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Why is there no fibre for microwave (or other) frequencies?

  1. Jan 16, 2013 #1
    Optical fibre communication depends on total internal reflection at the core-cladding boundary. Why not use the same principle for other portions of the spectrum? After all, total internal reflection occurs at all frequencies.
     
  2. jcsd
  3. Jan 16, 2013 #2
    If I remember correctly your typical rectangular or cylindrical waveguide for microwave frequencies operates on the same principles as optical fibers.
     
  4. Jan 16, 2013 #3
    No, the typical rectangular and cylindrical (metallic) waveguides do not work on total internal reflection; they depend on ordinary reflection from a conducting surface.
     
  5. Jan 16, 2013 #4

    dlgoff

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    They are used for microwave frequencies.

    Dielectric rods for microwaves
     
  6. Jan 16, 2013 #5
    In the 1980s and 1990s I worked analog cellular and there were instances when we modulated the light in a fibre optic directly with the 900 MHz carrier. I wonder if the reason it isn't done more often isn't due to the scarcity of modulators and detectors at frequencies much above 1 GHz.
     
  7. Jan 16, 2013 #6

    nsaspook

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    It's not exactly "fibre for microwave" but a dielectric/photo optic antenna (ADNERF) system is an interesting photonic-assisted RF system.

    http://www.photonics.ucla.edu/comp_pub/micr_phot/MWPS122.pdf [Broken]
     
    Last edited by a moderator: May 6, 2017
  8. Jan 17, 2013 #7
    An optical fiber is actually a waveguide. I'm quoting Cheng's Fundamentals of Engineering Electromagnetics "A type of waveguide of particular importance for optical frequencies consists of a very thin fiber of a dielectric material, typically glass, cladded with a sheath . . . Such optical waveguides are generally called optical fibers."
    The difference is diameter. Optical frequencies have wavelengths in the nm range, which means your waveguide can be roughly in the same order of magnitude (optical fibers are hair-thin). Microwaves have wavelengths typically in the cm range, so your waveguide also has to be in the dimensions. So to answer your Q, you can't use an optical fiber for microwave simply because the wave won't fit; the waveguide's just too small for it.
     
  9. Jan 21, 2013 #8
    At THz frequencies, metals are too lossy to make a line or a guide, and researchers use dieletric guides and antennas.

    Use at lower frequencies is uncommon just because of the dimensions. But "cigar antennas" are dielectric guides, of reduced diameter so they radiate. A few materials (PS, PP, PE, PFTE) might bring lower losses than a waveguide.

    A nice benefit of waveguide and coax cables is that they shield very well, which a dielectric wouldn't naturally. This would require a much marger cladding and some metal on it.
     
  10. Jan 22, 2013 #9

    sophiecentaur

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    As the wavelength is so much bigger than light, the actual diameter of any dielectric guide would mean that you needed a lot more high quality material than is needed for optical fibre. I bet this would make the economics very different -i.e. very expensive.
    I can't see why a long distance microwave guide would actually be attractive. Optical fibre has massive potential for information carrying; would it need to be replaced by a more expensive system? If we are talking about carrying power then DC is better for long distance links (or mains AC). If we are talking about taking power from transmitters to antennae, waveguide losses are not embarrassing, afaik.
     
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