28 Gbaud PAM4 Real Time Optical Datacom Link up to 40 km (2019)


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 optical link that operated at 28 Gbaud (56 Gbps) error-free over link lengths ranging from 10 m to 40 km with Pulse Amplitude Modulation 4-level (PAM4) signals. The links operated with link margins greater than 10 dB due to the wide optical dynamic range performance which will be evident from the sensitivity plots presented. The link was implemented with a commercial PAM4 transceiver chip with built-in FEC driving a Low Noise Amplifier (LNA) which in turn drove a 1550 nm Externally Modulated Laser (EML). The link could have been implemented with a 1310 nm laser and produced comparable results. The optical signal was transmitted over dispersion compensated fiber, fed to a low-noise 28 GHz Linear InGaAs photoreceiver having variable gain, and routed back to the PAM4 transceiver chip. Real time bit error rate measurements demonstrate the advantage of employing linear receivers having Automatic Gain Control (AGC).


Optical datacom links predominantly utilize NRZ-ASK format, where a binary stream of 1's & 0's are encoded in two analog levels of optical amplitude. 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 data throughput while maintaining the same baud rate. Therefore, 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. This modulation format has been recognized as the path forward to achieve data rate of up to 400 Gbps by the IEEE 400 GbE standard [1, 2].

We have previously demonstrated 28 Gbaud PAM4 link, having a data throughput of 56 Gbps, over 10 km single mode fiber span, which addresses Long Reach (LR) Ethernet applications [3]. New measurements presented here improved upon prior work, and extended the link length to 40 km, thus covering Extended Reach (ER) Ethernet requirements. The same link can support legacy NRZ-ASK modulation format without requiring any hardware changes. This scalability enables interconnections ranging from a few meters to a kilometer within aircraft, ships and datacenters, or when two or more datacenters are connected over many kilometers.


Event: SPIE Optical Engineering + Applications, 2019, San Diego, California, United States


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