Fig. 1. Configuration of optical fiber assisted circular thin film waveguide and generation mechanism of thin film waveguide modes. (a) Cross view, optical fiber assisted circular thin film waveguide and refractive index distribution; (b) side view, mode coupling principle of thin film waveguide mode excited by TFBG

Fig. 2. Mode distribution of approximation-based planar waveguide varying with thickness of circular thin film waveguide. (a) Propagation constant of the first 11 modes, inset gives the variation of propagation constant around refractive index of circular thin film waveguide; (b) propagation constant around cutoff region

Fig. 3. Full-vector modes of fiber assisted circular thin film waveguide and mode field distribution varying with thickness of circular thin film waveguide. (a) Effective refractive index of low-order modes; (b) effective refractive index of high-order modes; (c) mode field of TE_0,3, inset shows mode field of the first thin film guided mode; (d) mode field of TE_0,7; (e) mode field of TE_0,46; (f) difference in effective refractive index of adjacent TE or TM modes

Fig. 4. Transmission spectra of TFBG integrated circular thin film waveguide varying with thickness of circular thin film waveguide. (a) Evolution of transmission spectrum; (b) definition of wavelength separation between two adjacent P-and S-polarized resonances; (c) change of resonance wavelength and amplitude

Fig. 5. Enhanced polarization effect of optical fiber induced by circular thin film waveguide. (a) Resonance wavelength of adjacent P- and S-polarized state varying with thickness of circular thin film waveguide, inset shows the selected resonance bands; (b) periodic change of wavelength separation of adjacent P- and S-polarized state