# Off-Axis Diffractive Optics for Compact Terahertz Detection Setup
## 6 Summary  
The off-axis diffractive lens-like structures for THz spectral range have been designed, manufactured and tested. The applicability of the higher-order kinoforms has been shown. They offer a better manufacturing quality at the expense of the increased losses introduced by the structure thickness. The further work aims at solving this issue by changing the used PA12 material for the one with the lower absorption coefficient (paraffin). The paraffin lenses manufactured by filling 3D-printed molds are currently considered to be the most promising option and have shown satisfactory results.  
Figure 8: The dependence of the circularity of the focal spots (orange line) and the beam deflection (purple line) on the focal length of the designed lens. The circularity of the spot is defined as the ratio of its diameters measured along horizontal and vertical axes (a/b). The beam deflection denotes the tangent of the angle between the lens surface and the deflected beam.  
Additional study on the shape of focal spots circularity for different focal lengths has been conducted. Summarizing, the changing point between off and on-axis approaches can be set for the focal length of the structure equal to the shift of the structure from the main optical axis.  
All simulations and experimental evaluation have allowed determination that the smallest focal length of structures is equal to 50 mm for the configuration of the optical setup proposed here for in-reflection skin cancer scanners. At the same time, it needs to be remembered that in the proposed setup the middle of the structure is shifted 60 mm from the main optical axis.  
The analysis of the structures allowed determination of to what extend the setup can be minimized. The off-axis focusing is challenging, as tremendous aberrations are observed. The beam deflection angle for the proposed lens is equal to almost 31\({}^{\circ}\). For the shortest focal length with acceptable distortions of the focal spot (\(f=50\) mm) the mentioned angle even reaches 51\({}^{\circ}\). It is well beyond the limits of paraxial approximation (which can be applied up to approx. 17-18\({}^{\circ}\)). The iterative algorithms proposed in this article allow significant reduction of the undesired aberration effects and obtain higher quality of the off-axis focal spots.  
Conceptualization, P.K., A.S.; methodology, P.K., A.S., software, P.K.; investigation, M.W., P.Z.; writing--original draft, P.K.; writing--review and editing, M.S., A.S.; visualization, M.S., P.K.; supervision and idea, A.S.; project administration, A.S; funding acquisition, A.S. All authors have read and agreed to the published version of the manuscript.  
Research funded by the National Center for Research and Development under the LIDER program (LIDER /11/0036 / L-9/17 / NCBR / 2018).  
Authors would like to thank Orteh Company for providing LS 6.0 software for execution of light propagation simulations according to scalar diffraction theory.  
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:  
\begin{tabular}{l l} DOE & Diffractive optical element \\ HOK & Higher-order kinoform \\ FWHM & Full width at half maximum \\ PA12 & Polyamide 12 \\ \end{tabular}