Faraday Isolator for High Power Lasers
Faraday Isolator for Radiation with Average Power up to 10 kW
Tech Area / Field
- PHY-OPL/Optics and Lasers/Physics
3 Approved without Funding
Russian Academy of Sciences / Institute of Applied Physics, Russia, N. Novgorod reg., N. Novgorod
- VNIIEF, Russia, N. Novgorod reg., Sarov
- Universität Hannover / Institut fuer Atom und Molekillphysik, Germany, Hannover\nLaser Zentrum Hannover e.v., Germany, Hannover\nUniversity of Florida / Department of Physics, USA, FL, Caipesville
Project summaryPolarization optical isolators, based on the use of Faraday effect, are widely used in powerful many-stage solid laser facilities for decoupling of separate stages of facility and prevention of self-excitation.
With the recent increase in average powers of both CW and pulse-repetitive lasers, the study of thermal effects caused by the absorption of laser radiation in the bulk of optical elements of laser systems becomes ever more topical.
The light absorption in Faraday elements generates a temperature distribution which is nonuniform over a transverse cross section. This leads to thermal lens, nonuniform distribution of the angle of rotation of the polarization plane because of the temperature dependence of the Verde constant, and the linear birefringence due to the photoelastic effect. The self-induced depolarization of high-power radiation in the magneto-optical rod was first studied in Refs. [1-4], where it was shown that it is the photoelastic effect that makes the greatest contribution to depolarization, and the effect of the temperature dependence of the Verdet constant may be neglected. In Refs. [5, 6] this theoretical prediction was confirmed in experiment.
For high average power lasers, two novel designs of Faraday isolators were suggested and theoretically investigated for the rod geometry in Ref.. Further experiments [5, 6] confirmed the high efficiency of the novel designs. From Ref. [3-9] it follows that the most efficient and convenient design is the one with two Faraday elements, each rotating the polarization plane at 22.5o, and a reciprocal optical rotator by an angle 67.5î placed between them.
Unlike isolators, the Faraday mirror does not comprise any polarizers and is used not for optical isolation, but, generally, for compensation of birefringence in active elements (AEs) of high-power laser systems. It is clear that if the Faraday mirror itself inserts polarization distortions (depolarization), then the compensation of birefringence in AE will be incomplete. Despite the great similarity between the Faraday mirror and the Faraday isolator, there are two primary differences between them that make it ineffective to use the novel isolator designs for the mirror. First, the isolation is governed only by the depolarization in the second pass through the isolator, whereas in the mirror the polarization distortions are accumulated during the both passes. Second, the radiation that is incident on the isolator is always linearly polarized and, besides, in a certain direction. In contrast, the radiation that is incident on the Faraday mirror has been already depolarized. Therefore, for ideal compensation of depolarization in AE the mirror needs to be rotated during two passes by 90o, not distorting any polarization.
In Ref.  a novel design of the Faraday mirror was suggested, which can partially compensate for the depolarization in magneto-optic elements induced by heating due to laser light absorption. Further experiments  confirmed the efficiency of this technique.
Summarizing these results one can conclude that it is quite possible to construct reliable Faraday devices for radiation with average powers up to 1 kW. Advancing further to the multikilowatt range requires new approaches related to the use of either several thin discs cooled through an optical surface, or slabs and rectangular (elliptical) beams.
The objective of the proposed project is to determine, both numerically and experimentally, the maximum power that the Faraday devices (novel and traditional) can withstand when slabs are used. We will concentrate on two main issues: how the depolarization depends on the geometry of slabs or discs, and what yield in maximum power the novel designs provide in comparison with the previously investigated devices.
The implementation of this project will permit to theoretically calculate and experimentally develop underlying scientific principles of the creation and use of magneto-optic devices in laser systems with multikilowatt powers. The results will help come closer to the development of Faraday isolators and Faraday mirrors for technological lasers. Successful implementation of the project will open up an opportunity for creating inexpensive Faraday devices that will be able to operate at powers up to 10 kW. This will facilitate the enhancement of radiation quality of high-power laser systems used for either scientific or applied (technological) purposes.
A number of tasks should be solved to create Faraday isolators for laser radiation with average power of 10 kW.
Main of these are:
Task 1. Computation and optimization of thermo-optical parameters of magneto-optic medium for rectangular geometry and different heating and cooling conditions.
Task 2. Development and calculation of different slabs-based Faraday isolator designs (with compensation of thermally induced depolarization).
Task 3. Computation and optimization of parameters of magnet system.
Task 4. Experimental investigations of thermal effects in the slabs-based Faraday isolator.
Task 5. Experimental demonstration of the possibility of efficient optical isolation for radiation power up to 10 kW.
The accomplishment of these tasks is the main objective of the Project.
Principal executor of the Project is the Institute of Applied Physics of the Russian Academy of Science (IAP RAS). The IAP RAS cooperators have strong research experience in the area of theoretical and experimental studies of the thermal processes both in active elements of laser amplifiers and in magneto-optic media [1-11]. The IAP has state-of-the-art equipment for numerical modeling, technological processing and production of active elements and magnetic systems, which are needed for Faraday isolators operating at high average powers.
The co-executor of the Project is the RFNC-VNIIEF. This group participates in experimental and numerical investigations of nonlinear wave processes, in design, engineering and fabrication of separate units of experimental setups and prototypes to be created; participates in experimental investigations and analysis of results obtained. The RFNC-VNIIEF cooperators participating in the project have substantial expertise in studies of nonlinear optics and self-action of laser radiation, in the development and creation of high power laser systems with radiation conversion into second harmonic on nonlinear KD*P crystals, and in numerical modeling of wave processes and their experimental study. They are also experienced in the design of opto-electronic and opto-mechanical units of high power laser systems [12-14].
The Project mainly relates to basic research. In addition to the investigations, under this project it is intended to develop an original system of constant magnet arrangement, and to create a Faraday isolator prototype for radiation with average power up to 10 kW.
The Project will help implement a number of main ISTC objectives:
- A large group of scientists and experts previously involved in defense-related activity will reorient their work to peace activity. Twenty-nine arms scientists participate in the Project.
- A large amount of new important scientific information will be obtained. This information will be available for all parties concerned.
- The Project will involve rich experience, methodology and material resources accumulated by participating institutions.
- The Project will facilitate integration of Russian scientists into the international scientific community.
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