Integrated photonics and 400ZR drive next-generation DCI

2019-05-31  |  5 min read 

Cloud-based services, video streaming, Internet of Things (IoT) devices, 5G connectivity, and more put a strain on communications networks. Network infrastructure, in particular data center interconnects (DCIs), must evolve and transform to support these demands. Today’s DCIs need to offer higher bandwidth transfer rates and ensure energy efficiency between connected, distributed data centers.

While widely used in long-haul telecommunications networks, coherent optical technology has been cost prohibitive and impractical for use in shorter distances such as DCIs. Developments in integrated photonics technology and standards such as 400ZR will enable DCIs to reach a new speed class. Using coherent optical technology, DCIs can transport terabytes of information across a single fiber line and provide flexibility to address growing data demands.

Three key trends in data center interconnects:

1. Coherent optical moves to DCI

Distributed data centers need to communicate with each other to share data, balance workloads, provide backups, and scale data center capacity when needed. DCI connections are usually less than 80 km apart. The conventional means of data transfer through optical signaling using on-off keying (OOK) modulation was sufficient for speeds up to 100 gigabits per second (Gb/s). Today, many distributed data centers in a campus or metropolitan area need to significantly increase interconnection capacity. Coherent optical transmission technology offers the fastest and most efficient DCI transport.

Coherent optics enables higher rates of data transmission over the same fiber lines by using higher order modulation, such as quadrature amplitude modulation (QAM). QAM modulates the amplitude and phase of light to transmit the signal. This results in a significant capacity increase of a fiber optic cable through the associated increase of spectral efficiency. Using 16-QAM, a transceiver with a 64 gigabaud (Gbaud) raw symbol rate (or 50 Gbaud without the overhead) could transmit 400 Gb/s on a single optical carrier.

2. Integrated photonics enables terabit speeds

Typically, photonics use discrete components, i.e. they are physically separate and interact using different coupling procedures to create a complete optical circuit. Integrated photonics streamlines this process with the use of photonic integrated circuits (PICs). PICs integrate multiple photonic functions in a single device, similar to an electronic integrated circuit, and use light instead of electricity to transmit signals. PICs provide numerous advantages over conventional circuits including higher bandwidth, expanded wavelength division multiplexing, smaller size, lower power consumption, and improved reliability.

It is widely believed that Moore’s Law — the observation that the number of transistors that can be placed on silicon chips in integrated circuits doubles every two years — is nearing its end. Traditional chip technology cannot continue to get much smaller or keep pace with the exponential increase in processing speeds data centers need to support emerging technologies such as 5G, IoT, and autonomous vehicles. PICs offer an alternative to the limitations of silicon chip technology.

3. 400ZR provides a cost-effective alternative

Traditionally, optical coherent technology was only cost-effective over great distances, such as those in long-haul transport networks. However, new standards such as 400ZR and 400GBASE-ZR enable coherent optical technology to move to data center interconnects. The Optical Internetworking Forum (OIF) is developing the 400ZR implementation agreement. It will enable transmission of a 400 gigabit Ethernet (GE) payload over data center interconnect links up to 80 km using dense wavelength division multiplexing (DWDM) and higher-order modulation. The 400ZR specification recommends 16-QAM at a symbol rate of about 60 Gbaud. To achieve this rate with a maximum power consumption of 15 W and with space constraints in the target form factors, optical transceivers require dense electronic and photonic integration with tighter specifications and performance margins for all components. These small form factors need small component sizes and low electrical power consumption. These restrictions create challenges for digital signal processor (DSP) and component suppliers.

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