With the increasing demand for cloud-based services, internet content providers (ICPs) are pushing forward with the expansion of their data center clusters. Consequently, the need for simple and highly efficient data center interconnectivity (DCI) is growing to ensure that connectivity between metro-regional facilities doesn't become a bottleneck.

Energy usage, space, simplicity and cost effectiveness all have an impact on the efficiency of DCI infrastructure. The optimized synergy of these variables is essential. The need to consider watts per bit, rack space, simplified provisioning and operating expenditure has led to the choice between direct detection and coherent detection becoming a decisive question when building today’s DCI networks.

High-capacity single-span DWDM DCI links are crucial to give cloud-scale operators the capacity they need. These require Tbit/s networks that can cope with bandwidth demands and are optimized for distances of up to 80km.

Coherent Detection

In recent years, coherent detection has been the modulation format of choice for long-distance DWDM networks. Coherent detection uses a local oscillator at the receiver that tracks the phase of the optical transmitter enabling 100Gbit/s transport on a single wavelength for long distances. The need for the additional optical components as well as a complex digital signal processor (DSP) chip results in higher cost and higher power consumption.

Coherent detection has also been put to use in short-distance metro DCI networks, which are operated by relatively small teams. Coherent systems offer installation simplicity that is a highly attractive feature for ICPs and their relatively small numbers of operations personnel. Additionally, coherent detection enhances the performance and spectral efficiency of DCI networks but is less attractive in terms of cost and energy usage.

Direct Detection

Recent technological advances have increased the attractiveness of direct modulation for metro DCI networks. Direct detection appliances can now transport 100Gbit/s and, by leveraging PAM4 (pulse-amplitude modulation 4) technology, they can comfortably reach distances of up to 80km while complying with requirements for optical signal-to-noise ratio.

For the short distances of metro-regional DCI networks, direct detect systems offer ICPs a lower level of energy usage as well as lower costs for transmission. They are simpler and in this way more cost-efficient to deploy and operate as they are optimized for shorter distances. While coherent detection has more to offer in terms of optical bandwidth, direct detection convinces as an option for the expansion of data center clusters.

Current research is working to ensure direct detect technology continues to present an attractive alternative to coherent detection. New innovation is looking into possible solutions such as harnessing PAM4 with 50Gbaud technology or PAM8.

While its spectral efficiency makes coherent detection the dominant modulation in long-haul optical networks, new advances in direct detect technology will ensure that its attractiveness for operators of data center clusters will continue.