Amplifying optical fiber and optical amplifier

An optical amplifier comprising:

• an amplifying optical fiber including:
• a plurality of cores to which an active element is doped;
• a first cladding enclosing the plurality of cores with no gap;
• and
• a second cladding enclosing the first cladding;
• a pumping light source configured to emit pumping light for pumping the active element;
• a plurality of single core optical fibers to which communication light is entered;
• and
• an optical Fan-In configured to enter communication light incident on cores of the plurality of single core optical fibers to an end face of a core of the amplifying optical fiber corresponding to the core and to enter pumping light source emitted from the pumping light to a predetermined position on one end face of the amplifying optical fiber, wherein:
• the plurality of cores is disposed around a center axis of the first cladding in a state in which an inter-core distance between cores adjacent to each other is equal;
• a refractive index of the core is provided higher than a refractive index of the first cladding;
• the refractive index of the first cladding is provided higher than a refractive index of the second cladding;
• and
• an expression below is satisfied:
• 5.8≦Λ/MFD(2λc/(λc+λop))≦8
• where the inter-core distance is defined as Λ, a mode field diameter of the core is defined as MFD, a cutoff wavelength is defined as λc, and a wavelength of communication light incident on the core is defined as λop.