Contents 1. Introduction
With the expansion of social networking services, further rapid increases of transmission capacity in data centers are expected. To process large amounts of traffic, high-speed data communication in data center rack units is needed. For this purpose, low-power optical transceivers have been implemented on boards in data center racks, and multi-channel co-packaged optical devices [1] and photonic chiplets [2] have been reported. To increase the bandwidth density of these multi-channel devices, supply voltage terminals common to all channels are needed, since the larger the channels of devices are, the larger the supply voltage terminals are. However, since the terminals common to all channels receive crosstalk noise from adjacent channels, the impedance of the terminals should be extremely low. In particular, decreasing the impedance for the low-frequency range is important because pseudo-random bit sequence (PRBS)-31 signals, which are used for 100GbE [3], have a lower frequency range than low-order PRBS signals, such as PRBS-7, 11, 15, and 23 [4]. To decrease the impedance for the low-frequency range, decoupling capacitors with huge capacitance values are needed between the supply voltage terminals and ground in the transceivers. However, these capacitors take up huge spaces, which makes it difficult to implement them in the transceiver IC. We have reported a 4-ch transmitter that achieved 25-Gbps/ch NRZ PRBS-7 x 4 ch simultaneous error-free transmission by increasing the capacitance value of the decoupling capacitors as much as possible to decrease the impedance of the supply voltage terminals [5]. The transmitter was able to transmit only up to NRZ PRBS-7 because the increase in the capacitance value was not enough to transmit NRZ PRBS-31 signals in 4-ch simultaneous error-free operation.
