SALT

The number of possible applications of synchrotron radiation (broadband electromagnetic radiation) has increased exponentially since its discovery in 1947 [1]. Applications include materials science, life sciences and chemistry, but recently it has also been used in industrial processes, inspections and even cancer treatment [2,3]. Synchrotrons played an important role in the work that led to the 2009 and 2012 Nobel Prizes in Chemistry. It is therefore expected that this technology still has potential that will have a profound impact on science and, consequently, on future applications. However, synchrotrons have one major disadvantages: they are extremely large facilities and therefore very expensive and have very limited access. This significantly hinders the large-scale use of this radiation and thus the progress and development of many applications worldwide.
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 (see Fig.1). 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 [4]. In fact, Ytterbium doped fiber lasers are currently delivering record performances in both continuous and pulsed operation. For example, ultrafast fiber lasers at 1μm wavelength have produced average powers close to 1kW [8], pulse energies of up to 2mJ and peak powers of up to 7GW [9]. 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.

llustration of reachable spectral ranges and brightness levels with 1μm and 2μm laser driven HHG, DFG and two-color plasma in comparison to 3rd generation synchrotrons.

Picture: IAP (University Jena)