The timeframe between the first demonstration of novel laser parameters and the commercial availability of systems delivering those parameters has been shrinking over the recent years. This development makes femtosecond laser systems with unparalleled parameters available to a wide range of applicants. We present the progress in setting up such a FCPA laser system delivering 5mJ pulse energy at 100kHz pulse repetition rate (i.e. 500W of average power) and with pulse durations of 6fs. The system, the so-called HR2 laser, will be implemented at the ELI-ALPS facility in Szeged, Hungary. Furthermore, the laser is optimized to generate output pulses with a stable carrier-envelope-phase (CEP). Since 6fs requires a bandwidth not supplied by Yb-based gain-media, and thus nonlinear compression is necessary, the chirped-pulse-amplification (CPA) system itself must deliver a higher energy to compensate for the losses in these nonlinear-compression-stages. Hence, the CPA is operated at 12mJ and 1.2kW delivering sub 300-fs pulses with an almost diffraction limited beam quality. To achieve such a parameter-set, the output of 16 main-amplifier channels is coherently combined using a polarization-based filled-aperture combining scheme. Maintaining the CEP stability of the oscillator throughout the entire system is another key challenge of the project. We finally discuss further power-scaling approaches and possibilities to reduce system size, complexity and, therewith, cost. By using these revolutionary laser systems, even demanding applications such as laser-based particle acceleration or XUV-generation via high-harmonic generation will be able to perform the important step from the laboratory to real-world applications allowing to access repetition rates orders of magnitude higher than before.
In this contribution, we present a spatio-temporal coherent beam combining setup in a proof-of-principle experiment with an entirely fiber-coupled front-end. Unlike in previous experiments, where the temporal pulse division was achieved using free-space optical delay lines, the pulses are taken directly from the pulse train of the oscillator. Thereby, the free-space paths and the alignment requirement are cut in half. The combination inevitably remains in free-space considering application in high-power lasers. For the combination of 4 temporally separated pulses, a combining efficiency larger than 95% is demonstrated. The efficiency is largely independent of the combined pulse energy and temporal contrasts close to the theoretically estimated maximum are reached. Potentially, this approach allows for self-optimization of the combination due to the many degrees of freedom accessible with the electro-optic modulators.
State-of-the-art ultrafast fiber lasers currently are limited in peak power by excessive nonlinearity and in average power by modal instabilities. Coherent beam combination in space and time is a successful strategy to continue power scaling by circumventing these limitations. Following this approach, we demonstrate an ultrafast fiber-laser system featuring spatial beam combination of 8 amplifier channels and temporal combination of a burst comprising 4 pulses. Active phase stabilization of this 10-armed interferometer is achieved using LOCSET and Hänsch-Couillaud techniques. The system delivers 1 kW average power at 1 mJ pulse energy, being limited by pump power, and delivers 12 mJ pulse energy at 700 W average power, being limited by optically induced damage. The system efficiency is 91% and 78%, respectively, which is due to inequalities of nonlinearity between the amplifier channels and to inequality of power and nonlinearity between the pulses within the burst. In all cases, the pulse duration is ~260 fs and the M<sup>2</sup>-value is better than 1.2. Further power scaling is possible using more amplifier channels and longer pulse bursts.
The noise characteristics of high-power fiber lasers, unlike those of other solid-state lasers such as thin-disks, have not been systematically studied up to now. However, novel applications for high-power fiber laser systems, such as attosecond pulse generation, put stringent limits to the maximum noise level of these sources. Therefore, in order to address these applications, a detailed knowledge and understanding of the characteristics of noise and its behavior in a fiber laser system is required. In this work we have carried out a systematic study of the propagation of the relative intensity noise (RIN) along the amplification chain of a state-of-the-art high-power fiber laser system. The most striking feature of these measurements is that the RIN level is progressively attenuated after each amplification stage. In order to understand this unexpected behavior, we have simulated the transfer function of the RIN in a fiber amplification stage (~80μm core) as a function of the seed power and the frequency. Our simulation model shows that this damping of the amplitude noise is related to saturation. Additionally, we show, for the first time to the best of our knowledge, that the fiber design (e.g. core size, glass composition, doping geometry) can be modified to optimize the noise characteristics of high-power fiber laser systems.
Proc. SPIE. 9728, Fiber Lasers XIII: Technology, Systems, and Applications
KEYWORDS: Optical fibers, Fiber amplifiers, Energy efficiency, Mirrors, Beam splitters, Optical amplifiers, Oscillators, Polarization, High power lasers, Free space optics, Electro optics, System integration, Laser systems engineering, Coherent beam combination
We present a spatial and temporal coherent-beam-combination system based on a fiber-integrated front-end, electro-optical components, and optical delay lines. The system features a larger scaling potential, enhanced stability and reduced alignment sensitivity compared to known divided-pulse amplification schemes. In a proof-of-principle experiment combining 4 pulses, a combining efficiency larger than 95% and a high amplitude stability are demonstrated. The efficiency is largely independent of the combined pulse energy and the temporal pulse contrast is better than 20 dB.
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.
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.
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.
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.