205-240 GHz Free-space-to-fiber Mode Adapter with an 80% Mode Conversion Efficiency

In this work, we propose an integrated terahertz mode adapter that couples broadband terahertz radiation from free-space to hollow-core fiber with a high mode conversion efficiency (Gaussian beam-to- ) of up to 80%. The adapter consists of a pyramidal horn antenna, a broadband mode converter, and a conical horn. The simulation results indicate that the mode in the hollow-core fiber can be efficiently excited by the terahertz mode adapter. The terahertz mode adapter successfully achieved a wide operating bandwidth of more than 15.7% ranging from 205 to 240 GHz in our simulation.