Fiber-optic amplifier

A fiber-optic amplifier comprising:

• an optical gain fiber configured to amplify input light received from a seed laser, the optical gain fiber comprising:
• a first gain section comprising a single-clad gain fiber comprising a first fiber-optic core and a cladding that surrounds the core, wherein the first gain section is configured to:
• receive the seed-laser input light and co-propagating pump light;
• and
• amplify the seed-laser input light as it propagates along the first gain section, wherein the seed-laser input light and the co-propagating pump light are guided by and propagate substantially within the first core and along the first gain section in a same direction;
• a second gain section comprising a multi-clad gain fiber comprising a second fiber-optic core, an inner cladding surrounding the second core, and an outer cladding surrounding the inner cladding, wherein the second gain section is configured to:
• receive the amplified input light from the first gain section;
• receive counter-propagating pump light;
• and
• further amplify the amplified input light as it propagates along the second gain section, wherein the amplified input light is guided by and propagates substantially within the second core, the counter-propagating pump light is guided at least in part by the inner cladding and propagates substantially within the inner cladding and the second core, and the amplified input light and the counter-propagating pump light propagate along the second gain section in opposite directions;
• and
• a fiber-optic splice configured to couple the first gain section to the second gain section, wherein the fiber-optic splice provides a relatively low optical loss for the amplified input light and a relatively high optical loss for light propagating from the second core of the second gain section to the first core of the first gain section;
• a first pump laser diode configured to provide the co-propagating pump light to the first gain section;
• and
• a second pump laser diode configured to provide the counter-propagating pump light to the second gain section.