Enrico Seise

Femtosecond fiber CPA system emitting 830 W average output power

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.

High Repetition Rate Gigawatt Peak Power Fiber Laser Systems: Challenges, Design, and Experiment

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.

Coherently-combined two channel femtosecond fiber CPA system producing 3 mJ pulse energy

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.

Coherent addition of fiber-amplified ultrashort laser pulses.

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.

High average and peak power few-cycle laser pulses delivered by fiber pumped OPCPA system

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.

The impact of spectral modulations on the contrast of pulses of nonlinear chirped-pulse amplification systems

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.

Basic considerations on coherent combining of ultrashort laser pulses

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.

Fiber-amplifier pumped high average power few-cycle pulse non-collinear OPCPA.

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.

88 W 0.5 mJ femtosecond laser pulses from two coherently combined fiber amplifiers.

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.

Circular versus linear polarization in laser-amplifiers with Kerr-nonlinearity

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.