Reflections from Polished Single Mode Fiber Ends

When the end of a single mode fiber is polished and placed in an index-matching liquid, a reflection will occur. Its amplitude depends on two factors: (1) the difference between the liquid's refractive index and that of the fiber, and (2) the polishing procedure used. By proper choice of both, the end-reflection can be reduced to below -60 dB. When two well-polished fiber ends are joined together with a small, liquid-filled gap between them, the resulting reflection will be the coherent addition of the two individual end-reflections. BY adjusting the gap, this reflection can be made greater than or less than the individual end-reflections.