Design and optimization of mechanically down-doped terahertz fiber directional couplers.

We present a thorough practical design optimization of broadband low loss, terahertz (THz) photonic crystal fiber directional couplers in which the two cores are mechanically down-doped with a triangular array of air holes. A figure of merit taking both the 3-dB bandwidth and loss of the coupler into account, is used for optimization of the structure parameters, given by the diameter and pitch of the cladding (d and Λ) and of the core (d(c) and Λc) air-hole structure. The coupler with Λ = 498.7 µm, d = 324.2 µm, Λc = 74.8 µm, and d(c) = 32.5 µm is found to have the best performance at a center frequency of 1THz, with a bandwidth of 0.25 THz and a total device loss of 9.2 dB. The robustness of the optimum coupler to structural changes is investigated.