Single mode beam

An optical fiber structure, comprising:

• a first length of fiber having a first input end configured to receive a single mode input beam, the first length of fiber having a first refractive index profile (RIP) defined by a centrally located guiding region and an annular region coaxially encompassing the centrally located guiding region, the centrally located guiding region having a first diameter and a first index of refraction, and the annular region having a second index of refraction that is less than the first index of refraction;
• and
• a second length of fiber having a second input end coupled to an output end of the first length of fiber, the second length of fiber having a second RIP defined by a centrally located anti-guiding core and an annular guiding region coaxially encompassing the centrally located anti-guiding core, the centrally located anti-guiding core having a second diameter and a third index of refraction, the annular guiding region having a third diameter and a fourth index of refraction that is greater than the third index of refraction, the second diameter being less than or equal to half of the first diameter, and the third diameter being less than or equal to two times the first diameter,
• wherein the centrally located anti-guiding core is sized to support propagation of a single mode, saddle shaped output beam through the second length of fiber to a second output end of the second length of fiber.