Absolute Measurement of Dynamic Low-Finesse Fabry–Perot Cavity Using Phase-Shifting White-Light Interferometry

When the cavity changes during spectrum acquisition process, Doppler phenomena will occur and cause conventional scanning white-light interferometry (WLI) to fail. To absolutely demodulate the cavity length of the dynamic low-finesse Fabry-Perot sensor, a novel laser frequency modulated phase-shifting WLI (PS-WLI) is proposed. Two phase-shifted spectra can be obtained by introducing a fixed phase-shift after each scanning frequency. The phase results containing modulations of frequency scanning and dynamic signals are retrieved based on quadrature demodulation. Although the full-spectrum acquisition frequency of the proposed PS-WLI is only 50 Hz, 0.5~20 kHz vibration signals can be demodulated. It combines the advantages of frequency-switched two-step phase shifting interferometry (PSI) and scanning WLI to achieve absolute measurement with high sampling rate. Numerical simulations and experiments verify its effectiveness.