Fig. 1. Schematic diagram of the process of mode-selecting by the FBG and the subring compound cavity filter

Fig. 2. Schematic diagram of the optical Vernier effect principle^[10-11]

Fig. 3. Schematic diagram of the DCDR-CRC basedon two symmetric 3 × 3 optical couplers

Fig. 4. Transmission spectra of the dual-coupler double ring compound cavity at various cavity length differences，where the insets are the zoomed-in transmission spectra over one FSR of the simulated DCDR

Fig. 5. Schematic diagram of the TCDR-CRC

Fig. 6. Transmission spectra of the TCDR-CRC at various cavity length differences，where the insets are the zoomed-in transmission spectraover one FSR and the highest transmission peak of the simulated TCDR, respectively

Fig. 7. Transmission spectra of the TCDR compound cavity at different coupling ratios with certain cavity length and cavity length difference, where the insets are the zoomed-in transmission spectra over one FSR and the highest transmission peak of the simulated TCDR, respectively

Fig. 8. Transmission spectra of the TCDR compound cavity at different cavity lengths and certain coupling ratio and cavity length difference, where the insets are the zoomed-in transmission spectra over one FSR and the highest transmission peak of the simulated TCDR, respectively