All-Pass and Add-Drop Microsphere Resonator in a Suspended Dual-Core Hollow Fiber

In this paper, we demonstrate an in-fiber all-pass and add-drop whispering gallery mode microsphere resonator. We fusion splice a section of single mode fiber (SMF) to a section of suspended dual-core hollow fiber (SDCHF), into which a microsphere is then embedded. Most of the input light from the SMF can be coupled into the cores of the SDCHF, with a maximum coupling efficiency obtained when maintaining a balance between the cone-apex angle and the core size of SDCHF in the conical region. The optical spectrum can be clarified by reducing coupling paths between the microsphere and the SDCHF. It is observed that the linewidth of the resonance peak is halved with halved coupling paths in the coupling model. When the microsphere with a suitable diameter is located at a critical position in the SDCHF, the coupler turns from an all-pass into an add-drop coupling structure. The device also demonstrates good stability as a refractive index (RI) sensor with a sensitivity of -386.12 dB/RIU when the RI of the surrounding medium ranges from 1.3333 to 1.3714. This in-fiber resonator device is inherently robust, compact, and has a great potential for sensing applications.