Dynamic range expansion for optical frequency shift detection based on multiple harmonics

Sensors based on optical resonators often have their measurement range limited by their cavity linewidth, particularly in the measurement of time-varying signals. This paper introduces a method for optical frequency shift detection using multiple harmonics to expand the dynamic range of sensors based on optical resonators. The proposed method expands the measurement range of optical frequency shift beyond the cavity linewidth while maintaining measurement accuracy. The theoretical derivation of this method is carried out based on the equation of motion for an optical resonator and the recursive relationship of the Bessel function. Experimental results show that the dynamic range is expanded to 4 times greater than the conventional first harmonic method while still maintaining accuracy. Furthermore, we present an objective analysis of the correlation between the expansion factor of the method and the linewidth and free spectrum of the optical resonator.