Tops of optical fiber are polished to be detectable, the tops come into plane, which are perpendicular to fiber axis. A
cirque pattern is formed on the out top of fiber by skew rays in the fiber. The pattern is shot with a digital camera. A
bitmap format cirque pattern is obtained. The inside and outside boundaries are taken out with digital image processing
method. Three different pels on the boundaries are picked out to define a circle at every space. The maximum and the
minimum diameter are picked out from many circles acquired to calculate the fiber core no-circularity in according to
definition. A quartz fiber core no-circularity was measured, with numerical aperture 0.22, and diameter 100 micron.
When the optical fiber was placed straight, periphery of the pattern on the out top appear regular denotation, the core no-circularity
is less than 4.0%. But the fiber is made into a spiral cord in diameter 20cm, the periphery of the pattern is
smooth, measured the no-circularity is about 1.7%, the result is identical to which obtained by microscope measurement.
The result indicates that no-circularity measured is almost independent of wavelength of laser with modes being