# Total Internal Reflection Lens for Optical Wireless Receivers
## 1 Introduction  
The growth of the mobile data traffic [1, 2] challenges our wireless communication infrastructure. Moreover, new applications such as the tactile Internet [3, 4] demand low-latency data transfer. Spatially well-confined optical wireless communication (OWC) channels are able to mitigate interference issues and allow for real-time data transfer with high reliability. Therefore, for certain applications, OWC can be an alternative to radio frequency-based technologies. Optical wireless transceivers use photodiodes (PDs) such as P-i-N photodiodes (PIN PDs) [5] or avalanche photodiodes (APDs) [6]. To improve range and coverage, additional receiver lenses are used for optical concentration. For the sake of simplicity and cost, simple spherical and aspherical lenses have been used in industry and science [7, 8, 9, 10, 11, 12, 5]. A specific example is a hemispherical lens, which is well suited for large FOVs [13, 14, 15] (pp. 103-104). The restricted shape of simple lenses limits design freedom and concentration ratio. For instance, it is barely possible to get a constant concentration ratio over the FOV with steep FOV edges. Moreover, large lenses are not suited for most practical applications due to their costly production. Freeform lenses have the potential to overcome these limitations of spherical lenses. The concentration ratio of geometrical optics is limited by the conservation of Etendue [16] (pp. 18-22). Because fluorescence concentrators (FLC) can overcome the Etendue-caused concentration limit [17], there is high scientific interest in FLCs for OWC [17, 18, 19, 20, 21, 22]. However, current FLCs limit the modulation bandwidth [23] and the gain for small FOVs. Therefore, FLCs surpass conventional concentrators only for large FOVs and modulation bandwidths well below 100 MHz. Consequently, there is still a need for geometrical ray optics such as high-performance freeform lenses. Different freeform lenses have been studied over the last decades in the context of OWC. The compound parabolic concentrator (CPC) exhibits high efficiency and has been widely employed in science in various configurations [13, 14, 15] (pp. 104-106). A successor of the CPC is the dielectric totally internally reflecting concentrator (DTIRC),that was proposed in 1987 [24]. The main drawback of the CPC and the DTIRC is that their height and volume scale with increasing output aperture and decreasing acceptance angle [15] (pp. 104-122). Therefore, large PDs and small FOVs lead to bulky concentrators. [PERSON] et al. [25] and [PERSON] et al. [26] described catadioptric lenses in 2015 and 2018, respectively. These lenses use at least one surface for refraction and TIR at the same time. Other works dealt with fisheye lenses [27] in 2014 and fly-eye lenses [28] in 2015. Moreover, there were practical and theoretical works on different hybrid transmitter-receiver lenses for low-range OWC [29; 30] in 2018 and 2020, respectively. A TIR concentrator for OWC was proposed in 2019 [31]. However, the investigations were only theoretical and limited to simulations. Although a growing interest in freeform lenses for OWC exists, there is a substantial lack of experimental data. Only a part of the works describes the concentrators in detail and only a few provide an experimental characterization. An experimental characterization is crucial for identifying relevant practical problems. This paper considers freeform TIR lenses for OWC receivers. In this work, a lens prototype is fabricated and experimentally characterized. In contrast to previous works, we design the lens for a flat SMD package and mount it directly on top of it. For the first time, we show practical performance values in the context of OWC, as well as practical challenges and limits of the TIR lens approach. The rest of the paper is organized as follows. Section 2 considers the concept of the TIR lens and the assembly. Section 3 presents the measurement setup and the results from the optical performance measurements and the data transfer experiment.