US Patent 7557986 High power fiber amplifier

A high power fiber amplifier including a double clad fiber including a protective outer jacket (41), an outer clad (44), an inner clad (42, 35) and a doped core (43, 34, 32), and a source of pump power coupled to the inner clad through coupling optics (22) and at least one of a side-fiber coupling section and an end-fiber coupling section, wherein the inner clad includes a large diameter core portion (34), operative as a high power amplification stage, capable of absorbing the majority of the pump power, and a small diameter core portion (32), operative as a low power amplification stage, wherein both core portions, pumped through the inner clad (35), are serially connected through an optical interface point (37).