High Power Spectral Beam Combining of Fiber Lasers with Ultra High Spectral Density by Thermal Tuning of Volume Bragg Gratings

Lasers that produce 100 kW level diffraction limited power will require beam combining due to fundamental thermal and nonlinear limitations on the power of single aperture lasers. Towards this goal, we present high power, high spectral density beam combining by volume Bragg gratings of five 150 W beams with a spectral separation of 0.25 nm between beams, the narrowest to date for high power. Within 1 nm, 750 W of total power is combined with greater than 90 % efficiency. Combined beam quality is discussed including the effect of unequal individual beam divergences on the combined beam quality. The individual input beams may have unique divergences as they enter the system, and the heated volume Bragg gratings (VBGs) may introduce very slight changes in divergence to each beam. These small differences in beam divergence between the beams will not degrade the M of the individual beams, but the composite M after combination can be adversely affected if the beams do not have equivalent divergence at the output of the system. Tolerances on beam divergence variation are analyzed and discussed. High power beams transmitting through or diffracting from a VBG can experience different distortions resulting from thermal effects induced in the VBGs. Each beam also experiences a different aberration, as no two beams pass through the same number of identical VBGs. These effects are studied with experiment compared to modeling. Possible methods of beam quality improvement are discussed.