Observation and discrimination of the mode patterns in a micron-sized hollow optical fiber and its synthetic measurements: far-field micro-imaging technique

Abstract

We report a far-field micro-imaging technique that is used for the observation and discrimination of the mode patterns in a micronsized hollow optical fiber as well as for the synthetic measurement of the fiber. By using an M-20x microscope objective lens, we obtained images, magnified by a factor of about 460, from the mode patterns at an output end facet of the hollow fiber with relative measurement accuracy better than 3%. This method can be used for clear identification of the mode patterns in the hollow fiber and detailed study of the relationship between the excitation conditions and the excited modes in the hollow fiber. Moreover, it is useful for the measurement of the geometrical sizes of the hollow fiber and fortesting the coupling efficiencies of the core and cladding modes in their mixed mode pattern. In addition, this method can be also used in the generation of a dark hollow laser beam with 10-^m dark-spot size and the measurement of the focused-spot size of a Gaussian laser beam with about 1-µrn diameter.