Geometrical Aberration Suppression for Large Aperture Sub-THz Lenses

M Rachon,A Siemion,W Knap,D Coquillat,J Suszek,M Sypek,A Sobczyk,K Liebert,J Bomba

doi:10.1007/s10762-016-0342-1

M Rachon, A Siemion + Show 7 more

Open Access

https://doi.org/10.1007/s10762-016-0342-1

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Abstract

Advanced THz setups require high performance optical elements with large numerical apertures and small focal lengths. This is due to the high absorption of humid air and relatively low efficiency of commercially available detectors. Here, we propose a new type of double-sided sub-THz diffractive optical element with suppressed geometrical aberration for narrowband applications (0.3 THz). One side of the element is designed as thin structure in non-paraxial approach which is the exact method, but only for ideally flat elements. The second side will compensate phase distribution differences between ideal thin structure and real volume one. The computer-aided optimization algorithm is performed to design an additional phase distribution of correcting layer assuming volume designing of the first side of the element. The experimental evaluation of the proposed diffractive component created by 3D printing technique shows almost two times larger performance in comparison with uncorrected basic diffractive lens.

Highlights

THz beam shaping has vast applications in modern technology
In case of using a little bit thicker elements, when the phase retardation is a multiple of 2π for the design wavelength (DWL), a high order kinoform structures are generated that may work in a wide frequency range [8,9,10], so suppressing chromatic aberration in diffractive optical elements (DOEs) is possible
The correcting structure was calculated with new algorithm based on the comparison of the ideal thin structure phase distribution with the phase retardation introduced by real thick element

Summary

Introduction

THz beam shaping has vast applications in modern technology. Depending on the application and the material used for manufacturing, it can be practically realised as refractive elements [1], reflective mirror surfaces [2], diffractive structures [3] or. The first one describes the beam shaping providing the expected amplitude and phase distributions at a particular plane in space [5] It can be often used in THz systems with active illumination. Advanced diffractive optics (applied in THz range) has several advantages, for example, suppressing the geometrical and chromatic aberration [6] Such elements are relatively compact and thin, so the attenuation, the weight and the material cost is not significant [7]. In case of using a little bit thicker elements, when the phase retardation is a multiple of 2π for the design wavelength (DWL), a high order kinoform structures are generated that may work in a wide frequency range [8,9,10], so suppressing chromatic aberration in DOEs is possible. No matter what attenuation of the material we will assume, the errors occurring in the difference of designing thin and thick elements being only phase structures are causing decrease of the efficiency of the designed element

Theoretical Background

Numerical Modelling

Experimental Results

Summary