A method of forming an article, comprising: forming a melt pool; exposing the melt pool to an optical beam comprising at least one beam characteristic; forming a keyhole cavity in the melt pool, the keyhole cavity comprising at least one keyhole cavity property; characterized by modifying the at least one beam characteristic in response to a change in the keyhole cavity property; generating the optical beam in an optical beam delivery device including an optical fiber (100; 200) having a first length (104; 204; 1100; 1200; 1300; 1400; 1406; 1500; 1600) having a first refractive-index profile and a second length (108; 208; 1700; 1800; 1900; 2000; 2100; 2600; 2700; 2800) having a second RIP, wherein the first RIP differs from the second RIP, the second length having two or more confinement regions (216, 218, 220; 1704, 1706, 1708; 1808, 1810, 1812; 1904, 1906, 1908, 1910; 2004, 2006, 2008; 2104, 2106, 2108), the optical beam delivery device further including a perturbation device (110; 210) configured to physically act on the optical fiber (100; 200); launching the optical beam into the first length (104; 204; 1100; 1200; 1300; 1400; 1406; 1500; 1600); and guiding the optical beam into the second length (108; 208; 1700; 1800; 1900; 2000; 2100; 2600; 2700; 2800), wherein the modifying the at least one beam characteristic comprises: activating the perturbation device (110; 210) to modify one or more beam characteristics of the optical beam in one or more of the first length, and the second length, and confining at least a portion of the modified one or more beam characteristics of the optical beam within the two or more confinement regions (216, 218, 220; 1704, 1706, 1708; 1808, 1810, 1812; 1904, 1906, 1908, 1910; 2004, 2006, 2008; 2104, 2106, 2108) of the second length. forming a melt pool; exposing the melt pool to an optical beam comprising at least one beam characteristic; forming a keyhole cavity in the melt pool, the keyhole cavity comprising at least one keyhole cavity property; characterized by modifying the at least one beam characteristic in response to a change in the keyhole cavity property; generating the optical beam in an optical beam delivery device including an optical fiber (100; 200) having a first length (104; 204; 1100; 1200; 1300; 1400; 1406; 1500; 1600) having a first refractive-index profile and a second length (108; 208; 1700; 1800; 1900; 2000; 2100; 2600; 2700; 2800) having a second RIP, wherein the first RIP differs from the second RIP, the second length having two or more confinement regions (216, 218, 220; 1704, 1706, 1708; 1808, 1810, 1812; 1904, 1906, 1908, 1910; 2004, 2006, 2008; 2104, 2106, 2108), the optical beam delivery device further including a perturbation device (110; 210) configured to physically act on the optical fiber (100; 200); launching the optical beam into the first length (104; 204; 1100; 1200; 1300; 1400; 1406; 1500; 1600); and guiding the optical beam into the second length (108; 208; 1700; 1800; 1900; 2000; 2100; 2600; 2700; 2800), wherein the modifying the at least one beam characteristic comprises: activating the perturbation device (110; 210) to modify one or more beam characteristics of the optical beam in one or more of the first length, and the second length, and confining at least a portion of the modified one or more beam characteristics of the optical beam within the two or more confinement regions (216, 218, 220; 1704, 1706, 1708; 1808, 1810, 1812; 1904, 1906, 1908, 1910; 2004, 2006, 2008; 2104, 2106, 2108) of the second length. activating the perturbation device (110; 210) to modify one or more beam characteristics of the optical beam in one or more of the first length, and the second length, and confining at least a portion of the modified one or more beam characteristics of the optical beam within the two or more confinement regions (216, 218, 220; 1704, 1706, 1708; 1808, 1810, 1812; 1904, 1906, 1908, 1910; 2004, 2006, 2008; 2104, 2106, 2108) of the second length.