Theory analysis of mode coupling in tilted long period fiber grating based on the full vector complex coupled mode theory

The mode coupling in the tilted long period fiber grating (TLPG) is analyzed by using the full vector complex coupled mode theory (FV-CCMT). Compared with the non-tilted LPG, new resonance bands corresponding to the LP1m cladding modes are observed in the transmission spectrum of the TLPG. The coupling to LP1m cladding modes is the combination of the coupling to degenerate vector modes TE0m, TM0m and HE2m with different propagation constants. The maximum coupling between LP01 and LP1m modes occurs at a tilt angle of ∼87°. Numerical results show that the state of polarization (SOP) of the input light would induce the shift of the new resonance wavelength. Also, the RI sensitivity of the new resonance wavelength is much higher than that of the normal resonance wavelength corresponding to the LP0m cladding modes. Such TLPG would find application in chemical and biological sensing fields.

► Tilted long period fiber grating is investigated using the full vector complex coupled mode theory.
► New resonance bands corresponding to LP1m are found in tilted long period fiber grating.
► Coupling to LP1m is the combination of the coupling to degenerate modes TE0m, TM0m and HE2m.
► State of polarization of the input light would induce the shift of the new resonance wavelength.
► RI sensitivity of new resonance wavelength is higher than that of normal resonance wavelength.