OBSERVATION AND DISCRIMINATION OF THE MODE-PATTERNS IN A MICRON-SIZEDHOLLOW OPTICAL-FIBER AND ITS SYNTHETIC MEASUREMENTS - FAR-FIELD MICROIMAGING TECHNIQUE

We report a far-field micro-imaging technique that is used for the obs ervation and discrimination of the mode patterns in a micron-sized hol low optical fiber as well as for the synthetic measurement of the fibe r. By using an M-20x microscope objective lens, we obtained images, ma gnified by a factor of about 460, from the mode patterns at an output end facet of the hollow fiber with relative measurement accuracy bette r than 3%, This method can be used for clear identification of the mod e 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 s izes of the hollow fiber and for testing 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-mu m dark-spot size and the measurement of the focused-sp ot size of a Gaussian laser beam with about 1-mu m diameter. (C) 1998 Society of Photo-Optical Instrumentation Engineers.