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Composite Materials for Thin-Disk Lasers

#B-1435


Yb-doped Materials for High-Power Femtosecond Thin-Disk Laser

Tech Area / Field

  • PHY-OPL/Optics and Lasers/Physics

Status
3 Approved without Funding

Registration date
26.06.2006

Leading Institute
Belorussian National Technical University, Belarus, Minsk

Supporting institutes

  • Solix, Belarus, Minsk\nScientific-Practical Materials Research Centre NAS of Belarus, Belarus, Minsk

Collaborators

  • Universität Stuttgart / Institut für Strahlwerkzenge, Germany, Stuttgart\nUniversität Hamburg / Institut für Laser Physik, Germany, Hamburg\nTRUMPF Laser GmbH + Co. KG, Germany, Schramberg\nCoherent Inc., USA, NJ, East Hanover

Project summary

The project's objective is to develop Yb-doped composite crystalline materials for high power diode-pumped CW and femtosecond thin-disk lasers for material processing.

The state of the art. Femtosecond technologies have a great potential for practical applications in material processing. One of the most promising approaches to the high-power industrial solid-state ultrafast lasers is the thin-disk laser design proposed by Dr. A. Giesen, Stutgart University.

Crystals doped with trivalent ytterbium ions (Yb3+) are of great interest for ultrafast thin-disk lasers with direct laser diode pumping due to a low quantum defect between pump and laser emission and broad gain bandwidth and intense absorption lines in the area around 980 nm where powerful commercially available InGaAs laser diodes operate. Crystals which allow high doping level of ytterbium without distortion of crystalline structure are of special interest for thin-disk laser applications. Thermo-mechanical and thermo-optical properties of host crystalline matrices also influence sufficiently on of thin disk laser performance.

The highest output powers (up to 60 W) from mode-locked thin-disk lasers were demonstrated with Yb:YAG crystal [E. Innerhofer et. al] which has comparatively high thermal conductivity, however the gain bandwidth of this material limited pulse durations to 700-800 fs, or to 340 fs in a low-power laser. Recently, Yb-doped tungstates such as KYW, KGW and KLuW were shown to be the materials with the highest potential for efficient CW and mode-locked laser operation with diode pumping, including thin-disk configuration. A mode-locked Yb:KYW laser by using a semiconductor saturable absorber mirror (SESAM) as a passive shutter was demonstrated with as much as 24 W output power and 240 fs pulse duration. This approach appears to be power scalable, so that even higher mode-locked powers should be possible by using crystals with better thermo-mechanical properties. In bulk (not thin-disk) femtosecond lasers short pulses of about 100 fs and less were obtained with a number of crystals with broad emission band, such as Yb-doped KGW, GdCOB, BOYS etc. and with Yb:glass. However, these materials have a low thermal conductivity, which severely limits their potential for high power operation. Recently, the first demonstration of a femtosecond laser based on Yb-doped CaF2 was reported. Pulses as short as 150 fs were obtained. The thermal conductivity of CaF2 is practically the same as for YAG crystal.

Very recently groups from BNTU (the Institute for Optical Materials and Technologies), Minsk and University of Hamburg reported independently on crystal growth, spectroscopic properties and efficient CW laser operation of Yb-doped yttrium vanadate crystals with laser diode pumping. The laser related properties of this material are very attractive for generation of sub-100 fs pulses under direct diode-laser pumping with thin disk design. Moreover, YVO4 demonstrates moderate thermo-mechanical properties. Thermal conductivity of yttrium vanadate crystal is lower than in YAG, however approximately 40% higher than in KGW and KYW crystals. GdVO4 crystal is also of interest since its thermal conductivity is even higher than in YVO4 while spectroscopic and laser characteristics of Yb3+in these crystals should be comparable.

Typical thickness of Yb-doped crystalline disk is near 100 m and further reduction of the thickness by forming highly doped crystalline layers on undoped crystal substrates as composite materials is very attractive. It would result in better cooling efficiency of the laser elements also the serious technical problem of handling with such fragile laser disks will be eliminated.

Therefore fabrication technology for the new generation of composite laser materials is of high significance for the development of powerful femtosecond thin-disk lasers for industrial applications.

The impact of the proposed project on the progress in this field.

The project would allow to elaborate fabrication technology and make new efficient laser materials for thin disk-lasers. It will result in more robust design, power enhancement, pulse shortening and laser beam quality improvement of thin-disk lasers. As a result of the succesful project implementation thin-disk lasers with average power directly from the cavity more than 50 W and pulse duration less than 150 fs for industrial applications in precise material processing will be demonstrated.

Competence of the project team in the specified area.

The team from BNTU (Institute for Optical Materials and Technologies) is specialized in the development of new laser materials and saturable absorbers for diode-pumped solid-state lasers including ultrafast lasers. One of the main research activities of this Group is conventional and nonlinear spectroscopy of rare-earth-doped crystals and laser investigation in CW, Q-switching and mode locking regimes. A number of efficient laser materials including well-known Yb:KYW and Yb:KGW laser crystals were invented in this Group. Groups from the SOLIX and the Institute of Solid State and Semiconductor Physics (ISSSP) are material-growth-oriented teams with great experience in the growth of different oxide crystals, including tungstates and vanadates, as well as in the Liquid Phase Epitaxy and Diffusion Bonding Technique.

Expected results and their application.

  • Fabrication technologies of composite materials with high Yb-doping levels on undoped crystal substrates: tungstates (Yb:KYW/KYW and Yb:KLuW/KLuW); vanadates (Yb:YVO/YVO and Yb:GdVO/GdVO) and fluoride (Yb:CaF2/CaF2) by using Liquid Phase Epitaxy and Diffusion Bonding Techniques will be elaborated.
  • Heavily ytterbium doped laser crystals of KYW, KLuW, YVO, GdVO and CaF2 will be grown.
  • Diode-pumped thin-disk ultrafast Yb-lasers with an average output power more than 50W and pulse duration less than 150 fs are expected to be demonstrated.

The project will result in the development of new composite laser materials and new generation of thin-disk lasers with improved characteristics. All of this will promote production of commercially available laser materials with unique properties and laser devices with world record parameters for applications in industry for precise material processing.

Meetings ISTC Goals and Objectives

The project meets the ISTC goals and objectives.

  • The project execution will provide weapon project participants an opportunity to redirect their talents to peaceful activities.
  • Integration of project participants into the international optical community, especially in European scientific community, will be strengthened due to collaboration with European scientists.
  • Applied research and technology development for peaceful purposes in the field of high-power lasers will be supported.
  • The project will contribute to the solution of international technical problem: development of femtosecond solid-state laser for materials processing.
  • The transition to market-based economy responsive to civil needs will be reinforced since the results open new opportunities for companies in area of photonics to arrange mass commercial production of advanced laser materials for public applications.

Scope of activities

The project will be executed during 3 years by 40 scientists and engineers from participating institutions.

The Project activities are pided into four Tasks:


Task 1. Growth of laser single crystals with high concentration of ytterbium ions (ISSSP, SOLIX, BNTU).
Task 2. Synthesis of composite laser materials (ISSSP, SOLIX, BNTU).
Task 3. Investigation of laser related properties of single crystals with high Yb3+ doping level and composite materials (BNTU).
Task 4. Thin-disk laser experiments with single crystals and composite materials (ISSSP, SOLIX, BNTU).

The total Tasks efforts are 8,920 person*days, 1,719 person*days (Task 1), 1,723 person*days (Task 2), 968 person*days (Task 3), 4,510 person*days (Task 4).

Role of foreign collaborators

The project represents an international high-technology collaborative effort: Group of Prof. G.Huber from Institute for Laser Physics of Hamburg University (Germany) will perform low-temperature spectroscopic measurements with Yb-doped crystals; together with the group of Dr. A. Giesen, Institut für Strahlwerkzeuge (IFSW), University of Stuttgart, TRUMPF Laser, Rofin Sinar (Germany) and Coherent Inc. (USA) experiments with CW thin-disk lasers will be carried out; jointly with the group of Prof. U. Keller, Ultrafast Laser Physics Laboratory, Institute of Quantum Electronics, Swiss Federal Institute of Technology (ETH), Zurich, (Switzerland) laser experiments with femtosecond pulse generation from thin-disk lasers will be made. In the course of the project implementation information exchange with the collaborators will be fulfilled. The collaborators will provide comments to annual technical reports, share equipment for joint investigations and test result obtained during the project implementation.

Technical approach and methodology

Undoped and Yb-doped YVO and GdVO crystals will be grown by Czochralski technique. Yb:KYW, Yb:KGW and Yb:KLuW crystals and undoped substrates will be grown by the flux technique, CaF2 crystals will be grown by Bridgman technique. Composite laser materials will be fabricated by Liquid Phase Epitaxy (LPE) method and/or Diffusion Bonding Technique (DBT). Characterization of single laser crystals and composite laser materials will be performed by using conventional optical and spectroscopic methods as well as modern methods of nonlinear optics (Z-scan technique). Semiconductor saturable absorber mirrors (SESAMs) will be used for passive mode locking of a thin disk lasers with Yb-doped materials.


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