# The Role of the Directivity of Various THz Detectors in Multiplexing Systems
## 5 Conclusions  
Foremost, the results of this research are important from the perspective of the characterization methods of the THz optics and especially THz optics with small \(f\)-number values. Utilizing bulky antennas is likely to lead to distorted measurements. Moreover, the experiments have proven that 3D printed DOEs designed using the iterative algorithm and convolution method of light propagation can efficiently redirect THz radiation into three separate focal spots. The modified block-based extrusion method of 3D-modelinghas allowed for significant improvement of the uniformity of the intensity distributions between focal spots. It clearly demonstrates that in the case of more complex diffractive elements with a high variety of pixel height, the application of dedicated methods for the modeling of 3D-shapes from simulated bitmaps becomes necessary. Such structures can be used in future THz telecommunication systems as a way to multiplex signals. It should also be noted that any kind of setup, benefiting from the increment of the information capacity of the communication channel, could be seen as a potential point of application of multi-focal-point lenses.  
The results presented also point out the issue of detector directivity in systems with high deflection angles and a low focal length to diameter ratio. Although proper verification of the beam shapes can only be realized with high-angular-bandwidth receivers, the application of more directive ones is also possible. In such a case, individual adjustment of the detector's placement (including the angle of rotation) is necessary.  
Conceptualization, P.K., A.S. and P.Z.; methodology, P.K., M.W. and P.Z.; software, P.K. and P.Z.; validation, P.K., A.S., M.W. and P.Z.; formal analysis, P.K.; investigation, P.K. and M.W.; resources, M.W. and P.Z.; data curation, P.K. and M.W.; writing--original draft preparation, P.K.; writing--review and editing, A.S., M.W. and P.Z.; visualization, P.K. and M.W.; supervision, A.S. and P.Z.; project administration, A.S. and P.Z.; funding acquisition, A.S. and P.Z. All authors have read and agreed to the published version of the manuscript.  
The research was funded by the National Science Centre, Poland under the OPUS-18 programme (2019/35/B/ST7/03909).  
The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to the qualitative nature and all transcribed data are being kept in a different form.  
The authors would like to thank [PERSON] and [PERSON] from Faculty of Physics, Warsaw University of Technology for the help in the process of designing and manufacturing of the diffractive structures.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.  
The following abbreviations are used in this manuscript:  
DOE: Diffractive Optical Element  
DWL: Design Wavelength  
FET: Field-effect transistor  
QOD: Quasi-Optical Detector