Abstract: The present invention is directed at an all-fiber amplifier and laser which use an active multimode graded index fiber. In the amplifier, the active fiber is a multimode graded-index fiber having an axis and a length L. The length L is longer than the distance an input beam (from an input fiber) requires to relax into a stationary state of propagation along the axis of the active fiber. The active fiber is optically connected to an input and output fibers such that the input beam couples to the active fiber at a certain distance from the axis and that the axis of the output fiber coincides with the axis of active fiber. In a laser configuration, the active fiber has a complex gradient parameter g described by the gradient function:g=g.sub.r -ig.sub.i =.omega./n.sub.0wherein .omega. is the amplification gradient and equals (2n.sub.0 (n.sub.0 -n.sub.cl)).sup.1/2. The parameter g depends upon a propagation distance z and is described by the periodic function:g.sup.2 (z)=g.sup.2 (1+4h cos 2g.sub.r z), h<1g.