FIBER OPTIC COUPLER AND AMPLIFIER

Disclosed is a fiber amplifier system in which a gain fiber is operatively combined with a fiber optic coupler having first and second coupler optical fibers. The coupler fibers are fused together along a portion of their lengths to form a wavelength dependent coupling region whereby most of the light power of a wavelength .lambda.s couples between them, and most of the light power of a wavelength .lambda.p that is introduced into the first fiber remains in it. The mode field diameter of the first coupler fiber is substantially matched to that of the gain fiber and is smaller than that of the second coupler fiber. One end of the first coupler fiber is spliced to the gain fiber. The difference in mode field diameters between said couplers is sufficient create a difference in coupler fiber propagation constants in the coupling region which degrades the coupling of the .lambda.s signal. To overcome the difference in coupler fiber propagation constants in the coupling region, and thereby increase the coupled signal power to an acceptable level, a fiber characteristic, whose affect on MFD is insignificant, is modified in one of the coupler fibers.