Multilayer Infrared And Terahertz Hollow Fiber

The conventional silica optical fiber has large transmission loss in the mid-infrared, far-infrared, and terahertz wavelength regions because of the material absorption. Dielectric-coated metallic hollow fiber has an excellent performance in these wavelength bands because most energy is delivered in the hollow air core. The" low-loss transmission window can be obtained at a target wavelength by properly selecting the thickness of the dielectric layer. Since1980s, hollow fiber has experienced the development of theoretical analysis and fabrication techniques for metallic hollow fiber, single dielectric-coated hollow fiber, and multilayer hollow fiber.This paper focuses on the development of multilayer hollow fibers. Two model theories have been summarized. For the ray-optics theory, the transmission properties are evaluated for both single-layer and multilayer fiber by using the transmission matrix method. The influence of film surface roughness has been taken into consideration. For the transmission line theory, the transmission loss for HE11mode is calculated for multilayer hollow fiber with even and odd numbers of dielectric layers. The effect of dielectric absorption is introduced in the theoretical analysis.The film surface roughness is one of the imperfections that bring the additional loss. Based on the experimental results, the optimal value for structure and material parameters changes when the film roughness is considered. It is necessary to clarify the optimal design for multilayer fiber with the consideration of film surface roughness. This paper discusses the optimal design for inner radius, number of dielectric layers, dielectric thickness, and the refractive indexes. The roughness tolerance for fiber with various structure parameters is also discussed.The multilayer dielectric-coated metallic hollow fibers have been successfully applied to the near-infrared transmission. It has been a challenge to fabricate multilayer hollow fiber for low-loss transmission in the mid-infrared regions because thick film tends to have rough surface. In this paper, a three-layer SiO2and AgI-coated hollow fibers are fabricated for5.3μm wavelength transmission. Theoretical and experimental investigations were made on the optimal design and liquid-phase fabrication techniques for the multilayer infrared hollow fiber. Detailed discussions are given to precisely control of the layer thicknesses and effective reduction of the film roughness. Measured results show that the fabricated fiber has an obvious band gap effect around5.3μm wavelength.There are some reports on the single-layer hollow fiber for terahertz wave. This paper proposes the application of multilayer fiber for far-infrared and terahertz wave transmission. Same as that in the near infrared regions, the multilayer structure can effectively reduce the transmission loss and bending loss in terahertz region. This paper gives detailed discussion on the transmission property, parameter design, and fabrication method for terahertz multilayer fiber. The effect of dielectric absorption is analyzed. The optimal selection of inner radius, materials, and the number of dielectric layers is provided.