A pair of small diameter optical fibers are arranged in a side-by-side configuration, the first fiber providing a passive guide for pump light, and the second fiber formed of a material, such as Nd:YAG which has amplifying properties at the signal frequency. The signal to be amplified propagates through the second fiber from the active material, resulting in amplification of the signal. The first fiber is pumped from one or both ends, utilizing a cone shaped rod which is optically coupled thereto. A large diameter beam of collimated pump light enters the cone shaped rod, where total internal reflections on the cone walls compress the beam to a small, fiber-size diameter for absorption by the active fiber. The light input to the cone-shaped rod is produced by plural, high power laser diodes, and is collimated by microlenses. By utilizing the cone-shaped rod to couple light from the high power diode lasers to the pump fiber, high pump power densities can be achieved, yielding increased amplification.