A Design For Measuring The Speed Of Light In Fiber Based On FPGA

Velocity of light, as an important physical quantity, is closely related to many theory and laws of optics and electromagnetics. The light velocity measurement not only puts forward the development of physics, but also promotes the innovation of measurement technology.Optical fiber as a kind of optical waveguide has some advantages such as anti-electromagnetic interference, well stability and low attenuation, which has a wide application in optical fiber sensing technique, photoelectric signal detection, optical fiber communication, etc., so it is important to measure the velocity in optical fiber. In order to accurately measure the velocity in optical fiber, we designed a measurement system, based on Field Programmable Gate Array(FPGA), of light velocity in optical fiber.This paper explains the principle of light propagation in optical fiber and gives the equations of calculation of velocity in optical fiber. Additionally, we have discussed the structural principles and characteristics of light emitting diode (LED ) and photodiode (PD)and presented the method of phase detection. We also have designed a driving circuit of LED,signal regeneration and phase detection based on FPGA. The circuit of phase detection consists of phase, frequency doubling, filling pulse counting, control and display module. The part of filling pulse counting is designed based on input of principle image and Verilog HDL.The function and time sequence of the entire module has been simulated on Quartusll, then two simulated outputs were compared . We have set the measurement system of velocity in optical fiber based on the FPGA chip of EP2C8Q208C8N, Program was downloaded and debuged by it.In the experiment, we have measured and calculated the velocity of light((1.97±0.03)×108m/s) in optical fiber by using the FPGA, relative deviation about 3.3%compared to the nominal value velocity of light, then correlation measurements has been set on the oscilloscope, which has a relative deviation about 16%. The system based on FPGA not only excludes the external environment disturbance with good system stability, but also can realize high-frequency counting with high accuracy of measurement. The program has been coded by Verilog of hardware description language (HDL), and the circuit of phase detection with FPGA has versatility and portability, which can be used in electromagnetic wave detection in practice.