An apparatus and method that provide optical isolation by permitting substantially all forward-propagating light into a delivery fiber from an optical amplifier and substantially preventing backward-traveling light from the delivery fiber entering the optical amplifier without the use of a conventional optical isolator. Eliminating the isolator improves efficiency and reduces cost. Some embodiments use a delivery fiber having a non-circular core in order to spread a single-mode signal into multiple modes such that any backward-propagating reflection is inhibited from reentering the single-mode amplifier. Some embodiments amplify an optical signal in a gain fiber having an output end, output the forward-propagating amplified signal as a high-brightness optical beam (having a first Rayleigh range) into a removable delivery fiber having a non-circular waveguide, output the amplified signal from a distal end of the delivery fiber, and, without the use of a non-linear optical isolator, inhibit backward-propagating light from re-entering the gain fiber.