# Monolitic Hybrid Transmitter-Receiver Lens for Rotary On-Axis Communications
## 1 Introduction  
Reliable, real-time connectivity is the backbone of industrial automation. Data transmission over rotating parts is required in a broad range of applications such as wind turbines [1], industrial communications [2; 3], surveillance radars [4; 5; 6; 7], military [2; 8], aerospace [9] and many more. Table 1 gives an overview of the most important transmission principles used in rotary communication links. Slip rings were widespread in the past. However, due to mechanical contact they suffer wear and tear which limits their durability [1; 2; 5; 6; 9; 10; 11] and thus increases maintenance costs. Precious brush materials and lubricants are used to extend lifetime [12] at the expense of increased system complexity and higher costs. Therefore, contactless data transfer is favored nowadays [6; 10]. Life times of several hundreds of revolutions are common and rotation speeds in the order of magnitude of \(10^{3}\) rpm or even \(10^{4}\) rpm are reached with contactless methods. Capacitive-based near-field transmission links are known as reliable and cost-effective [11]. However, the system proposed by [PERSON] et al. [13] shows that system complexity of modern solutions is clearly not negligible. Data rates of up to several Gbit/s are possible [11; 13]. Practical systems support conventional Gigabit-Ethernet and industrial protocols like ProfiNET and EtherCAT for instance. Thus, devices with data rates ranging from \(~{}500\) kbit/s to \(1\) Gbit/s were developed [13]. The maximum transmission distance is in the range of \(1\) cm. Inductive coupling is mainly employed for power transfer and only rarely used for high-speed data transmission [14; 15]. Several systems using conventional low-power radio-frequency (RF) technologies were proposed [1; 10] with data rates in the range of tens of Mbit/s or below. Standards like 802.11 ac and 802.11 ad might be able to provide data rates in the range of Gbit/s. Their main problem is reliability and robustness in terms of ensuring a bandwidth and low-latency data transfer [2; 3; 13]. Future millimeter-wave based communication [20] standards like IEEE 802.11 ay might even reach data rates in the range of several Gbit/s to several tens of Gbit/s [11]. However, their practicality and cost-effectiveness has to be proven. Highest data rates are reached with fiber optical rotary joints (FORJ). Besides their superior data rate in the range of Gbit/s up to multiple tens of Gbit/s per channel [6; 7; 13], these links provide immunity against RF. Single-fiber systems only consist of an optical transceiver at both sides and optical fibers in-between. However, due to sophisticated mechanical alignment [8], these systems are expensive. Multi-fiber links offer even higher data rates but exhibit a very high complexity [6]. Optical wireless communications (OWC) aim to combine the advantages of rotary FORJ with relaxed mechanical tolerances, reduced system complexity and thus lower costs. Initially only light emitting diode (LED) based systems with data rates in the lower Mbit/s range [2; 3] or laser diode based uni-directional links were demonstrated [21]. [PERSON] et al. [19] introduced a full-duplex link for data rates of up to 10 Gbit/s for on-axis rotary data transfer. Similar to fiber-based communications, data rates are likely to increase in the future. The demonstrated rotation speed from 0 rpm to 1400 rpm [19] was limited by the test equipment. There is no OWC-exclusive factor limiting the speed. The lifetime of OWC links is expected to be similar to other contactless methods, since there are no significant aging effects. The optoelectronic components, i.e., light emitting diodes (LEDs), laser diodes (LDs) and photodiodes (PDs) are known for high reliability and long lifetime [22; 23; 24]. The key element of the transceiver in [19] is a monolithic hybrid lens that acts as transmitter (TX) and receiver (RX) optics in parallel and thereby relaxes the mechanical alignment to several millimeters. The form factor of the transceiver is only 5 mm \(\times\) 5 mm [19].