We establish a spectral beam combing structure with two distinct diffraction gratings. A theoretical analysis of the beam quality degradation of this beam combining system is given. A six-channel spectral beam combining experiment is constructed based on conical diffraction structure by transmission and reflection gratings. Over 19 kW combined power is achieved with a combining efficiency of about 92%. The six incident fiber amplifiers, ranging from 1040 nm to 1082 nm, can each deliver a 3.5 kW output power laser with about 60 GHz 3 dB linewidth. Both transmission and reflection diffraction gratings, with different grating periods (1000lines/mm, 1170lines/mm), are employed in this work. The combined beam quality factor is controlled to be M2 similar to 2 by effective dispersion compensation. To the best of our knowledge, this is the first time to demonstrate a high power beam combination by two gratings with different types and periods, which verifies the dispersion capability of this structure and provides an alternative approach for high power beam combining.
