In multimode fiber optics, a technique used to modify the modal distribution of a propagating optical signal. Note: A cylindrical rod wrap consists of a specified number turns of fiber on a mandrel of specified size, depending on the fiber characteristics and the desired modal distribution. It has application in optical transmission performance tests, to simulate, i.e., establish, equilibrium mode distribution in a launch fiber (a fiber used to inject a test signal in another fiber that is under test. ) If the launch fiber is fully filled ahead of the mandrel wrap, the higher-order modes will be stripped off, leaving only lower-order modes. If the launch fiber is underfilled, e. G. , as a consequence of being energized by a laser diode or edge-emitting LED, there will be a redistribution to higher-order modes until modal equilibrium is reached.