Fiber fuse terminator with optical fiber with a core and a cladding having longitudinal holes therein

A fiber laser comprising:

• a pumping light source;
• a rare-earth doped optical fiber;
• and
• a fiber fuse terminator having an optical fiber which includes a core and a cladding having holes extending in a longitudinal direction thereof,
• wherein:
• a refractive index of the core of the optical fiber is higher than a refractive index of a portion of the cladding excepting portions of the holes;
• when it is assumed that a mode field diameter at a used wavelength of the optical fiber is MFD, and a distance, in a cross section perpendicular to the longitudinal direction of the optical fiber, between a center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a value expressed by 2×Rmin/MFD is no less than 1.2 and no more than 2.1;
• when it is assumed that a width, in a diameter direction, of a region where the holes present in the cladding is W, a value expressed by W/MFD is no less than 0.3;
• and
• when it is assumed that a diameter of the cladding of the optical fiber is Dfiber, W≦0.45×Dfiber is satisfied;
• each end of the optical fiber is fusion-spliced to an optical fiber having no holes, and the fusion-splicing loss per one point thereon is no greater than 0.50 dB;
• a portion of a surface of the optical fiber, excepting a fusion-splice portion between the optical fiber and the optical fiber having no holes and a periphery thereof, is covered with a resin coating;
• and
• the fusion-splice portion and the periphery thereof of the surface of the optical fiber are covered with a flameproof protective layer.