A High-Precision Method for the Determination of Cavity Length of a Fabry-Perot Interferometer

A novel method to obtain the cavity length of a Fabry-Perot interferometer (FPI) precisely based on the Vernier effect via a virtually variable reference FPI is proposed in this paper. Compared with the traditional method based on the measurement of wavelength spacing between two adjacent peaks in the spectrum of a single FPI, or enlarge the wavelength spacing with the Vernier effect by incorporating a real reference FPI in the sensor part, the present scheme is more flexible and applicable. By scanning the cavity length of the virtual reference FPI digitally, the accurate cavity length of the real FPI can be obtained when the free spectrum range (FSR) of the spectrum envelope of the combined FPIs reaches a maximum. Both theoretical analysis and experimental results show that the proposed scheme is superior to traditional methods, and with the present measurement condition, more than 20 times accuracy improvement for the determination of cavity length can be achieved.