This paper describes the fabrication process and coupling principle of a single-mode fiber coupler. Precise etching properties of the fibers used are presented. Fiber etching is shown to result in smooth surfaces. Coupling is seen to vary with the refractive index of the material separating the fiber cores. Coupling efficiency is shown to be variable in a controlled and reversible manner after coupler fabrication. For a 3-dB coupling efficiency, <1-dB insertion loss has been obtained.
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