Near field measurement of fiber mode field diameters: effects of defocusing

An instrument has been built to measure the mode field profile of optical waveguides by the near field method. It incorporates a confocal microscope that allows accurate focusing of the waveguide end. The instrument has been used to measure the effect of defocus on measured fiber mode field diameters (MFD's). The amount of defocus must be less than a few micrometers if errors in the measured MFD are to be kept less than 0.1 /spl mu/m. Other focusing criteria can give errors of 2% or more.<
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