The circuit board consists of a simple oscillator, LED transmitter and LED detector. A repeating pulse travels through the plastic fiber cable and is detected. An oscilloscope allows for timing between transmitted and received pulse.

A rather short length of fiber is hooked up first. The electronic delay is determined from the trans/rec pulse timing. The long fiber is then hooked up and timing is determined. Difference is taken between this measurement and the first. The calculation is then finished by including the index of refraction of the fiber (1.5) to arrive at c.

This demo can be found in cabinet G on shelf 2 of the UCSD PUL Demo Room (WLH 2015).