Fig. 1. (a) Output response of the optical resonator when the optical frequency changes within its linewidth; (b) output response of the optical resonator when the optical frequency changes beyond its linewidths.

Fig. 2. (a) Experimental system of the multiple harmonics method; (b) tuning factor of PZT to the fiber resonant cavity; (c) output signals of the fiber resonant cavity after different optical frequency shift inputs.

Fig. 3. (a) Variation of the first harmonic amplitude with gradually increasing optical frequency shift input, where Ω₍₁₎ = 197 kHz; amplitude of the first three harmonics after Fourier transform of the output signals with optical frequency shift of (b) 118 kHz; (c) 198 kHz; (d) 354 kHz; (e) 512 kHz.

Fig. 4. (a) Experimental result of the first harmonic method; (b) experimental result of the multiple harmonics method; (c) resolution test of the first harmonic method in its linear range; (d) resolution test of the multiple harmonics method in its linear range.

Fig. 5. (a) Theoretical limitation of dynamic range expansion with the FRR in our experiment; the inset of the figure shows the measurement range of the first harmonic method. (b) The expansion factor varies with the linewidth when the FSR is fixed; the expansion factor varies with the FSR when the linewidth is fixed.