of 830 W compressed average power from a femtosecond fiber chirped pulse amplification (CPA) system. In the high-power operation we achieved a compressor throughput of about 90% by using high-efficiency dielectric gratings. The output pulse duration of 640 fs at 78 MHz repetition rate results in a peak power of 12 MW. Additionally, we discuss further a scaling potential toward and beyond the kilowatt level by overcoming the current scaling limitations imposed by the transversal spatial hole burning. (C) 2010 Optical Society of America

We report on a novel approach of performance scaling of ultra-fast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. Splitting and combination is achieved by polarization cubes, i.e. the approach bases on polarization combining. A Hänsch-Couillaud detector measures the polarization state at the output. The error signal (deviation from linear polarization) is used to stabilize the synchronization of different channels. In a proof-of-principle experiment the combination of two femtosecond fiber-based CPA systems is presented. A combining efficiency as high as 97% has been achieved. The technique offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.

We report on the performance of a 60 kHz repetition rate sub-10 fs, optical parametric chirped pulse amplifier system with 2 W average power and 3 GW peak power. This is to our knowledge the highest average power sub-10 fs kHz-amplifier system reported to date. The amplifier is conceived for applications at free electron laser facilities and is designed such to be scalable in energy and repetition rate.

Based on an analytical model for the impact of SPM on stretched ultrashort pulses, the influence of SPM is experimentally controlled in a fiber-based CPA-system. Nearly transform-limited pulses with energies as high as 1 mJ are produced at a B-integral as high as 8 rad. The value of the B-integral is measured by the phase-compensation method. The technique neither relies on complex phase-measurements nor on time-consuming optimization routines.

We experimentally present the advantage of using circularly polarized light during laser amplification to reduce the accumulated Kerr-nonlinearity.The theoretical value of 2/3 for the reduction of the B-integral is verified in a fiber CPA-system. The demonstrated technique is very attractive since it can be employed in nearly all kind of ultrafast laser systems to scale their performance, when they are limited by Kerr-nonlinearity.

In this work we present a new method for peak-power scaling in nonlinear CPA-systems. In conventional CPA systems temporal stretching is used to avoid high peak powers developing during the amplification process. However, if the peak power of the stretched pulse is high enough, a considerable amount of nonlinear spectral phase shift (as described by the B-integral) can be accumulated due to self-phase modulation. Consequently, reduced peak powers and pulse shape degradation will be obtained after compression. Such systems are referred to as nonlinear CPA-systems.

The generation of ultra-short pulses is typically governed by the interplay of dispersion and nonlinearity. Ultrashort pulses have high peak-powers which cause nonlinearities. In particular, nonlinearity can cause wave-breaking of ultra-short pulses. This effect imposes limitations on the generation and amplification of femtosecond laser pulses. Chirping of the pulses reduces the peak-power during amplification, and thus, increases the onset of nonlinearities. Since the chirping of the pulses is limited due to practical reasons, the impact of nonlinearity plays a role again with increasing pulse-energies. To further scale the energy of such systems, there is need for operation in the nonlinear CPA-regime.

Pulse-contrast degradation at the CPA-system's output is analyzed. If Kerr-nonlinearity is present, weak initial spectral phase and amplitude modulations are responsible for the decrease. The pulse is split into several sub-pulses. Bessel-functions describe the intensities of the side-pulses relative to the principal pulse. We provide the governing physical quantities.

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.

A high-average-power Yb-doped fiber chirped-pulse amplification system is presented. Compressed average power of 325 W at 40 MHz repetition rate corresponding to 8.2 mu J pulse energy is extracted. Compression in a highly efficient dielectric-grating-based compressor yields pulses as short as 375 fs, resulting in 22 MW of peak power.

Phase-compensation in ultra-compact nonlinear CPA systems employing chirped volume Bragg gratings as stretcher and compressor is demonstrated. It is based on compensation of nonlinear phase with phases due to the dispersion of the amplifiers.

In this contribution it is reported that circularly polarized light is advantageous if the Kerr-effect has to be minimized during laser-amplification. The experimental demonstration is based on a fiber CPA-system. The different polarization states 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 verified. In laser-amplifiers circularly polarized light reduces the detrimental impact of the Kerr-nonlinearity, and thus, increases the peak-power and the self-focussing threshold.

We analyze the impact of small amplitude and phase modulations of the input pulse on the pulse contrast at the output of nonlinear CPA-systems. The Kerr-effect precludes nonlinear amplification if the initial pulse-quality is low.

An average power of 325-W is extracted from a high repetition rate (40MHz) fiber-CPA-system. The compressed pulse duration of 375-fs leads to a peak-power of 22-MW. Further scaling potential towards kW-level will be discussed.

A detrimental pulse distortion mechanism inherent to nonlinear chirped-pulse amplification systems is revealed and analyzed. When seeding the nonlinear amplification stage with pulses possessing weak side-pulses, the Kerr-nonlinearity causes a transfer of energy from the main pulse to side pulses. The resulting decrease in pulse contrast is determined by the accumulated nonlinear phase-shift (i.e., the B-integral) and the initial pulse-contrast. The energy transfer can be described by Bessel-functions. Thus, applications relying on a high pulse-contrast demand a low B-integral of the amplification system and a master-oscillator that exhibits an excellent pulse-contrast. In particular, nonlinear fiber CPA-systems operated at B-integrals far beyond pi have to be revised in this context.

The generation of 0.5 mJ femtosecond laser pulses by coherent combining of two high power high energy fiber chirped-pulse amplifiers is reported. The system is running at a repetition frequency of 175 kHz producing 88 W of average power after the compressor unit. Polarizing beam splitters have been used to realize an amplifying Mach-Zehnder interferometer, which has been stabilized with a Hänsch-Couillaud measurement system. The stabilized system possesses a measured residual rms phase difference fluctuation between the two branches as low as λ/70 rad at the maximum power level. The experiment proves that coherent addition of femtosecond fiber lasers can be efficiently and reliably performed at high B-integral and considerable thermal load in the individual amplifiers.

We report on a high power optical parametric amplifier delivering 8 fs pulses with 6 GW peak power. The system is pumped by a fiber amplifier and operated at 96 kHz repetition rate. The average output power is as high as 6.7 W, which is the highest average power few-cycle pulse laser reported so far. When stabilizing the seed oscillator, the system delivered carrier-envelop phase stable laser pulses. Furthermore, high harmonic generation up to the 33(th) order (21.8 nm) is demonstrated in a Krypton gas jet. In addition, the scalability of the presented laser system is discussed.

Using phase shaping, the impact of the Kerr effect in a fiber-based chirped-pulse amplification (CPA) system is experimentally controlled. The technique is based on an analytical model describing the spectral phase owing to self-phase modulation in CPA systems. The method relies neither on complex phase measurements nor on time-consuming optimization routines. Nearly transform-limited pulses with energies as high as 1 mJ are produced, and a B integral being as high as 8 rad is accumulated in the main amplifier. The value of the B integral is determined by the method itself.

We experimentally demonstrate compensation of the impact of Kerr-nonlinearity with positive dispersion in chirped-pulse systems. The condition for the phase-compensation is derived and design guidelines are presented. The technique is shown with a fiber-based system employing conventional diffraction gratings as well in a system that is based on chirped volume Bragg-gratings. Practical requirements on the stretching unit are discussed.

It is analytically shown that weak initial spectral phase modulations cause a pulse-contrast degradation at the output of nonlinear chirped-pulse amplification systems. The Kerr-nonlinearity causes an energy-transfer from the main pulse to side-pulses during nonlinear amplification. The relative intensities of these side-pulses can be described in terms of Bessel-functions. It is shown that the intensities of the pulses are dependent on the magnitude of the accumulated nonlinear phase-shift (i.e., the B-integral), the depth and period of the initial spectral phase-modulation and the slope of the linear stretching chirp. The results are applicable to any type of laser amplifier that is based on the technique of chirped-pulse amplification. The analytical results presented in this paper are of particular importance for high peak-power laser applications requiring high pulse-contrasts, e.g. high field physics.

We report on the generation of 830 W of 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.

We report on a fiber pumped high power few cycle OPCPA system delivering 8 fs pulses at 96 kHz repetition rate. The amplified pulse energy is as high as 70 μJ which corresponds to 6.7 W average power and the pulse peak power is as high as 5 GW. Additionally, the seed oscillator is CEP stabilized and the CEP stability of the amplified pulses is measured. Furthermore, high harmonic generation up to the 35th order (20.9 nm) in demonstrated in a Krypton gas jet.

An optical parametric chirped pulse amplifier with few-cycle pulses and high average power is demonstrated. This is the precursor of a system that will work at higher average power at the FLASH free electron laser.

We report on a 96 kHz repetition rate sub-10 fs, optical parametric chirped pulse amplifier system with 6.7 W average power and >5 GW peak power. Furthermore the oscillator is CEP stabilized and the CEP of the amplified pulses is measured.