Short-range Optical Communications using 4-PAM

Licentiate thesis, 2017

As the demand for ever higher throughput short-range optical links is growing, research and industry has shown increased interest in multilevel modulation formats, such as the four leveled pulse amplitude modulation, referred to as 4-PAM. As on-off keying persists to be the choice for low latency applications, such as high performance computing, datacenter operators see 4-PAM as the next format to succeed current OOK-based optical interconnects. Throughput can be increased in many ways: parallel links can be deployed, multicore fibers can be used or more efficient modulation formats with digital signal processing is an alternative. Cost- and power efficiency and available physical volume are the main aspects considered when designing and building a new datacenter. Therefore, to improve link data rates, the introduction of new modulation formats are primarily considered for this task. 4-PAM provides double spectral efficiency and double data rate at the same symbol rate as on-off keying, but, as with any technology transition, new challenges emerge. Considerations for this transition is the topic of this thesis. Since the vast ma jority of optical interconnects use the vertical cavity surface emitting laser as the transmitter and multimode fiber as the transmission medium, simulated and experimental work relied on links with these devices. Many techniques are presented in this thesis to improve and analyze such links. The pre-emphasis of signals is a powerful tool to increase link bandwidth at the cost of modulation amplitude. This has been investigated in this thesis for on-off keying and has shown 9% and 27% increase in bit rate for error-free operation with two pre-emphasis approaches. Similarly, pre-emphasis of a 4-PAM electrical signals has enabled 71.8 Gbps transmission back-to-back with lightweight forward error cor rection and 94 Gbps net data rate was achieved with the same pre-emphasis and post-processing using an offline least-mean-square equalizer. Links using 850 nm vertical cavity surface emitting lasers still dominate today’s 25 Gbps lane rate datacenter interconnect links. Introducing 4-PAM in these links creates new challenges and it is important to know what design considerations are needed during this transition. Detailed investigation of legacy 25G class VCSELs has shown that devices with moderate damping are suitable for 4-PAM operation.