[21a-P6-1~17] 3.9 Terahertz technologies

[21a-P6-3] Use of Overshoots of Signal at Knife Edge Position in Imaging using THz-TDS with Spiral Antennas

〇Osamu Morikawa¹, Kohji Yamamoto², Masahiko Tani², Kazuyoshi Kurihara², Fumiyoshi Kuwashima³ (1.JCGA, 2.Univ. Fukui, 3.FUT)

Keywords:spiral antenna,THz-TDS,overshoot

The signal variation is sometimes not monotonic in the knife edge measurement of the focused THz beam using the THz-TDS system with a fixed time delay. For example, when a metal edge gradually block the THz beam, the signal increase and then decrease and cross the 0-value line and finally converge to 0 (this feature is hereafter referred to as overshoot). The overshoot is often seen if spiral antennas are employed in the THz-TDS system. The overshoot is caused, probably by superposition of components with different frequency and opposite signs. If there is the overshoot, the 10%-90% width of the signal variation caused by a metal edge is expected to be narrower than those of the components the superposition of which causes the overshoot. Therefore the overshoot is preferable for imaging. We obtained a 2-dimensional image of a star-shaped metal pattern under the conditions where there is the overshoot. In the image, the overshoots are observed around the metal edges, which makes the image sharper. Further, we obtained an image of the star-shaped metal pattern with the time delay position where the time-domain signal is 0. The image consists only of the overshoot since each frequency component is not 0 although the sum is 0. In such image, the edge of the metal pattern is positioned between the peak and dip of the overshoot, we can obtain an image of edge lines of metal patterns through a simple process.