Dielectric-coated metallic hollow fiber (DMHF) is an increasingly popular fiber for the delivery of terahertz (THz) wave. There is a need for small-diameter fibers in a variety of THz wave applications. Transmission characteristics of DMHF with subwavelength diameter in the THz region are numerically investigated. The effective refractive indices, attenuation constants, and power distributions are presented. The effect of the metal layer on enhancing the modal field confinement is demonstrated by comparing the power distribution of the fiber to that of the polymer tube. Attenuation dependence on the dielectric layer is studied concerning the layer thickness and the refractive index. Both the optimal thickness and the optimal refractive index for the dielectric layer are dependent on the core size. Fiber with a smaller core requires a thicker dielectric layer and lower dielectric refractive index to achieve low attenuation. The results are important to the design of subwavelength diameter DMHF for the transmission of THz waves.
Transmission characteristics at terahertz (THz) frequencies are numerically analyzed for elliptical dielectric-coated
metallic hollow fibers. Effective refractive indices of the two polarizations of the HE11 mode, the modal power fraction
in the air core and the birefringence of the fiber are presented. The impact of the metallic layer on the field confinement
is investigated by comparing the modal profiles of the dielectric-coated metallic hollow fiber (DMHF) to that of the
polymer tube (PT). Effects of dielectric absorption on the transmission properties are demonstrated. Total transmission
loss of about 2 dB/m and birefringence in the order of 10<sup>-2</sup> are predicted. Owing to the high reflectivity of the inner
coatings, more than 99% of the fundamental mode power is confined in the air core.
We report the optimal design for hollow fiber inner-coated with metallic and multi-dielectric layers by using ray-optics
theory. Transmission characteristics of the multilayer hollow fiber are more dependent on the film surface roughness in
infrared region. Comparisons of fibers with smooth and rough films are made and discussed in detail. The optimal design
for film thickness, inner radius, the number of layers and refractive indices is presented. The calculation results are
important for structure design, material selection and further fabrication of metallic multilayer hollow fiber when
considering imperfections in film coating techniques.
Hollow fiber with internal metal and dielectric coating films is one of the promising media for THz
transmission. Although the dielectric layer can effectively reduce the transmission loss, it brings additional
loss due to its absorption. It has been shown in mid-infrared region that the optimum thickness of the
dielectric layer becomes smaller due to absorption. For terahertz hollow fibers, the film thickness of the
dielectric layer become much larger and the transmission characteristics are more dependent on the
dielectric absorption. The influences of dielectric absorption on the structure parameters of the
dielectric-coated metallic hollow fibers are discussed. Calculation results show that the optimum refractive
index of the dielectric layer, which is 1.41 for perfect transparent dielectrics, turns out to be greater. The
absorption tolerance is also investigated considering the factors of inner diameter, the refractive index, and
the transmission wavelength. It is shown that absorption tolerance decreases when the inner diameter
becomes smaller or when the transmission wavelength becomes larger. In extreme cases of small
inner-diameter or large transmission wavelength, the absorption tolerance is not existent. Because the loss
of the dielectric-coated metallic hollow fiber is larger than that of metallic hollow fiber even the dielectric
layer has no absorption. The calculation results are helpful to the structure design and material selection in
the fabrication of terahertz hollow fibers.