DETERMINATION OF MODAL POWER DISTRIBUTION IN GRADED-INDEX OPTICAL-WAVEGUIDES FROM NEAR-FIELD PATTERNS AND ITS APPLICATION TO DIFFERENTIAL-MODE ATTENUATION MEASUREMENT

A technique is introduced that determines power distribution in fibers from the measured near-field pattern, assuming that: (1) the optical power distributes uniformly among degenerated modes with the same propagation constant, (2) enough modes are excited to ensure the validity of calculation by geometrical optics, and (3) the phase of each propagation mode has no correlation. Experiments verifed that the fibers have the function of flattening power distribution among modes with the same propagation constant. This fact shows that assumption (1) does not severely limit the applicability of the technique. Wave optical calculation is done to determine the numbers of modes that must be excited to satisfy assumption (2). As an example of application of the technique, differential mode attenuation of graded-index fibers is determined from longitudinal variation of the measured near-field pattern. © 1979 Optical Society of America.