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High-Flux Synchrotron Alternatives Driven by Powerful Long-Wavelength Fiber Lasers

Periodic Reporting for period 2 - SALT (High-Flux Synchrotron Alternatives Driven by Powerful Long-Wavelength Fiber Lasers)

Reporting period: 2021-01-01 to 2022-06-30

The visionary goal of the SALT project is to develop high-flux table-top alternatives to synchrotrons in three application-relevant spectral regions: the THz, the mid infrared(mid-IR) and the soft X-rays. The success of this endeavour will have a profound impact on society by making access to synchrotron-like radiation widely available. Accordingly, this will not lead to a general acceleration in the development of applications, but a number of cross-disciplinary applications are also expected to enable seminal discoveries.
For this goal, the frequency conversion of a high-power driving solid-state laser seems to be the most elegant solution. However, replication of the extremely wide spectral bandwidth of synchrotron radiation requires three parallel developments of radiation sources for the THz range, the mid IR range and the soft X-ray parts of the spectrum. With this approach, however, it is possible to obtain a high photon flux and high brightness radiation in these important spectral ranges in a table-top format. This makes industrial applications more cost-effective, and scientific applications benefit from an increased signal-to-noise ratio and a shorter acquisition time, which then makes it possible to observe what could not be seen before. Moreover, the approach followed in the SALT project will also allow obtaining frequency combs in all spectral regions, which is extremely interesting for high-precision spectroscopic applications.
To achieve a high photon flux with this frequency conversion approach, powerful driving solid-state lasers are required. Directly diode-pumped, double-clad Ytterbium-doped-fiber laser systems are particularly interesting for the generation and amplification of high-power ultrashort pulses due to several unique properties. In fact, Ytterbium doped fiber lasers are currently delivering record performances in both continuous and pulsed operation.
Shifting the emission to longer wavelengths can unleash an unexpected performance scaling potential of fiber laser systems, which is essential for the success of the project. In particular, 2μm fiber lasers based on Thulium-doping have the potential to revolutionize today's laser technology with an effect that will surpass that of Ytterbium-doped solid-state lasers. In fact, lasers with longer wavelengths are gathering momentum, because there is a wide variety of direct applications. Through frequency conversion, these lasers open up ranges to spectral regions of enormous interest such as the mid-infrared, the THz range and the soft X-ray range.
The aim of the SALT project is to investigate novel approaches for high-power coherent light sources (with parameters that are orders of magnitude above the current state of the art) that will serve as viable table-top alternatives to synchrotrons.
Main tasks:
A) to revolutionize the performance levels of ultrafast lasers by unlocking the potential of Thulium-doped fiber lasers,
B) to demonstrate new realms of flux in selected wavelength regions by frequency-converting these high-power 2μm sources and, therewith, pave the way for a number of frontier applications allowing for seminal discoveries.
Within the project SALT several technological accomplishments have been achieved. A new generation of Tm-doped fiber lasers was developed. With the source realized in this project period, the performance level of state-of-the-art Tm-doped fiber laser technology and OPCPA technology could be surpassed. The laser delivers a pulse energy of 1.7 mJ and 170 W of average output power at 100 kHz repetition rate. Additionally, the output pulse duration was 100 fs and the beam quality was close to the diffraction limit.
This renders this source the highest average power mJ-level SWIR source to date. These parameters are generated by a table-top source which is the ideal platform to scale frequency conversion processes into the soft X-ray, THz and mid infrared spectral region.

Regarding frequency conversion into the soft X-ray spectral region, first high-order harmonic generation experiments, driven by the established laser technology, have been conducted [1].
Here, a novel concept of simultaneous pulse self-compression and high-harmonic generation in a single hollow-core fiber has been demonstrated. Due to the tight confinement of light in the fiber, this design allows using moderate peak power driving laser pulses, which can potentially enhance both integration and the availability of these sources. Numerical modelling of the experiment additionally revealed important findings regarding the interaction length of the driving field with the generated high harmonic radiation in the waveguide. These findings are an important cornerstone for the next experimental steps.

For the preparation of driving a THz generation with the recently completed Tm-laser system an experiment using a 1 µm wavelength driving laser [2] has been carried out. By using an unprecedented average power of 640 W to drive the gas-plasma based THz generation an average power of 640 mW of broadband, single-cycle THz pulses has been generated [3]. This result is the current state-of-the-art of laser-driven THz sources in terms of average power as well as conversion efficiency for a 1 µm wavelength driving laser.
In alignment with the project goals, the same THz generation technique will be addressed with now established 2 µm wavelength laser technology. The experimental setup for this is currently under construction and results can be expected in the near future. It is expected that the change to longer driving wavelengths significantly increases the conversion efficiency of the generated THz radiation.

[1] M. Gebhardt et al., Light Sci Appl 10,36(2021).
[2] H. Stark et al., Opt. Lett. 46, 969-972 (2021).
[3] J. Buldt, et al., Opt.Lett.46(20) 5256-5259 (2021).
The developed Thulium fiber CPA system defines the new state of the art and has pushed the available performance by roughly an order of magnitude in the 2 µm wavelength region. Worth to be mentioned, the architecture is highly scalable and until the end of the project SALT another performance step will be targeted. Also noteworthy, the demonstrated THz source driven by an 1 µm ultrafast fiber laser is by far the most powerful in the world. The high-flux soft-Xray source employs nonlinear pulse self-compression in a waveguide and simultaneous phase-matched HHG up to the carbon K-edge for the first time.
The Thulium fiber laser system is the perfect platform to perform frequency conversion towards application relevant spectral ranges, such as the soft Xray, the mid-IR and the THz range. Hence, the focus of the upcoming period within SALT is on harvesting the potential by combining the Thulium fiber laser system with existing frequency conversion stages and beat flux records obtained with Ytterbium based ultrafast lasers, which are record values to date, by a considerable factor.