Leaky Mode Combs in Tilted Fiber Bragg Grating

Leaky modes in tilted fiber Bragg grating (TFBG) present attractive properties but it has been generally underestimated and ignored. Herein, we systematically investigate the enhancement mechanism and the sensing performance of the enhanced leaky mode resonance (eLMR) combs in `dielectric graphene' integrated TFBG in theory. The investigation is conducted using the Fresnel reflection theory and the effective refractive index method. The graphene induces a highly polarization-dependent influence on the internal reflection and mode coupling of the TFBG, resulting in the s-polarized cladding guided mode being highly lossy and the leaky modes becoming guided. This is verified by the propagation length and extinction ratio. The p-/s-polarized coupling coefficient κ with the core guided mode presents a sudden large increase at the position where the mode transition occurs from the cladding guided modes to the leaky modes in both bare and coated TFBG. But the s-polarized κ is further increased by the graphene. Therefore, the mode coupling associated with the s-polarized guided leaky modes is greatly enhanced whereas it is efficiently suppressed for the lossy cladding guided modes, leading to the eLMR combs in the TFBG. It is found that the sensing performance of the eLMR combs appears a quasi-periodic property, evidenced by two groups of amplitude evolution with the surrounding refractive index (SRI). By choosing the optimal eLMR combs, a quasi-continuous SRI sensing is realized. The results demonstrate that the eLMR combs present highly sensitive performance with the average sensitivity up to 5180.4 dB/RIU and 12227.8 dB/RIU for these two groups respectively. Moreover, the eLMR combs present several advantages including insensitive wavelength shift, high figure of merit and wide application fields. This research extends our knowledge and will serve as a base for future studies on the leaky mode resonance.