Analysis of transmission properties of terahertz hollow-core optical fiber by using time-domain spectroscopy and application for remote spectroscopy

The behavior of terahertz pulse propagation in hollow optical fibers is investigated using terahertz time-domain spectroscopy. The transmission loss spectra of hollow optical fibers made of a flexible polycarbonate tube with an inner silver layer are measured at the wavelength range from 0.2 to 3 THz. The spectra of fibers with an inner diameter of 3 mm and length of 42 cm show some interference peaks around 1-2 THz, and it was found that these are due to mode interference between the lowest-order TE11 mode and second-order TM11 mode. The mode mixing is also explained from the results of time-frequency analysis conducted using a short-time Fourier transform, and it is confirmed that the traces of the TE11 and TM11 modes clearly appear at 0 to 10 ps after the first signal's detection. The dispersion properties of the transmission modes are derived from the measured phases of transmitted pulses, and it was found that group velocities in hollow optical fibers decrease in the low-frequency region. The group velocity curve coincides well with the theoretical result of the TE11 mode in low frequencies, suggesting that the TM11 mode has little effect on the propagation constant of hollow optical fibers. Finally, terahertz wave remote spectroscopy was conducted using the hollow optical fiber, and a clear spectrum with absorption peaks of the theophylline at around 1 THz was acquired.