Abstract
We propose two designs of effectively single mode porous polymer fibers for low-loss guiding of terahertz radiation. First, we present a fiber of several wavelengths in diameter containing an array of sub-wavelength holes separated by sub-wavelength material veins. Second, we detail a large diameter hollow core photonic bandgap Bragg fiber made of solid film layers suspended in air by a network of circular bridges. Numerical simulations of radiation, absorption and bending losses are presented; strategies for the experimental realization of both fibers are suggested. Emphasis is put on the optimization of the fiber geometries to increase the fraction of power guided in the air inside of the fiber, thereby alleviating the effects of material absorption and interaction with the environment. Total fiber loss of less than 10 dB/m, bending radii as tight as 3 cm, and fiber bandwidth of approximately 1 THz is predicted for the porous fibers with sub-wavelength holes. Performance of this fiber type is also compared to that of the equivalent sub-wavelength rod-in-the-air fiber with a conclusion that suggested porous fibers outperform considerably the rod-in-the-air fiber designs. For the porous Bragg fibers total loss of less than 5 dB/m, bending radii as tight as 12 cm, and fiber bandwidth of approximately 0.1 THz are predicted. oupling to the surface states of a multilayer reflector facilitated by the material bridges is determined as primary mechanism responsible for the reduction of the bandwidth of a porous Bragg fiber. In all the simulations, polymer fiber material is assumed to be Teflon with bulk absorption loss of 130 dB/m.
Highlights
Terahertz radiation, with wavelengths from 30 to 3000 microns, has big potential for applications such as biomedical sensing, noninvasive imaging and spectroscopy
In this paper we present two designs of highly porous fibers that rely on two different guiding mechanisms – the total internal reflection (TIR) and photonic bandgap (PBG) guidance
Let us emphasize that a periodic array of holes is not necessary as the guiding mechanism remains total internal reflection, and not the photonic bandgap effect
Summary
With wavelengths from 30 to 3000 microns, has big potential for applications such as biomedical sensing, noninvasive imaging and spectroscopy. PVDF is a semi-crystalline polymer that has many phases and a complicated poling procedure is required for achieving the ferroelectric state Another hollow core design was discussed by Yu et al [17] is of a hollow Bragg fiber where solid layers are separated by air and supported by a network of solid supports, to the air/silica Bragg fibers for the near-IR applications described in [18]. Main disadvantage of rod-in-the-air subwavelength designs is that most of the power is propagated outside of the waveguide core, resulting in strong coupling to the environment, which is typically unwanted in power guiding applications. In this paper we present two designs of highly porous fibers that rely on two different guiding mechanisms – the total internal reflection (TIR) and photonic bandgap (PBG) guidance The geometries of these structures are optimized to increase the fraction of power guided in the air inside of a fiber, thereby reducing the absorption losses and interaction with the environment.
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