ABSTRACT
In this Letter, we investigate the transition of the well-known Fabry-Perot (FP) and antiresonant (AR) mechanisms via a single-mode fiber (SMF)-capillary-SMF structure. The critical length for this transition is analytically found as a linear relation with the capillary inner diameter based on the ray optic method, which shows the agreement with both numerical simulations and experiments. Evolutions of the transmission and reflection spectra verify that FP and AR mechanisms are closely related to the critical length. An observed AR envelope modulated by the FP mechanism in the reflection strengthens gradually with the increase of the capillary length, which is expected to be a novel method for potential applications in multi-parameters sensing because of its combined mechanisms. The transition and critical lengths can be also found and explained using the same method in other types of AR fibers or waveguides with a hollow core.
ABSTRACT
In this paper, we demonstrate a cone-shaped inwall coupler for excitation of the whispering-gallery modes (WGMs) of a microsphere resonator. The coupler is composed of a single mode fiber (SMF) and a capillary with an inner diameter of 5 µm. After immersing the capillary front end vertically into Hydrofluoric acid to obtain a cone inside the capillary, light in the SMF couples into the capillary efficiently while the hollow core is wide enough for a microsphere to be inserted. Because the front end face of the capillary acts as a reflector, a Fano resonance with an asymmetric line shape and a Q-factor of 2.57 × 104 is observed in the reflection spectrum using a barium titanite glass microsphere with a diameter of 45 µm. The integrated resonator structure has the advantages such as the reflective type, alignment-free and mechanically robust, making it have great potential in sensing applications and optical switching.
