Characterization of hollow-core-metal waveguide using broadband THz time domain spectroscopy for high-pressure and temperature sensor

Terahertz (THz) technology has matured over the past few decades and recently innovative applications beyond spectral sensing and imaging, and industrial quality control are being pursued. Earlier we had proposed a high-pressure-hightemperature (high-PT) THz waveguide sensor for harsh environments using hollow-core-metal-waveguides (HCMWs). The modal response in terms of change in dispersion due to applied pressure and/or temperature is highly sensitive for these structures and could be calibrated directly in the time-domain. Moreover, for sensing purposes one requires very short lengths of HCMWs which could be replaced easily with minimal interference to the sensing system. Towards realizing this sensor, however, one requires systematic study of the loss and dispersion of these structures over a broad frequency range. Here we have characterized HCMWs of Copper and Silver claddings having lengths between 20 to 160 mm with inner (core) diameters of 1.5, 2 and 3 mm using photoconductive antenna-based pulsed THz system with bandwidths up to 5.5 THz. Employing hybrid THz-optics, careful cut-back measurements were performed to obtain a loss ~ 34 dB/m for 2-mm core Silver waveguide, and we found that the coupling efficiency is highest for TE11 mode. Using Short-Time Fourier Transform Analysis, complete modal mapping revealed that this mode is the only principal supported mode over the range of 0.8 to 1.6 THz. The waveguide dispersion of this mode directly relates to the core diameter which in turn, is related to the application of external high-PT and could be calibrated in future over this frequency range for sensing purposes.