Far Field Measurement
Measuring the far field of an optical fiber involves quantifying the distribution of light at distances from the fiber’s facet much greater than the fiber’s diameter.
The far field angle Θmax can be calculated from distances B (edge of fiber core to edge of light distribution) and D (fiber facet to measurement screen). Typically, the edge of the light distrubution in the pattern on the measurement screen is defined as when the relative intensity of the light falls below 5%.
Using the far field angle, it is possible to calculate the fiber’s numerical aperture.
A fiber’s far field distribution can be mapped using an array of receptors arranged in a semicircle, centered at the end of the fiber. This way, the amount of light emitted from all angles on a plane can be measured. Normally, the measurement can be assumed to be radially symmetric, but if a three-dimensional map is required, the array can be continuously rotated 180° while taking measurements.
References
O. Ziemann et al., POF Handbook. Berlin, Germany: Springer-Verlag Berlin Heidelberg, 2008.