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Grating Reflectors Enabled laser Applications and Training

Periodic Reporting for period 1 - GREAT (Grating Reflectors Enabled laser Applications and Training)

Reporting period: 2019-03-01 to 2021-02-28

Improving the control of the properties of lasers has led to their rapid commercial adoption, through high-value manufacturing in the automotive and consumer electronics markets, in digital communications for increased bandwidth, to medical applications with enhanced biological imaging and surgical techniques. Tailoring the temporal, spectral and spatial properties of light is enabling new advances across scientific frontiers, for example in nuclear physics to the recent successes in the measurement of gravitational waves.
A key optical element that enables the control of these properties of light is the diffraction grating. In the case of high power laser light, a combination of a planar waveguide and sub-wavelength gratings, also known as Grating Waveguide Structure (GWS), is known to be a powerful solutions for tailoring the polarization and for realizing spectral and spatial beam shaping. The development of GWS, from concept through fabrication, qualification and implementation, forms the platform upon which the project GREAT is built.
The overall aims of GREAT are to conceive and produce GWS which are – by design – responding to the end-users’ needs and products, to develop and apply controlled production processes of GWS for different applications such as micro and macro materials processing as well as for relativistic science and to develop and implement precise measurement and qualification tools.
The concepts of the grating waveguide structures for all planned applications were defined and the structures for the first generation GWS were designed. Tolerance analysis is ongoing and will be fully implemented after input about the fabrication tolerances provided by the partners after the fabrication of the 1st generation of GWS.
The first mapping, definition, and development tasks have put together the fabrication resources in the consortium and allow for establishing such fabrication chains that utilize the strengths of each institute, both in infrastructure and know-how. Therefore, for the fabrication of the devices the needed lithography and etching processes where determined for each application also with regard to the final design of the first generation of GWS, which resulted in a comprehensive list with all available processes (thin film deposition, patterning, and etching) and process flows.
Additionally the properties of the optical coating were measured using grating-based M-lines spectroscopy, spectrophotometry and ellipsometry. Optical characterization of 10 different coating materials was prepared for the project and is available for GWS manufacturing. A laser damage test station dedicated to the GREAT project was implemented and the measured induced laser damage threshold (LIDT) at 1030 nm / 500 fs of main coating materials for fabrication of GWS (HfO2, Nb2O5, SiO2) was obtained and matches the state of the art performances.
Finally, the laser architectures for all targeted applications were determined and for each of them the experimental design for the demonstration were defined. The main achievement at this day is the validation of the tight specifications of the GWS produced within GREAT (central wavelength, bandwidth, energy handling, pulse width, beam size, polarization, diffraction efficiency, fluence and environmental constraints) for all beneficiaries and partners of the GREAT project. Additionally, the laser experiments have been designed and deployed for the relevant characterization in realistic conditions of GWS for all applications addressed in GREAT.
In order to strive for beyond state-of-the-art results the complete development chain from design, fabrication and implementation of GWS structures is conducted within GREAT. Robust designs of the GWS for the different application will be developed together with characterization tools to allow for an reproducible and reliable characterization process, which will be used in a feedback loop to optimize the different fabrication processes. The overall aim is the reliable and reproducible production of highly efficient, low-cost grating waveguide structures which will lead to more efficient and more compact laser systems for different applications, which are the pulse compression for wavelengths in the 1 µm as well as the 2 µm region, the spectral stabilization and wavelength multiplexing of solid-state lasers at wavelengths of 1030 nm (e.g. Yb:YAG thin-disk laser) and 2000 nm (Tm-doped fiber laser) as well as for diode lasers emitting between 900 and 1000 nm, and the generation of cylindrical vector beams with radial and azimuthal polarization in continuous wave as well as pulsed operation.
Grating waveguide output coupler (GWOC) for the generation of a radially polarized beam at 1030 nm
Reflective pulse compression grating for a wavelength of 1030 nm