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21815 3rd Dr. SE
Bothell, WA 98021
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FBT Based Components >
Fused Biconical Tapered (FBT) Components - Overviews
In its simplest form, an FBT fiber coupler consists of two optical fibers in which the optical cladding has been fused together. The fusion of two or more fibers into a new wave-guide structures in which several fiber cores share a common optical cladding. Tapering this structure changes the core modes of the fibers, broadening the modal fields in the cladding.
 In the "down" taper direction, modes can escape the cores to become common cladding modes that overlap all the fiber cores present.
In the "up" taper region, the cladding modes are converted back into fiber-core modes. Which particular cores recover power depends on the relative phase between the cladding modes.
  
FBT couplers are fabricated by placing two standard single-mode fibers side by side, twisting them together to bring them into strong contact and thermally fusing (burning) them with flame or heater while elongating (tapering) the fused region.
After the tapering process, the coupler is bonded to a quartz substrate with a special adhesive. The substrate with the coupler is inserted into a metal tube and the ends of the tube are closed with elastic epoxy glue. FBT technique of fabrication has proved to be an effective method for coupler construction because the resultant fused couplers can be potted and mounted in small and rugged packages.
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