Optical fiber amplifier

An optical fiber amplifier comprising a multicore fiber doped with erbium, wherein

• the multicore fiber is helically wound to form a fiber coil,
• the multicore fiber includes, in a cross section intersecting a longitudinal direction of the multicore fiber, a center core located at a center of the cross section and outer cores located around the center core, and
• a minimum angle φ formed by a binormal vector extending in an axial direction of the fiber coil and a vector extending from the center core toward one of the outer cores located outside the center core in a radial direction of the helix is at least 0.3°.