A fringe jump counting method for the phase measurement in the HCN laser interferometer on EAST and its FPGA-based implementation

The electron density is usually diagnosed by the laser-aided interferometers. The phase difference signal obtained after phase demodulation is wrapped, which is also called fringe jump. A method has been developed to unwrap the phase difference signal in real-time by using FPGA, specifically designed to handle fringe jumps in the Hydrogen Cyanide (HCN) laser interferometer on the EAST superconducting tokamak. The method is designed for phase demodulator using the Fast Fourier Transform (FFT) method at the front end. The method is better adapted for hardware implementation compared to complex mathematical analysis algorithms, like Field Programmable Gate Array (FPGA). The method has been applied to process the phase measurement results of the HCN laser interferometer on EAST in real-time. Electron density results show a good confidence on the fringe jump unwrapping method. Further possible application on other laser interferometers, like the POlarimeter-INTerferometer (POINT) system on EAST is also discussed.