56 Gbaud PAM4 Real Time Optical Datacom Link (2020)


John Pertessis, Shubhashish Datta, and Abhay Joshi
Discovery Semiconductors Inc., 119 Silvia Street, Ewing, NJ 08628, USA


We present a Forward Error Correction (FEC) - assisted, 56 Gbaud PAM4 optical link having error free performance with adequate link margin of better than 6 dB. The link was formed by driving a 1550 nm Externally Modulated Laser (EML) with a commercial PAM4 transceiver chip. The optical signal was transmitted over various link lengths of dispersion compensated fiber, fed to a low-noise Linear InGaAs Photoreceiver having Automatic Gain Control (AGC), and routed back to the PAM4 transceiver chip with ADC & DSP capability. Real time bit error rate measurements demonstrate the advantage of employing linear receivers having AGC.


The motivation for utilizing 56 Gbaud PAM4 modulation in optical data links is due to the ever-increasing bandwidth demand in communication networks used within datacenters that span from a few meters to 2 km. Present day networks utilize an NRZ-ASK format also known as binary which is a stream of 1's & 0's. This method has reached its practical limits at 25 Gbps for serial communications due to bandwidth limitations of optical & electronic components. PAM4 modulation doubles the bandwidth of the data throughput while maintaining the same baud rate, which means the bandwidth required for components in optical links does not have to be doubled as well. This makes PAM4 modulation a better format for transmitting data at 50 Gbps or higher in optical data links. Linearity & FEC are the key features for the success of PAM4 data communication and the reason for increased complexity of components. PAM4 modulation is especially useful in upgrading scalable systems [1]. This modulation format has been recognized as the path forward to achieve a data rate up to 400 Gbps by the IEEE GbE standard [2].

We have previously demonstrated FEC assisted 28 Gbaud PAM4 links, having a data throughput of 56 Gbps, over 10 km & 40 km single mode fiber spans, which addresses Long Reach (LR) Ethernet applications [3, 4]. New measurements presented here improved upon prior work by increasing the baud rate to 56 Gbaud with a data throughput up to 112 Gbps. Previously, we presented a paper demonstrating a 56 Gbps PAM4 optical link that operated up to 40 km intended for interconnecting datacenters [4]. Some of the results from that paper, shown in Figure 1, include a sensitivity plot which indicates link margins greater than 10 dB for FEC assisted operation. This is more than adequate for forming optical links in practical applications such as interconnecting datacenters. In this paper, we have doubled the data rate to 112 Gbps for a PAM4 optical link that operated up to 2 km intended for interconnections within datacenters. Additionally, we have developed a photoreceiver which could be used for evaluating PAM4 optical links that operate at wavelengths from 1300 nm to 1600 nm over rates ranging from 25 to 56 Gbaud.


Event: SPIE OPTO, 2020, San Francisco, California, United States


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