A modal instability of a fiber amplifier may be reduced by coupling, e.g. splicing, a length of passive multimode optical fiber to an active multimode optical fiber of the fiber amplifier. Upon launching light into the passive optical fiber, some higher order transversal modes may be excited in the passive optical fiber. The higher-order modes may interfere with the fundamental mode in the passive multimode optical fiber. However, the intermodal interference of the launched modes does not cause thermal gradients in the passive optical fiber. Upon propagation in the passive multimode optical fiber, the excited optical modes may lose mutual coherence, causing a reduction of contrast of the intermodal interference pattern along the doped core of the active optical fiber, effectively reducing modal instability in the active optical fiber.