Experimental observation of mode evolution in single-mode tapered optical fibers

Step-index single-mode optical fibers were tapered and then cleaved by ion beam milling to allow direct examination of the mode evolution in the taper with a microscope objective and CCD camera. Observation of HE/sub 1m/ modes is reported in tapered fibers cleaved at diameters ranging from 8 to 35 /spl mu/m. Theoretical scalar mode fields are fit to the experimental data and the mode transitions are explained by considering coupled mode theory.