Crystals for Diod-Pumped Lasers
Laser Crystals for Diod-Pumped Solid-State Tunable Mid-Infrared Medical Lasers
Tech Area / Field
- PHY-OPL/Optics and Lasers/Physics
3 Approved without Funding
Belorussian State Polytechnic Academy / International Laser Center, Belarus, Minsk
- Scientific-Practical Materials Research Centre NAS of Belarus, Belarus, Minsk
Project summaryThe objectives of the Project are to elaborate the methods of preparation of high optical quality single crystals of ZnSe doped with Cr2+, to characterize their spectroscopic properties and laser performance at different types of pumping including diode-laser pumping, and to develop solid state tunable mid infrared prototype laser for potential medical applications in neurosurgery, ophtalmology, urology, and plastic surgery.
The state of the art in the field of researches. The Tunable mid-infrared lasers emitting near 2.2-2.7 mm are of great interest for medical applications in neurosurgery, ophthalmology, urology, and plastic surgery. Tunability of laser wavelength in this spectral region enables to control over the depth of penetration of laser radiation in tissues due to the overlap with the absorption bands of water. In ophthalmology it permits to exert influence selectively at different tissues of the eyes thus giving the possibility to address a number of diseases which can be treated using laser radiation (not only by laser coagulation). In neurosurgery the use of laser radiation near 2.7 mm can reduce the area of irreversible damages of brain tissues down to a few tens of microns. Transition metal-doped crystals are very attractive for the development of tunable mid-infrared solid state lasers owing to their reliability, simplicity and compactness. A commercial transition metal laser operating in the mid-infrared wavelength region (1.8-2.4 mm) is Co2+:MgF2, but its performance and mode of operation remain limited due to the poor fluorescence quantum efficiency of the Co2+ emitting ion in this material (less than 3% at room temperature). Presently available mid-infrared lasers include lead-salt diode lasers, which produce milliwatt powers and require cryogenic techniques, some rare-earth-ion lasers, whose tunability is limited, and nonlinear-optical devices like Raman shifters, frequency mixers, and optical parametric oscillators, the latter nonlinear-optical schemes remaining too complex for medical applications. Recently mid-infrared laser action near 2.5 mm was reported for Cr-doped ZnS, ZnSe and Cd1-xMnxTe [Refs. 1-3]. Laser demonstrations with Cr2+-doped zinc chalcogenides gave slope efficiencies up to 30% using a Co2+:MgF2 laser pump source  and 60% with CW Tm-laser pumping . A tuning range of about 600 nm was obtained around 2.5 mm with the prediction that it can be extended. Spectroscopic parameters of Cr:ZnSe are very comparable to that found in the case of Ti-sapphire, a well-known laser medium, thus indicating high potential of these crystals as mid-infrared laser materials. In addition, ZnSe host material exhibit useful thermal and mechanical properties (thermal conductivity and thermal shock parameter higher than in YAG).
The impact of the proposed project on the progress in the field of researches. Вoth detailed spectroscopic studies and synthesis of materials by using various crystal-growth techniques are necessary to estimate the amount of intrinsic optical losses and reduce as much as possible what is due to extrinsic reasons such as unwanted impurities and lattice defects. Good reproducibility of high quality crystals with low losses at laser wavelength around 2.5 mm will be obtained. So, the elaboration of the methods of high quality Cr2+:ZnSe crystals preparation will enable to create a compact, efficient, diode pumped continuous wave solid-state laser source tunable between 2.2 and 2.7 mm, since InGaAsP laser diodes emitting around 1.8 mm are now commercially available.
Project teams. International Laser Center of BSPA is specialized in development of new laser materials and lasers. One of the main research activities of this Group is spectroscopic and laser investigations of transition metal and rare-earth-ion (Cr2+, Cr4+, Ni2+, V3+, Co2+, Pr3+, Yb3+, Er3+) doped crystals promising for laser applications including high power visible and IR lasers. A number of new solid-state laser crystals and saturable absorber Q-switches based on Cr4+-, V3+-, Co2+-doped oxide crystals and efficient solid-state diode-pumped lasers based on Yb3+-doped tungstates have been proposed, investigated and developed by this Group [5-8]. The Institute of Solid State and Semiconductor Physics, National Academy of Sciencies of Belarus is material-oriented with good crystal growth facilities including equipment for crystals growth by vapor phase travelling heater and Bridgman methods and for characterization of them by double crystal X-ray diffraction, chemical etching and optical polarization microscopy as well. This team has a great experience in the field of AIIBVI and AIVBVI chalcogenides compounds growth and in preparation of semiconductor structures and IR-devices for special applications based on these structures [9-12].
Currently the equipment for crystal growth have been prepared and the first experimental samples of pure and by diffusion-doping methods doped ZnSe single crystals are obtained in ISSSP. The set up for investigation of Cr:ZnSe laser performance has been created in ILC. Preliminary laser experiments with Cr:ZnSe crystals has been performed, which demonstrate laser slope efficiency exceeding 45% for the best samples.
Expected results of the project:
— preparation methods of high-quality (Figure of Merit higher than 150) Cr2+:ZnSe laser crystals will be elaborated to the end of Task 1;
— comprehensive characterization of spectroscopic and laser properties of this material will be performed to the end of Task 2;
— Cr2+:ZnSe laser materials (more than 5 sets) with high laser efficiency (slope efficiency not less than 50%) will be produced to the end of Task 3;
— diode-pumped solid-state prototype laser tunable between 2.2 and 2.7 mm will be developed, produced and tested for potential applications in medicine to the end of Task 4.
The Cr2+:ZnSe crystals and prototype laser developed in course of the project will be tested and evaluated in independed laboratories including foreign Collaborator.
Application of the project results. The project results will provide an opportunity for production of laser materials and lasers for medical applications in neurosurgery, ophthalmology, urology, and plastic surgery. Such lasers can be produced by laser manufacturing companies in Belarus (BelOMO, Solar TII, Lotis TII, Solar Laser Systems, etc.), and in other countries (Germany, U.K., USA, Japan, etc.).
Meeting ISTC Goals and Objectives
- The project execution could provide 9 weapon scientists and engineers from Belarus (CIS state) an opportunity to redirect their talents to peacefull activities: investigation of materials and lasers for medical applications.
- Integration of scientists from Belarus into the international optical community, especially in Europian scientific community, will be strengthened due to extention of long-term collaboration with Group of Prof. G. Huber from Hamburg University.
- Applied research and technology development for peacefull purposes in field of new types of IR solid state laser materials and efficient diode-pumped lasers for medical purposes will be supported.
- The project will contribute to the solution of national and international technical problem: development of cw laser light sources tunable in the mid-IR spectral range.
- The transition to market-based economy responsive to civil needs will be reinforced.
Scope of Activities. The proposed experimental research and development programme includes the following Tasks:
Task 1. Elaboration of the crystal growth methods. The goal is to elaborate the methods of preparation of Cr2+-doped ZnSe single crystals of high optical quality.
Task 2. Investigation of the spectroscopic properties of synthesized materials. The goal is detailed spectroscopic characterization of the Cr:ZnSe and optimization of the growth conditions and parameters of the materials.
Task 3. Laser experiments. The goal is to characterize laser performance of the Cr:ZnSe crystals with pulsed laser pumping and CW diode laser pumping and to propose suitable pump schemes and cavity configurations for prototype laser.
Task 4. Development of diode-pumped solid state prototype laser tunable in the Mid Infrared. The goal is to develop diode-pumped solid state prototype laser with high slope efficiency (not less than 50%) and broad tuning range (2200-2700 nm).
Role of Foreign Collaborator. Since 1994 year International Laser Center have a fruitful cooperation on the research of new laser materials [6-8] with the Group of Prof. Dr. G. Huber from the Institute of Laser Physics, Hamburg University. This Group is going to collaborate with the project Participants in:
· shared use of some equipment for ESA mesurements and laser experiments;
· testing and evaluation of the laser crystals and laser, developed in course of the project;
· assistance for project participants to join international meetings;
· invitation of Scientific Leader to Hamburg University for information exchange, specific experiments and seminars.
Technical approach and methodology
Task 1. Laser crystals will be prepared by using a two stage process. At the first stage undoped ZnSe single crystals of high optical quality will be grown by sublimation traveling heater method (STHM) without seeding, which is a closed tube physical transport process. At the second stage the introduction of the chromium impurity into the host material will be performed by the solid phase metallic source diffusion process. Advantages of this two stage process are a high quality of the final products and ability to control over impurity doping level by adjustment of the diffusion time and temperature.
Task 2. Spectroscopic measurements will be performed using standard absorption and luminescence technique. ESA measurements will be performed by using a specially dedicated experimental arrangement based on a particular pump-probe double-modulation technique. Experimental equipment of Collaborator will be used for the ESA measurements.
Task 3. Laser experiments will be carried out using experimental arrangement of International Laser Center and purchased for the Project execution. Particular emphasis will be made on laser experiments with longitudinal diode-laser pumping. Some laser experiments will be performed in cooperation with Collaborator using their experimental facilities.
Task 4. The prototype laser is expected to be compact, efficient and robust. The use of newest high-power InGaAsP diode lasers operating around 1800 nm as pump laser sources is planned. Computer simulations of optical pump schemes, laser cavity configurations and output laser parameters will be applied.
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