Fiber laser based on 4-core Yb-doped fiber and multimode Bragg grating

We have realized and studied the fiber laser based on 4-core Yb-doped fiber and multimode Bragg grating as the input reflector. The lasing efficiency slope of 55% was measured. An investigation of the output emission has shown that the far-field pattern constitutes an interference picture. That allows us to conclude that the multimode Bragg grating provides phase locking of 4 lasers utilizing the different active cores. A coherent length of laser is longer than possible difference between optical ways of different active cores.