Characteristics of embedded-core hollow optical fiber.

We propose a novel embedded-core hollow optical fiber composed of a central air hole, a semi-elliptical core, and an annular cladding. The fiber characteristics are investigated based on the finite element method (FEM), including mode properties, birefringence, confinement loss, evanescent field and bending loss. The results reveal that the embedded-core hollow optical fiber has a non-zero cut-off frequency for the fundamental mode. The birefringence of the hollow optical fiber is insensitive to the size of the central air hole and ultra-sensitive to the thickness of the cladding between the core and the air hole. Both thin cladding between the core and the air hole and small core ellipticity lead to high birefringence. An ultra-low birefringence fiber can be achieved by selecting a proper ellipticity of the core. The embedded-core hollow optical fiber holds a strong evanescent field due to special structure of thin cladding and therefore it is of importance for potential applications such as gas and biochemical sensors. The bending losses are measured experimentally. The bending loss strongly depends on bending orientations of the fiber. The proposed fiber can be used as polarization interference devices if the orientation angle of the fiber core is neither 0° nor 90°.