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15 May 2012 Applying refractive beam shapers for spectral beam combining with volume Bragg gratings
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The technique of combining the laser beams with proximate wavelengths by a train of volume Bragg gratings operating as narrow band spectral mirrors allows reaching extremely high resulting power. Performance of these volume Bragg gratings in terms of reflectivity, stability of spectral characteristics depends on their temperature, especially on the temperature profile being a result of interaction of the incident or passing through radiation of powerful laser with material of the grating. The most dangerous effect of thermal lensing appears as a result of heating by laser radiation. The temperature profile in a grating is determined by the intensity profile of a laser beam applied, conditions of thermal conductivity, and energy exchange with environment. The Gaussian intensity distribution in typical laser beams leads to higher temperature in the central part of a grating and, hence, causes nonuniform shift of Bragg wavelength across the aperture and thermal lensing. Homogenizing of the temperature profile over the working field of a volume Bragg grating would mitigate radial gradient of temperature and increase brightness of a combined beam. This can be realized through applying the beam shaping optics, for example refractive field mapping beam shapers. They provide high flexibility in building various optical setups due to their unique features: almost lossless intensity profile transformation, providing flattop, super-Gauss or inverse Gauss profiles. Different profile shapes can be achieved with the same beam shaper, saving of the beam consistency, high transmittance and flatness of intensity profile, extended depth of field, capability to adapt to real intensity profiles of TEM00 and multimode laser sources. Combining of the refractive field mapping beam shapers with other optical components, like beam-expanders, relay imaging lenses, anamorphic optics makes it possible to generate the laser spots of necessary shape, size and intensity distribution. This paper describes a comparison of quality (M2) of Gaussian and super-Gaussian beams diffracted and transmitted by reflecting volume Bragg gratings used for spectral beam combining. Both, mathematical modelling of thermal lensing and experimental results with high density spectral beam combining of 150 W laser beams are described. It was found that the use of super-Gaussian beams results in smaller gradient of temperature across the aperture and, therefore, smaller thermal lensing.
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Alexander Laskin, Derrek Drachenberg, Sergiy Mokhov, George Venus, Leonid Glebov, and Vadim Laskin "Applying refractive beam shapers for spectral beam combining with volume Bragg gratings", Proc. SPIE 8433, Laser Sources and Applications, 84330L (15 May 2012);

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