Fused fiber couplers

Fused fiber coupler is one of the lowest-priced but with high performance fiber optical devices widely used in optical communication systems. It is also called biconical tapering coupler.

The single mode fiber confines most of optical energy in the fiber core. When the fiber is tapered, its core diameter is reduced. Then the optical energy spreads more and more into the cladding region of the fiber waveguide. If there is another same optical fiber core/cladding close by, the optical energy will be guided into the neighbour fiber core. Hence optical power is coupled into the other fiber.

The same coupling process happens for optical power from the neighbour optical fiber. Depending on the length that those two tapered optical fibers run side by side with each other, optical power couplers into and back from each of those two fibers.

When fabricating an optical fused fiber coupler, a pair of fibers are twisted together (protection jacket removed), heated to the softening point by flame, and slowly pulled. Optical power is fed into one of the fiber, and at the two outputs, the optical power is monitored. The stretch distance, the heated region, the heating time, etc., all contribute to make the coupler with different coupling ratios, bandwidth, excess loss, with (or without) wavelength dependent coupling, etc.