Flexible terahertz fiber optics with low bend-induced losses

We developed a metal hollow waveguide as a flexible delivery medium of terahertz waves. Theoretical evaluation shows that a metal waveguide mainly supports TE modes and that the TE₁₁ mode shows a high coupling coefficient to linearly polarized light. Also the TE₁₁ mode brings less mode conversion to high-order modes than the TE₀₁, mode giving the lowest loss. We measured transmission losses in the terahertz region of hollow waveguides with inner silver coating; the losses were ∼7.5-8 dB/m at the wavelengths from 190-250 μm for waveguides with an inner diameter of 1 mm. These losses coincide well with theoretical ones, and this shows that in these waveguides, the TE₁₁ mode is dominant when a linearly polarized beam is launched into them. The waveguides are flexible because we use a thin-wall glass capillary as the base tubing. Our experiment revealed low additional losses due to bending even when complicated bends were applied to the waveguides. The metal-coated hollow fiber with an inner diameter of 1 mm is sufficiently small and flexible for use in endoscopic applications.