Near field patterns(NFPs) of a plastic optical fiber (POF) are studied in this paper. NFPs are
found to be inhomogeneous across the cross section of the POF and vary irregularly over short POF
distances. They also vary when light incident angles are varied. The irregularity and inhomogeneity
are found to come not from the irregularity of the POF end surfaces but from the POF bulk property.
They are also found to affect the far field pattern(FFP),which in turn affects the impulse response
waveform and frequency transfer characteristics of the POF.
The use of a photodiode in combination with a POF of a large diameter and large numerical aperture
is also studied in this paper. Frequency transfer function of a POF can be measured precisely only
when all of the exiting lights from the POF are received at a photodiode(PD). Such a condition is
not met for the POF under consideration. POFs of 35m and 10m in length and with light incident
at zero degree both show the best frequency characteristics when the distance between the POF end
surface and the APD is the largest as we have expected. However,POF of 10m in length and light
incident at 25 degrees shows the best frequency characteristics when the distance is small. Both
characteristics are discussed in terms of varying angle distributions of the light that the APD can
accept with the varying distances.