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High Performance Alexandrite Crystals and Coatings for High Power Space Applications

Periodic Reporting for period 2 - GALACTIC (High Performance Alexandrite Crystals and Coatings for High Power Space Applications)

Reporting period: 2021-01-01 to 2021-12-31

Spaceborne Earth observation missions with laser-based instruments are essential to gain deeper insight in large-scale atmospheric dynamics, to improve the climate modelling and to enhance the monitoring of the planet’s surface regarding the impact of climate change e.g. on the vegetation. The design of currently used laser systems relies on the well-established Nd:YAG laser technology. Alexandrite is investigated as potential substitute due to its broad spectral bandwidth and thus tunable output wavelength and the high optical efficiency. Besides, it features excellent material properties such as high hardness, high thermal conductivity and good breaking strength.
Non-European companies dominate the global Alexandrite laser crystal market, so far. Furthermore, only a Technology Readiness Level (TRL) of 4 can be attributed to the currently available Alexandrite crystals, i.e. the technology or component is only validated in a laboratory environment, while a TRL 6, i.e. a verification under space environmental conditions, is needed.
The Horizon 2020 project GALACTIC (“High Performance Alexandrite Crystals and Coatings for High Power Space Applications”) aims at pushing the Alexandrite laser crystal technology within the EU up to TRL 6 and at establishing a fully European-based supply chain for high-quality, functionally coated Alexandrite laser crystals.
For this, the project partners Optomaterials S.r.l. (Italy), Altechna Coatings (Lithuania), and the Laser Zentrum Hannover e.V. (Germany) work closely together to enhance the European state-of-the-art Alexandrite crystal and crystal coating technology. Therefore, Optomaterials S.r.l. is refining the crystal growth process and improving the machining and optical polishing quality. Altechna Coatings develops high-quality, low loss, and high laser-induced damage threshold coatings specifically tailored for Alexandrite laser crystals. The Laser Zentrum Hannover e.V. designs, assembles and characterizes two laser demonstrator prototypes, which mimic typical Earth observation laser source requirements to assess the functional performance of the developed Alexandrite crystals.
Finally, the GALACTIC consortium will qualify the coated laser crystals to TRL 6 on component level via an environmental test campaign comprising thermal cycling as well as proton and gamma irradiation tests.
Altogether, GALACTIC will enable non-dependence of Europe on coated Alexandrite laser crystal technology and guarantee long-term European availability of this technology.
After the status’ evaluation of the European and non-European Alexandrite laser crystal and coating technologies, detailed specifications were deduced before starting the manufacturing process and the procurement of crystals from two leading non-European suppliers. The various growth runs performed within GALACTIC yielded high-quality crystals which could be already used for extensive characterization and functional tests. The results show the comparable if not better performance of the GALACTIC samples.
The second aspect was the review of relevant space missions to compile a detailed components test plan for the TRL 6 qualification campaign. Besides, the international state-of-the-art of Alexandrite lasers was assessed to consolidate the requirements for the two laser demonstrator prototypes set up at the Laser Zentrum Hannover e.V. (project coordinator). The first demonstrator in the cavity-dumped, q-switched operational mode (low energy, high repetition rate) was successfully commissioned and optimized leading to record output parameters that by far exceed the target values. The setup of the second laser demonstrator (high energy, low repetition rate) has started with the characterization of the pump diode stack.
Optomaterials S.r.l. researched the optimal growth conditions for high-quality crystals and thoroughly evaluated crystal structure data from X-ray diffraction and Raman measurements. They synthesized thirteen boules using different growth parameters and already achieved a sufficiently high doping concentration. Around 180 shaped crystal components were extracted, machined, optically polished and comprehensively characterized.
Altechna Coatings processed the samples from Optomaterials S.r.l. by various surface cleaning and etching methods and studied their influence on the surface characteristics and the durability against intense laser radiation via different characterization tests. In parallel, they designed two different coating systems which they could realize and optimize in several coating runs. Thus, a significant enhancement of the laser-induced damage threshold (LIDT) could be achieved.
Significant effort was spent on increasing the visibility of the project e.g. via a website, press releases and engagement in social media according to the dissemination plan. The project and its outcome has been presented at five scientific conferences and in the corresponding proceedings’ papers so far.
To address the progress beyond the state-of-the-art, a distinction between the European and the worldwide status of the Alexandrite crystal manufacturing and coating technology needs to be made. While the European manufacturer and project partner Optomaterials S.r.l. already offers low doped crystals in good quality for industrial applications, a primary GALACTIC goal is to further improve the overall fabrication and characterization capabilities. The finally fabricated crystals should not only show the same quality as the ones from the non-European suppliers, but even surpass them regarding the relevant requirements for future space missions.
The availability of TRL 6 space-qualified crystals will be a worldwide unique feature of the components produced within the GALACTIC project. In addition, these high-quality, shaped and coated Alexandrite crystals will be available reproducibly manufactured by solely European suppliers. The LIDT shall also be significantly enhanced due to the improved bulk material, the developed surface treatment processes and the optimized coating.
The main impacts of the GALACTIC project’s results will be first to reduce the dependence on critical technologies and capabilities from outside Europe for future space applications. For the Alexandrite crystal and coating technology this means even industrial independence due to the completely European-supplier-based supply chain. GALACTIC will help to develop or regain in the mid-term the European capacity to operate independently in space.
Besides, the use of functionally coated Alexandrite crystals is not restricted to laser systems for space missions. Another application is e.g. medical skin treatment. Since the consortium is continuously looking for ways to improve the value chain of optical components, the GALACTIC results can be used to gain competitive advantage in various R&D-intensive markets of optics for high-energy laser systems.
To date, there are no Earth observation missions based on Alexandrite crystal technology. Different ESA-driven studies are undertaken, but all efforts rely on non-European technology, so far. The production of European high-performance, space-qualified Alexandrite crystals will simplify the design of such laser-based missions. The results of GALACTIC will push the development of space observation missions with optical payloads, which are a crucial building block in the understanding of our planet and its climate.
Alexandrite crystal boule
Alexandrite crystals as used in the GALACTIC project