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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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