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.
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 10-2 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.
The possibility of spectroscopic gas sensing using infrared hollow fiber as the absorption cell was demonstrated. By
optimizing the coating dielectric material and fiber structure parameters, hollow fiber obtains low-loss property at the
absorption wavelength band of various gases. Using low-loss infrared hollow fiber as the gas absorption cell has the
potential advantages of miniaturization, high sensitivity, small gas sample volume, and thus rapid response. Several
kinds of dielectric-coated silver hollow fibers were fabricated and tested for corrosive gas sensing. The durability of the
fibers, with inner coating film of silver and silver iodide (AgI), cyclic olefin polymer (COP), SiO2, polycarbonate (PC),
as well as Ag tube were discussed experimentally. We used NH3, HCl, and ammonia, hydrochloric acid as the corrosive
gas or liquid. By flowing the corrosive material through the hollow fiber, durability of the fiber on these corrosives were
measured. Experimental results showed that AgI/Ag and COP/Ag hollow fiber have high durability.
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