Design theory, materials selection, and fabrication of hollow core waveguides for infrared to THz radiation
Creators: name: Bowden, Bradley
Publisher: name: No Publisher Supplied
Year: 2007

description: Hollow core waveguides (HCWs) are comprised of a central hole surrounded by a highly reflective inner wall. The core can be filled with air, inert gas, or vacuum, allowing these waveguides to transmit a broad range of wavelengths with low attenuation. HCWs are of particular interest for the transmission of infrared (IR) to THz radiation, where it is otherwise difficult to find materials that have the optical, thermal, and mechanical properties required for use in solid core optical fibers.Ray optics calculations are used to predict the attenuation of the low-loss Gaussian-like HE11 mode propagating in two types of HCWs: hollow Bragg fibers (HBFs) and metal/dielectric hollow glass waveguides (HGWs). These calculations provide guidance on the materials selection and design of HCWs optimized for CO2 (10.6 μm) IR laser radiation and CO2 pumped CH3OH (119 μm) THz laser radiation.An all-chalcogenide glass HBF is proposed for the delivery of CO2 laser radiation. Such a fiber would combine a high refractive index contrast (ratio of the high to low refractive index) with low materials absorption, characteristics that are critical to the design of a low loss HBF. Ge20Se80 glass (ñλ=10.6 μm = 2.46 + i9.7e-7) is identified as an excellent candidate for the low refractive index composition due to its thermal stability and relatively low refractive index among chalcogenide glasses that transmit 10.6 μm radiation. To identify a high refractive index glass to combine with Ge20Se80, several glass compositions in the Ag-As-Se glass forming system are characterized using FTIR spectroscopy, CO2 laser variable angle reflectometry, and CO2 laser calorimetry. Of the compositions investigated, Ag25As40Se35 glass (ñλ=10.6 μm = 3.10 + i1.7e-6) has the best thermal and optical properties for this application. Ray optics calculations show that a HBF made from alternating layers of Ge20Se80 and Ag25As40Se35 glass could have orders of magnitude lower loss than any IR waveguide that has been demonstrated to date.A metal/polymer coated HGW is proposed for the transmission of THz radiation. Ray optics calculations show that silver (Ag) has the best optical properties for use in this application. In theory, a polystyrene (PS) layer added over the silver coating can reduce the waveguide's attenuation by over an order of magnitude. Ag/PS HGWs are fabricated by extending coating techniques that were originally developed for IR transmitting HCWs. FTIR spectroscopy is used to determine the PS film thicknesses and confirm coating uniformity. Uniform PS films are deposited with thicknesses up to 16.7 μm, which is approximately ten times greater than what had previously been demonstrated using these techniques. The mode structure, attenuation, and coupling loss of Ag/PS HGWs are characterized using a CO2 pumped CH3OH THz laser tuned to emit 119 μm radiation. The best waveguide demonstrated in this study, a 2.2 mm bore diameter Ag/PS HGW with a 8.2 μm PS film thickness, has a loss of 0.95 dB/m and a coupling efficiency of 80 %. These attributes are the best that have been demonstrated for any THz waveguide to date.