# The Role of the Directivity of Various THz Detectors in Multiplexing Systems
## 4 Discussion  
First, it should be underlined that all designed three-focal-spot diffractive lenses have been positively verified experimentally. The structures focus the THz radiation of a given frequency onto three separate focal spots lying in the same focal plane. The first two structures (\(f=700\) mm, \(\
u=140\) GHz in Figure 8 and \(f=100\) mm, \(\
u=140\) GHz in Figure 9) generate focal spots of unequal intensity. As already explained in [23], it is most likely related to the 3D model rendering method. In this case, pixels of the 2D bitmap obtained in the simulations have been extended and interpolated to form a popular triangle mesh. The last DOE (\(f=100\) mm, \(\
u=260\) GHz in Figure 10) was manufactured with the new method, rendering the 3D model as a set of cuboids, which complies more closely with the simulated bitmap. The experimental results have shown that this method allows us to obtain three well-defined focal spots of almost equal intensities. The issue lies, however, in the experimental validation of the structures and the characterization of the focal spot distribution. Commonly used THz detectors, equipped with the horn antenna or a silicon hemisphere, have a rather narrow angular acceptance band of the incoming radiation (16\({}^{\circ}\) and 12\({}^{\circ}\), respectively). Thus, obtaining reliable results would require the whole DOE to be placed within the acceptance cone of the particular detector. This in turn means that the ratio of the diameter of the structure and/or the shift of the focal spots to the focal length should be sufficiently small. In fact, this happens for the \(f=700\) mm structure, where the redirection angles are as low as 2\({}^{\circ}\). The experiment has shown that, in this case (Figure 8), the influence of the illumination angle is relatively insignificant. A setup that is over 70 cm long, although very convenient during laboratory experiments, most probably shall be too bulky for practical applications. Therefore, we also designed DOEs with a focal length \(f=100\) mm for much more compact setups. In these projects, the proper separation of the focal spots could have been realized only at the cost of much higher redirection angles. Moreover, from the distance of 100 mm only part of the structure lies within the acceptance cone of the bulky detectors (WR34, WR51 and QOD). This means that moving the detector changes not only the average illumination angle, but also the area of the DOE observed by the detector. The influence of these two effects on the detected focal spots can clearly be seen in Figures 9a,b and 10a,b. Not only do the relative intensities of the focal spots change for different detector positions, but also the maximal intensity value of a particular focal spot is not always registered for the theoretically favored configuration with the detector aiming at the center of the lens from the focal spot position. Therefore, the reliable characterization of the THz diffractive optics with a high \(f\)-number can only be performed with a detector much less sensitive to the direction of the incoming radiation, such as, for example, a field effect transistor with a planar antenna. An experiment has shown that such a detector allows for a much more certain verification of the three-focal-spot structure (Figure 10). The detector used is characterized by a 3 dB angular acceptance band of 69\({}^{\circ}\) and registered an almost uniform focal spot distribution with the maximum intensity deviation between detector configurations equal to 11%.