# A Roadmap for Gigabit to Terabit Optical Wireless Communications Receivers
## 4 Discussion
### Using Diodes to Add SiPMs  
Operating at OOK data rates whose bit time is significantly shorter than the duration of SiPMs' output pulses incurs a significant power penalty. This suggests that the primary characteristic to consider when selecting a SiPM to use at higher data rates, e.g., OOK rates about 2 Gbps, should be the duration of the fast output pulses. Unfortunately, the results in Table 1 show that these SiPMs have a smaller area and therefore fewer microcells. The results in Figure 8 show that this means that a J30020 is a better choice than a C10010 at eye-safe irradiance; however, it is possible to use a pair of Schottky diodes on the fast output of each SiPM to add their fast output pulses without increasing the width of the output pulses [27]. Figure 9 shows a schematic diagram of this idea, where Skyworks  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Name**} & **Maximum Count** & **Fast Output Pulse Width** & \begin{tabular}{c} **Estimated Equivalent** \\ **Gaussian Fast Output** \\ **Pulse Width (ns)** \\ \end{tabular}  & \ \begin{tabular}{c} **Data Rate** \\ **(i) Gbps** \\ \end{tabular}  & \begin{tabular}{c} **Data Rate** \\ **(ii) Gbps** \\ \end{tabular}  \\ \hline RB10010 & 162.7 & 2.3 & 1.5 & 12.5 & 0.7 \\ RB 10020 & 34.4 & 2.0 & 1.3 & 2.6 & 0.8 \\ RB 10035 & 3.9 & 3.7 & 2.5 & 0.3 & 0.4 \\ \hline C10010 & 261.8 & 0.6 & 0.4 & 20.1 & 2.5 \\ C10020 & 25.6 & 0.6 & 0.4 & 2.0 & 2.5 \\ C10035 & 2.8 & 0.6 & 0.4 & 0.2 & 2.5 \\ C30020 & 217.4 SMS7621 24 GHz Schottky diode pairs are used to combine the outputs of the SiPMs. In this circuit the SiPMs are biased by connecting a bias voltage, Vbias, to their cathode. The fast output of each SiPM is connected to the center of a pair of Schottky diodes, which are forward-biased by the bias source, Vss, such that each diode pair passes approximately 1 mA. An output pulse on one SiPM will cause the current through the associated diode pair to vary. This current then flows on a common line where it is added to the current flowing from other SiPMs. Any variation in this total current is converted into a voltage by the resistor connected to Vss. The high-frequency content of this voltage passes through a capacitor to Fast+, which is the shared output from all of the SiPMs. The potential advantages of using more SiPMs with narrow fast output pulses has been investigated by assuming that it is possible to increase the number of microcells in a C10010 and a C10020 without changing any of the other parameters. The results in Figure 10 confirm that increasing the effective area of a SiPM reduces the irradiance required to support a particular data rate. The results for the combination of nine C10020 SiPMs show that they might be the best choice for data rates up to approximately 3 Gbps; however, their significantly lower maximum count rate then causes a rapid increase in the required irradiance at approximately the data rates in Table 2. In contrast, their large maximum count rates mean that both a J30020 and the combination of nine C10010 SiPMs can operate a data rates well above those in Table 2. The difference now is that an increase in area means that the combination of C10010 SiPMs is a better choice at data rates of approximately 1.5 Gbps, which is close to the data rate at which ISI caused by the fast output pulses is expected to impact the performance of a J30020. Figure 9: Schematic showing a method of combining multiple SiPM fast outputs together using diode pairs [27]. The possible benefits of using even more SiPMs in parallel are shown in Figure 11. This figure confirms that adding more SiPMs in parallel will reduce the irradiance required to support a particular data rate. The ideal conditions assumed in these estimates, for example, the absence of ambient light and an infinite extinction ratio on the transmitter, mean that the same number of photons per bit have to be detected.