THz technology has become increasingly significant in a diverse range of applications such as spectroscopy, imaging and communication as a result of a plethora of significant advances in this field.
Photonic-based THz systems (where the THz signal is generated by illumination of the photoconductive materials in the presence of a strong static electric field) are one of the promising areas for employing the THz technology in industrial application; a compact, robust, stable and versatile THz system can be applied in industrial environments. In the research laboratories, a THz module with the aforementioned features would be helpful in making measurements more reliable and less time consuming. The main challenge in using photoconductive antennas is the difficulty in the excitation of the THz antenna in a free space optical setting; this problem is usually addressed by directly coupling the light on the antenna using optical fibers.
In CW THz systems where two optical laser beams with frequency difference in THz range should be combined and focused on the photoconductive antenna, applying the fiber optics realizes a compact and less complicated system due to the fact that all the required fiber optics components have been developed for both 800nm and 1550nm wavelength. However, utilizing fiber optics in THz pulse systems is more challenging as the dispersion and non-linear effects can change the properties of the high power femtosecond pulse propagating in optical fiber. To address this problem and retrieve the optical pulse after propagating in the fiber, usually a dispersion compensation technique should be applied. One approach is to control optical pulse shape by tuning the laser structure. However, to realize a modular system, which is independent of lasers module, a dispersion compensation unit should be integrated to the system. In 1550nm THz system, an all fiber compensation unit can be realized by using Disperson Compensation Fibers (DCF). On the other hand, the compensation technique usually is implemented in a free beam setup by using a pair of grating structures in 800nm THz systems due to the large amount of dispersion at this wavelength.
In addition to the compensation technique for optical beam propagation in fiber optics, THz fiber-coupled transceiver sensors have been developed to focus the optical beam on the THz antenna structure. In the current developed sensors, either the fiber optics is directly attached to the antenna or the output beam of the fiber is focused on the antenna by a small lens in a compact enclosure. THz systems with fiber-coupled sensors can be robust, reconfigurable, and versatile which are the key requirements for industrial applications.