# A Roadmap for Gigabit to Terabit Optical Wireless Communications Receivers
## 4 Discussion
### Designing Application-Specific SiPMs
At the end of the dead time the digital circuits rapidly reset the APD bias voltage. Since this minimizes the probability that a photon is detected whilst a SPAD is being recharged, it reduces the risk that the SPAD can be paralyzed at high irradiances [33]. Depending upon the application, the digital signal generated by an avalanche event can be processed in one of several different ways. An example of the circuit complexity that can be achieved is a low-light image sensor [34]. In this case, the ancillary circuit associated with each SPAD included a quenching circuit, as well as a nine-bit counter to count the detected photons; however, this might overflow, and so it also contained an additional five-bit latch which, together with the nine-bit latch from the counter, can store a fourteen-bit code that represents the time at which the counter overflows. It also includes a 15-bit multiplexer to connect the contents of these 14 latches and an overflow flag to a shared 15-bit bus. All of this functionality was achieved in a 12.24 \(\upmu\)m pitch. This example system indicated the functionality that can be achieved in an area that is smaller than the area of the microcells in most existing SiPMs; however, the challenge when supporting multi-Gbps OOK data rates is that data have to be obtained from each array element in a small fraction of a nanosecond. Fortunately, the location at which a photon is detected is not important. This means that each SPAD can be allowed to transmit its response to a detected photon as soon as it occurs. Furthermore, the method of combining SPAD outputs should support the simultaneous detection of a few photons and different modulation schemes. One way to accommodate the simultaneous detection of photons would be to generate a narrow output current pulse when an avalanche occurs. In the exemplar system, the quenching and digital logic circuits were manufactured in a 40 nm CMOS process [34]. A 1 \(\upmu\)m-wide transistor manufactured in a 40 nm process can pass a current of approximately 400 \(\upmu\)A [35]. In addition, the maximum frequency at which these transistors can amplify a signal is at least 200 GHz [35]. This suggests that it should be possible to generate 10 ps current pulses when a photon is detected. Furthermore, it should be possible to create transimpedance amplifiers on the same chip that create an analogue output voltage that is proportional to the sum of these short current pulses. Table 4 contains a comparison between a J30020 and a possible stacked SPAD receiver. The two systems are assumed to have the same area. The pitch of the stacked receiver is assumed to be the same as the pitch of the exemplar system [34]. The result is a four-fold increase in the number of microcells. In addition, it is assumed that the dead time of the stacked system is 8 ns. This is the time previously used to suppress afterpulsing [36]. Finally, although it may be possible to create 10 ps output pulses, a conservative estimate of 100 ps is included in the table. If the assumed fast output pulse width can be achieved, then the stacked system would not suffer from ISI caused by the receiver for data rates of less than 10 Gbps. At lower data rates it would therefore require an average of approximately 2.6 photons per bit to achieve a BER of 3.8 \(\times\) 10\({}^{-3}\). A data rate of 10 Gbps would then be achieved when the average irradiance from a 405 nm transmitter is 1.8 mW\({}^{-2}\). This is less than the irradiance that can be obtained from an eye-safe 405 nm transmitter in a typical office [12]. This receiver therefore has the potential to deliver data rates of up to 10 Gbps in a typical office. A potential hurdle to achieving 10 Gbps using 405 nm is the bandwidth of 405 nm transmitters. In this case, it may be prudent to change to 850 nm transmitters designed to deliver data rates of more than 10 Gbps. At 850 nm the SPAD could have a PDE of 30% or more [32]. In this case, the lower energy of 850 nm photons means that 10 Gbps could be achieved at an irradiance of 1.1 mWm\({}^{-2}\).