In various examples, a semi-guiding optical fiber includes a core having a high aspect ratio including first and second wide surface interfaces, and first and second narrow edge interfaces. The core has a slow axis parallel to the first and second wide surface interfaces, and a refractive index. The fiber also includes first and second signal claddings positioned in contact with the first and second wide surfaces of the core, respectively. The first and second signal claddings have a spatial refractive index profile having: i) a minimum value at a longitudinal axis of the core, ii) a maximum value substantially equal to the refractive index of the core at the first and second narrow edge interfaces of the core, and iii) intermediate values gradually increasing from the minimum value to the maximum value as distance from the longitudinal axis increases along the slow axis of the fiber.