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Thin-disk lasers for the 2-micron spectral range based on Ho-doped monoclinic double-tungstate epitaxial structures

Periodic Reporting for period 1 - TDL2Ho (Thin-disk lasers for the 2-micron spectral range based on Ho-doped monoclinic double-tungstate epitaxial structures)

Reporting period: 2015-06-01 to 2017-05-31

Eye safe lasers emitting in the 2-µm spectral range have potential applications in medicine, material processing and defence and security among others. Short pulse (ns) lasers are employed as pump sources for optical parametric oscillators (OPOs), for extension to the mid-infrared (MIR) spectral range. All such applications will benefit from the power scaling, increased efficiency and compact design, aimed in this project and provided by the thin-disk laser concept.
Reports on 2-µm lasers with thin-disk geometry are scarce and describe complex architectures. In previous experiments, negative temperature of the heat sink was applied in the crystal with only 4 bounces of the pump. Room temperature operation required 12 bounces of the pump beam for high absorption in the crystal.
The research activities planned in the present project were devoted to development, test and characterization of novel thin-disk structures based on epitaxially grown monoclinic double tungstate (MDT) crystals for use in high average and peak power all-solid-state laser systems operating in the important 2-µm spectral range. The laser gain is based on the Holmium (Ho) trivalent ion operating slightly above 2-µm. The Ho ion can be excited directly into the upper laser level, producing less heat and enabling power scaling with diode pumping.
The general objective was to realize novel compact and efficient Ho-based epitaxial lasers with thin-disk design and greatly simplified pump geometry based on a single bounce of the pump due to its advantageous spectroscopic features. It consisted of 4 specific objectives: Preparation of thin disk elements based on KLu(WO4)2 (KLuW) epitaxies and characterization, laser operation with epitaxial co-doped Tm,Ho:KLuW and singly-doped Ho:KYW thin disks, laser operation in different temporal regimes with Ho thin disks, and high-power and high efficiency operation in comparison with other designs and pump concepts.
The initial part of the project was related to the epitaxial growth and the fabrication of laser elements. The growth of the epitaxial layers as active media has been successful as well as their processing and characterization to have them ready for the laser experiments.
The second part of the project dealt with the laser characterization. The first Ho thin-disk laser based on MDT crystals has been realized and described. The thin-disk laser was pumped via a single bounce of the pump. This is the simplest and most compact pump scheme ever reported for a 2 micron thin-disk laser. The output power was scaled up to the Watt-level in the CW operation mode representing a record slope efficiency of 60%. Further power scaling was also demonstrated with a double-bounce of the pump, still preserving the simplified and compact pump geometry. After studying several doping levels, the key role of the Ho-ion concentration for the output characteristics was established and analyzed.
As for the pulsed laser operation, the Ho thin-disk laser has also been passively Q-switched with a GaSb-based quantum-well semiconductor saturable absorber mirror (SESAM). This laser showed a conversion efficiency with respect to the CW mode as high as 93%.
The results achieved in this project related exclusively to the development of the Ho:KYW thin-disk laser have been or will be disseminated in the form of scientific publications (below) and parallel presentations at conferences.

Scientific publications:
X.Mateos S. Lamrini, K. Scholle, P. Fuhrberg, S. Vatnik, P. Loiko, I. Vedin, M. Aguiló, F. Díaz, U. Griebner, and V. Petrov. Holmium thin-disk laser based on Ho:KY(WO4)2 / KY(WO4)2 epitaxy with 60% slope efficiency and simplified pump geometry. Submitted to Optics Letters.
X. Mateos, P. Loiko, S. Lamrini, K. Scholle, P. Fuhrberg, S. Vatnik, I. Vedin, M. Aguiló, F. Díaz, U. Griebner, and V. Petrov. Power scaling and thermo-optics of Ho:KY(WO4)2 thin-disk lasers: Effect of Ho3+ concentration. To be submitted to Optical Materials Express
X. Mateos, P. Loiko, S. Lamrini, K. Scholle, P. Fuhrberg, S. Suomalainen, A. Härkönen, M. Guina, S. Vatnik, I. Vedin, M. Aguiló, F. Díaz, Y. Wang, U. Griebner, and V. Petrov. Ho:KY(WO4)2 thin-disk laser passively Q-switched with a GaSb-based SESAM. To be submitted to Optics Letters.
The results obtained in the present project highlight two major achievements:
The first one is the proof of the suitability of the studied materials (i.e. Ho:KYW epitaxial layers on KYW substrates) for the thin-disk geometry due to advantageous spectroscopic properties.
As a second achievement, this laser shows great potential to open new market opportunities, especially for medium output powers needed in many applications. This is due to a great extent to the very simple pump geometry, making this laser very compact and thus attractive to companies for commercialization. Indeed, LISA Laser Products OHG, one of the world leading manufacturers of 2-µm lasers, where the secondment of the researcher took place, has shown enormous interest in the results achieved and wishes continuation of the collaboration for further improvement and development.
Besides the impact of the scientific results, the educational aspect for the professional career of the fellowship holder is very important. The experience gained in industrial environment related to the activity carried out during the secondment at LISA Laser Products OHG was unique. He acquired new know-how concerning high power 2-µm lasers and their real world application to medical therapies. He learnt about different aspects of engineering and quality management for medical devices, including requirements for regulatory purposes. All this information and skills are far from being taught in universities.
The supervision of a Master student was very motivating and was an excellent opportunity to learn management of human resources contributing to the future professional career where more people are expected to be supervised, including PhD students.
As for managing of projects, the researcher had the chance to apply for a Laserlab-Europe staff exchange project with MBI as sending institution and HILASE, Prague, Czech Republic as host institution. Among the achievements, some have recently been presented at a conference (CLEO Europe) held in June 2017 in Munich.
The researcher was elected to organize the next Advanced Laser Technologies conference, ALT’18 (http://altconference.org/) in Tarragona, Spain. He will be the Programme-Committee co-chair of the ALT’18 conference in September 2018. This has been already confirmed and currently he is involved in the preparation/organization of this event. He was invited to join the Journal of Optical Society of America, JOSA B, Optical Society of America, OSA as a Topical Editor for Lasers and Laser Materials, which he accepted and he was also invited and participated in the organisation of the Europhoton Conference in 2016 as a member of the Fiber & Waveguide Active Devices Committee.
Ho epitaxy after crystal growth
Ho thin-disk laser