What’s All this Coherent Stuff?
With the emergence of 400G Ethernet we have seen the wide deployment of higher order modulation in client interfaces. We have moved from the mature NRZ of 100G and below to PAM-4 modulation both optically and electrically. This has allowed 400G links to be deployed for reaches of 10km. But what about applications like data center interconnect (DCI) which need a reach of closer to 80km?
The IEEE have been working on an emerging standard, IEEE 802.3ct, which will use coherent optical modulation to carry 400G over reaches of 80km and greater. We move from the traditional direct-detection modulation (where we measure light intensity) to a coherent system where the phase and polarization are used to convey information. The transmitter and receiver become more complex (although a lot of the “magic” is hidden inside the digital signal processor IC) and more complex photonics (including a tunable laser) are required. However, a coherent system brings many potential benefits including higher performance, electronic dispersion compensation and fewer lasers vs. more complex ICs.
Coherent modulation has been used for many years by classic transport NEMs. But in early 2020 we will start seeing compact pluggable modules from multiple vendors inter-operating across an open ecosystem for broad applications. VIAVI will be central to enabling this ecosystem. The VIAVI ONT family has supported pluggable coherent optics since the early CFP-DCO days. Our MAP photonic range will be heavily used for the photonic validation and test (we move from simple optical parameters like attenuation to more complex concepts like optical signal to noise ratio- OSNR) while the ONT 800, already established as the reference for client optics, has been enhanced with new features to support the test challenges of coherent pluggable optics.
Find out more about these topics by visiting the VIAVI website and talking to our experts across the globe. Join us next week for “Making all the Right Mistakes”, the next blog in this series which talks about advanced error analysis