Spectral Beam Combining of Fiber Lasers by Using Reflecting Volume Bragg Gratings

By employing three reflecting volume Bragg gratings, a near-infrared 4-channel spectral-beam-combining system is demonstrated to present 720W combined power with a combining efficiency of 94.7%. The combined laser beam is near-diffraction-limited with a beam factor M-2 similar to 1.54. During this 4-channel beam-combining process, no special active cooling measures are used to evaluate the volume Bragg gratings as combining elements are under the higher power laser operation. Thermal expansion and period distortion are verified in a 2 kW 2-channel beam-combining process, and the heat issue in the transmission case is found to be more remarkable than that in the diffraction case. Transmitted and diffracted beams experience wave-front aberrations with different degrees, thus leading to distinct beam deterioration.