To inject a signal using an identical multiplication coupler by pumping a lasing material and causing an inversion distribution in a doped material so that a signal is amplified by induction radiation in passing through the doped material thereby realizing symmetrical bi-directional amplification.

A pumping illumination source 42 is coupled with the fiber 12a of a coupler and provides the port A of a multiplication coupler with pumping illumination. A signal to be amplified is coupled with one end of a fiber 12b at port C of the coupler. Pumping illumination from the light source 42 and the signal to be amplified are coupled at port D of the coupler through amplifying function of the coupler. The pair of signal wavelengths are delivered to an ND:YAG crystal 44 to be coupled with the fiber 12b. A signal input at port C is amplified in the ND:YAG crystal 44 and coupled with an optical fiber 46 before being transmitted through an optical fiber system.