A method of materials processing using an optical beam includes: launching the optical beam into a first length of fiber having a first refractive-index profile (RIP); coupling the optical beam from the first length of fiber into a second length of fiber having a second RIP; modifying one or more beam characteristics of the optical beam in the first length of fiber, in the second length of fiber, or in the first and second lengths of fiber; and/or generating an output beam, having the modified one or more beam characteristics of the optical beam, from the second length of fiber. The first RIP can be the same as or differ from the second RIP. The modifying of the one or more beam characteristics can include changing the one or more beam characteristics from a first state to a second state. The first state can differ from the second state.