# Monolitic Hybrid Transmitter-Receiver Lens for Rotary On-Axis Communications
## 2 Materials and Methods
### Fundamental Concepts
#### 2.1.3 Geometrical Arrangements  
Figure 2 shows three geometrical approaches for rotary OWC. In Figure 2a TX and RX are placed next to each other separated by a spacing \(d\). In Figure 2b, TX and RX are radially separated regarding the optical axis. Third, both elements arranged along the optical axis and the front element are transparent as illustrated in Figure 2c. Approach (a) is used by commercial low data rate IrDA transceivers like Vishays _TFBS4711_[33] but also by a high-speed transceiver demonstrated by [PERSON] et al. [34]. The axis of rotation is placed trough one of the elements or between them. This leads to misaligned FOVs. The designer tries to minimize the spacing \(d\). Next, both FOVs are chosen large enough to ensure a sufficient \(\epsilon_{\text{c}}\). As we learned from Equation (4), \(P_{\text{PD}}\) scales with \(1/\tan\theta_{\text{TX FOV}}^{2}\) and \(1/\sin\theta_{\text{RX FOV}}^{2}\). This penalty is typically significant. A numerical example is given in Chapter 3. This penalty is not present for the radial separation from Figure 2b. The approach directly provides aligned FOVs and a high \(\epsilon_{\text{c}}\). TX can be placed in the center surrounded by RX as shown in Figure 2b or vice-versa. The approach shown in Figure 2c avoids shadowing of the back component by designing the front element transparent. If the front component is TX, it has to have separated emission and absorption bands, i.e., it must exhibit a Stokes shift similar to the fluorescence materials [35]. The issue here is the back plane of the emitter: on the one hand, it has to be reflective to direct the transmitted signal towards the other transceiver. On the other hand, it has to be transparent for the incoming signal. This contradiction does not seem to be easily resolved. Figure 1: Ideal rotary arrangement: transmitter (TX) (blue) and receiver (RX) (green) of a transceiver are placed on the optical axis at the same position. The field of view (FOV) of TX and RX overlap ideally. Figure 2: Schematic illustration of arrangements of separated TX (blue) and RX (green). (**a**) TX and RX are placed next to the rotation axis. The FOV of TX and RX overlap only partially. (**b**) TX and RX are placed at the optical axis and they are separated in radial direction. (**c**) TX and RX are placed in a line along the optical axis and the front element is transparent. Clearly the radial separation from Figure 2b has superior performance over Figure 2a and has no fundamental concept issue like architecture Figure 2c. However, what is the best way of realizing radial separation? Figure 3 illustrates three principles. Designing a transmitter element surrounded by one or more high-speed PDs or vice versa on a single chip, like it is shown in Figure 3a, comes with many design challenges including process compatibility. Thereby, it seems easier to produce the chips separately and stack them afterwards. A small emitter die is bonded onto a large area PD chip as it is depicted in Figure 3b. The emitter is contacted with bonds or directly trough the PD chip. The PD could be separated into multiple parts to ease the contacting and to reduce the transit time of the electrons and holes as they might limit the bandwidth [26]. The main issues of this approach include crosstalk between TX and RX, shadowing of the PD by the emitter and the fact that both chips are custom designs. By using an optical system like it is shown in Figure 3c, conventional emitter and receiver chips can be used. Those dies are placed next to each other and a hybrid TX-RX lens is used to redirect the rays to achieve radial separation. The spatial separation of both chips is favorable to reduce electrical crosstalk. Injection molding allows the fabrication of the lens in high volume [36] and low cost. Ultra-precise drilling and milling [36] is used to produce the mold tool. Since a part of the lens is used to direct the TX rays, the maximum theoretical gain \(g_{\text{max}}\) cannot be reached.