A multicore fiber includes a plurality of unit multicore fibers each including: a plurality of core portions; and a clad portion which is formed in an outer circumference of the core portions and has a refractive index lower than a maximum refractive index of the core portions. The plurality of the core portions have substantially same refractive index profile and different group delays at same wavelength in same propagation mode. The core portions of the multicore fiber are configured so that the core portions of the plurality of the unit multicore fibers are connected in cascade, a maximum value of differential group delays between the core portions of the multicore fiber is smaller than a reduced value of a maximum value of differential group delays between the core portions of each unit multicore fiber as a value in terms of a length of the multicore fiber.

A multicore fiber includes a plurality of unit multicore fibers each including: a plurality of core portions; and a clad portion which is formed in an outer circumference of the core portions and has a refractive index lower than a maximum refractive index of the core portions. The plurality of the core portions have substantially same refractive index profile and different group delays at same wavelength in same propagation mode. The core portions of the multicore fiber are configured so that the core portions of the plurality of the unit multicore fibers are connected in cascade, a maximum value of differential group delays between the core portions of the multicore fiber is smaller than a reduced value of a maximum value of differential group delays between the core portions of each unit multicore fiber as a value in terms of a length of the multicore fiber.

A method of drawing a material into sheet form includes forming a preform comprising at least one material as a large aspect ratio block wherein a first transverse dimension of the preform is much greater than a second transverse dimension substantially perpendicular to the first transverse dimension. A furnace having substantially linearly opposed heating elements one spaced from the other is provided and the heating elements are energized to apply heat to the preform to create a negative thermal gradient from an exterior surface along the first transverse dimension of the preform inward toward a central plane of the preform. The preform is drawn in such a manner that the material substantially maintains its first transverse dimension and deforms across its second transverse dimension.

Provided are a high-efficiency parallel-beam laser optical fiber drawing method and optical fiber, the method including the steps of: S1: providing base planes on the side surfaces of both a gain optical fiber preform and a pump optical fiber preform, inwardly processing the base plane of the gain optical fiber preform to make a plurality of ribs protrude, and inwardly providing a plurality of grooves on the base plane of the pump optical fiber preform; S2: embedding the ribs into the grooves, tapering and fixing one end of the combination of the ribs and the grooves to form a parallel-beam laser optical fiber preform; S3: drawing the parallel-beam laser optical fiber preform into parallel-beam laser optical fibers. The process has high repeatability, and the obtained parallel-beam laser achieves peelability of pump optical fibers in a set area, thus facilitating multi-point pump light injection of parallel-beam laser optical fibers.

A method of drawing a material into sheet form includes forming a preform comprising at least one material as a large aspect ratio block wherein a first transverse dimension of the preform is much greater than a second transverse dimension substantially perpendicular to the first transverse dimension. A furnace having substantially linearly opposed heating elements one spaced from the other is provided and the heating elements are energized to apply heat to the preform to create a negative thermal gradient from an exterior surface along the first transverse dimension of the preform inward toward a central plane of the preform. The preform is drawn in such a manner that the material substantially maintains its first transverse dimension and deforms across its second transverse dimension.