Fabrication and THz loss measurements of porous subwavelength fibers using a directional coupler method

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Abstract
We report several strategies for the fabrication of porous subwavelength fibers using low density Polyethylene plastic for low-loss terahertz light transmission applications. We also characterize transmission losses of the fabricated fibers in terahertz using a novel non-destructive directional coupler method. Within this method a second fiber is translated along the length of the test fiber to probe the power attenuation of a guided mode. The method is especially suitable for measuring transmission losses through short fiber segments, a situation in which standard cutback method is especially difficult to perform. We demonstrate experimentally that introduction of porosity into a subwavelength rod fiber, further reduces its transmission loss by as much as a factor of 10. The lowest fiber loss measured in this work is 0.01cm-1 and it is exhibited by the 40% porous subwavelength fiber of diameter 380 μm. For comparison, the loss of a rod-in-the-air subwavelength fiber of a similar diameter was measured to be ~ 0.1cm-1, while the bulk loss of a PE plastic used in the fabrication of such fibers is ≳ 1cm-1. Finally, we present theoretical studies of the optical properties of individual subwavelength fibers and a directional coupler. From these studies we conclude that coupler setup studied in this paper also acts as a low pass filter with a cutoff frequency around 0.3THz. Considering that the spectrum of a terahertz source used in this work falls off rapidly below 0.25THz, the reported loss measurements are, thus, the bolometer averages over the ~ 0.25THz –0.3THz region.

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