A Study on the Fiber Dispersion Effect for the Generation of Quasi-Sinusoidal Terahertz Modulations on Optical Pulses

This paper provides a comprehensive study of the fiber dispersion effect for the generation of quasi-sinusoidal terahertz modulations on optical pulses, which are especially beneficial for the applications of terahertz photonic generation and low-frequency coherent Raman spectroscopy. Instead of compensating the fiber group-velocity dispersion in a fiber-delivery system, we propose to make a good use of this fiber dispersion effect, so as to simultaneously chirp and beat the optical pulses. We have not only derived the formulas, but also experimentally elucidated the dispersion effect on the generation of quasi-sinusoidal terahertz modulations via different fiber lengths. The maximum modulation frequency on top of an optical pulse was experimentally demonstrated up to 3.75 THz. To show its application, a low-temperature-grown gallium arsenide-based bow-tie photoconductive antenna was excited by the fiber-delivered light source, and the corresponding frequency quasi-continuous-wave terahertz photons were successfully excited. The generated terahertz frequency was linearly tunable from 0.1 to > 2.1 THz. The linewidth and the spectral intensity of the modulated optical pulses, as well as the impulse response and the bandwidth saturation effect of the antenna, were also experimentally and analytically investigated. Because the laser source can be readily decoupled from the terahertz emitting end due to fiber delivery, this optical fiber-delivered configuration possesses the advantages of simplicity and higher flexibility, which are, therefore, suitable for tabletop biomedical applications or even for clinical uses. The dispersion analysis in the present study is also beneficial for the future manipulation of terahertz radiation, such as the direct generation of chirped terahertz waves.