Terahertz Fibres and Functional FibreI-Based Devices

Hualong Bao

    Research output: Book/ReportPh.D. thesis

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    The area of Terahertz (THz) radiation has been proved to be a very promising utility for a wide range of applications. However, since current THz systems predominantly utilize freespace propagation, the large size and requirement of careful alignment thus increasing the complexity are the drawbacks on using such systems. Consequently, it is in urgent need to develop waveguides/devices, similar with the fiber waveguides in infrared region, in THz region, which holds great promise for driving this technology further. In this thesis, we have investigated several different dielectric waveguides/devices that rely on different waveguiding mechanisms to guide THz radiation.
    We first focus on Photonic bandgap gap (PBG) THz fibers. To overcome the fabrication problems of traditional PBG type fibers, which caused by the imbalance of hole dimensions, we investigate a novel porous-core honeycomb bandgap type THz fiber. The fabrication and experimental characterization of such a PBG THz fiber are also performed. The fiber is made of polymer TOPAS and confirm that it allows to fabricate long lengths of fiber with a near-perfect periodic structure and thus very clear bandgap guidance. The fundamental bandgap at 0.75-1.05 THz is found to have losses lower than 1.5 dB/cm, whereas the loss is below 1.0dB/cm in the reduced bandgap 0.78-1.02 THz. The particular fiber we present has an outer diameter of 3.65 mm, and is thus already flexible. The outer diameter can be further reduced and thus these fibers may also be bent and cleaved.
    We then focus on tube waveguides. Three different methods are used to improve the transmission bandwidth and dispersion properties, while the propagation loss can be kept generally low. The first way is to deliberately introduce high material absorption to the cladding material, thus efficiently removing the interfering fields that bounce through the cladding and back into the core in a traditional low-loss ARROW tube waveguide. The same effect has been obtained by adding a thin layer of a suitable absorber around the tube surface, here exemplified with water. We also designed and demonstrated another kind of tube waveguides consist of a uniform air-core and a cladding layer with tapered thickness. Results show that the same effect of the highly absorbing cladding material tube waveguides can also be obtained, thus breaking the cladding material limitation.
    Finally, we investigate a special design of a broadband THz fiber directional coupler, which uses mechanical down-doping of the two cores. We show how the proposed coupler provides a broad bandwidth with relative low device loss and perform detailed optimizations of the coupler design to maximize bandwidth and minimize loss. Optimum parameters have been found. Moreover, we verify that the optimum coupler is single-moded and we investigate the robustness of its performance to structural changes.
    Original languageEnglish
    Place of PublicationKgs. Lyngby
    PublisherTechnical University of Denmark
    Number of pages124
    Publication statusPublished - 2014


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