A stable reproducible optical standard source for measuring multimode optical fiber attenuation is required as recent
round robin measurements of such fibers at several international companies and national standards organizations showed
significant variation when using a source having only the encircled flux in the near field emerging from it defined. The
paper presents and compares the far field modal power distributions for (i) 2 km and 3 km step-index multimode Hard
Plastic Cladding Fibers, HPCF, (SI-MMF) with 200 μm silica core diameter, 0.37 numerical aperture (NA) and polymer
cladding, (ii) a 10 m silica graded-index multimode fiber (GI-MMF) with 50 μm core diameter and 0.2 NA, and (ii) a
near field Encircled Flux Mode Convertor or “modcon”. A free space method for measuring the far field using a Lightemitting
diode (LED) centered at 850 nm wavelength with 40 nm 10 dB-bandwidth and a charge-coupled device (CCD)
camera is compared with a f-theta multi-element lens based far field pattern (FFP) system. Mandrels of different
diameter and different numbers of turns of the fiber around them were used to achieve an equilibrium mode distribution
(EMD) for the GI-MMF. The paper defines encircled angular flux (EAF) as the fraction of the total optical power
radiating from a multimode optical fiber core within a certain solid angle in the far field. The paper calculates the EAF
when the solid angle increases from the far field centroid.
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