Analysis of Terahertz Waves in Waveguides
Authors
Jensen, Simon Faurholt ; Jørgensen, Kasper Hoffmann
Term
4. term (FYS10)
Education
Publication year
2020
Submitted on
2020-06-03
Pages
65
Abstract
Dette projekt modellerer, hvordan terahertz (THz)-bølger udbreder sig i to almindelige bølgeledere: enkeltmodede trinindeksfibre og slabbølgeleder. Med en kombination af analytiske metoder og numeriske simuleringer opstiller vi den relevante teori og undersøger (i) modeprofiler (felternes form i lederen), (ii) modekobling via evanescent-bølger—svage, hurtigt aftagende felter uden for lederen—samt (iii) transmission og refleksion i flere geometrier. For en luft–silica trinindeksfiber med 200 µm i diameter finder vi, at der kun understøttes én mode for frekvenser under 0,607 THz. For to identiske silica-slabbølgeledere med 200 µm tykkelse spænder koblingslængden (den afstand, hvor effekt overføres mellem lederne) fra nogle hundrede mikrometer til flere meter på grund af markante evanescent-felter. Refleksionen i flere slabbølgeleder-geometrier er lav, typisk under 1 %. Derimod er transmissionen af den guidede mode mellem et bølgelederlag ophængt i luft og et lag med skum i stedet for luft stærkt afhængig af skummets brydningsindeks; det samme gælder i den omvendte retning (skum til luft).
This project models how terahertz (THz) waves propagate in two common waveguides: single-mode step-index fibers and slab waveguides. Using a combination of analytical methods and numerical simulations, we develop the relevant theory and examine (i) mode profiles (the shapes of the guided fields), (ii) mode coupling via evanescent waves—weak, rapidly decaying fields outside the guide—and (iii) transmission and reflection in several geometries. For a 200 µm-diameter air–silica step-index fiber, we find that only a single mode is supported for frequencies below 0.607 THz. For two identical silica slab waveguides of 200 µm thickness, the coupling length (the distance over which power transfers between the guides) ranges from a few hundred micrometres to several meters due to substantial evanescent fields. Across several slab-waveguide geometries, reflections remain low, usually below 1%. In contrast, the transmission of the guided mode between a waveguide layer suspended in air and a layer with foam instead of air depends strongly on the foam’s refractive index; the same sensitivity appears in the reverse direction (foam to air).
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