Chapter 19 - Liquid-Core Optical Fibers

Abstract

The liquid-core fibers are multimode waveguides and the operation of these waveguides is based on multimode propagation of light within the liquid core, which is contained by a hollow glass or capillary tube used as fiber cladding. Because the liquids used as core materials have a higher refractive index than that of the glass tube, the guided beams propagate by multiple total internal reflections at the core–cladding interface. The liquid-core fibers do not exhibit stress birefringence, which is commonly observed in glass core fibers. This yields polarization-maintaining behavior for straight fibers as long as single-mode operation is sustained. Nonetheless, bends in the fiber can create axes of birefringence, which are orthogonal to each other, so polarization of the guided beam can be adjusted mechanically. The phase shift induced by these axes can be compensated by rotating the plane of curvature, as is commonly done with fiber polarization controllers with rotatable paddles. Liquid-core fibers are fabricated using a hollow glass tube that serves as a cladding. Because of the high numerical aperture and the modal effects observed in these fibers, there are no stringent requirements regarding losses in the glass.