Low-loss single-mode optical couplers

The fabrication of very-low-loss single-mode optical-fibre couplers is described. The biconical fusion method has been used to investigate three types of single-mode fibre (i.e. depressed cladding, matched cladding and raised cladding), and losses of less than 0.05 dB have been obtained. Experimental evidence also shows that power transfer in the biconical fusion coupler cannot be supported by evanescent wave coupling between the cores of the fibres. The fabrication of polished-type couplers is also described for fixed and mechanically tunable output power splitting ratio couplers. Typically losses of less than 0.2 dB are possible.