Multimode interference is fairly known from the 1D case of slab waveguides. We present for the first time to our knowledge the reconstruction of the 2D radial symmetric structure of a multicore fiber laser in a multimode fiber. In the concept of multicore fiber, rare earth-doped single mode waveguides (micro cores) are placed on a ring inside a big pump core. The situation of injecting radiation from N incoherent emitting sources into a multimode waveguide is described analytically. Experimental and numerical results for various multimode diameters and fiber lengths dealing with the reconstruction of the injected near-field pattern and the corresponding far-field patterns are presented. We propose that the reconstructed field could be re-injected into the multicore fiber-laser in order to introduce parallel coupling of all emitters. Additionally, using the multimode fiber as a passive element, without re-injection, the on-axis intensity of the multicore laser radiation is significantly increased by a single pass through a multimode fiber with a certain length. This effect takes place without any loss of energy.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.