Near Field Measurement
An optical fiber’s near field describes how the light power is distrusted at the facet of the fiber. This is typically measured using an image of the fiber’s facet taken by an array of lenses, or a fiber optic scanning tool shown in the following figure.
Using this tool, the fiber under test is connected to one end, and a small-diameter, singlemode fiber sensor is attached to the other and moved horizontally across the facet of the fiber under test, sampling the relative optical power level at specific increments.
Step index fibers are characterized by a fairly uniform distribution light across the face of the core of the fiber, while graded index fiber exhibits a pattern that increases in intensity toward the center of the core.
O. Ziemann et al., POF Handbook. Berlin, Germany: Springer-Verlag Berlin Heidelberg, 2008.