Fiber integrated laser systems are attracting increasing interest. They are indispensable in a diversity of applications from macroscopic industrial material processing to scientific applications like gravitational wave detection and astronomical spectrometer calibration. The advancement of fiber lasers is coupled to the progress in the development of fiber integrated components, especially optical couplers which are capable of multiplexing different wavelengths and transversal modes. Novel optical couplers are enabling new laser architectures which can tackle the needs of demanding applications. We used numerical methods to study light coupling between non identical fibers. Our investigations enabled us to develop novel couplers which represent a significant improvement beyond the state of the art. A first coupler type is consisting of two strongly fused single- or few mode fibers. It targets applications, in which different wavelengths have to be multiplexed or a conversion from a LP01 transversal mode to a higher order mode is desired. A second coupler type consists of weakly fused multimode fibers. Such couplers are widely used in monolithic high power fiber lasers.