Socionext invited to talk at the 43rd European Conference on Optical Communication
17th- 21st September, 2017, Gothenburg, Germany
The European Conference on Optical Communication (ECOC) is the leading European conference on optical communications and one of the most prestigious and long-standing events in this field. Selected papers, keynotes, presentations and special symposia update conference delegates on the latest developments in Optical communication technologies.
Socionext were invited to talk at the conference and produced a presentation on ‘DSP for Short Reach Optical Links’. The presentation by Jens C. Rasmussen, discussed Digital Signal Processor (DSP) topologies for optical transceivers using data-rates ≥100Gbit/s to enable SMF transmission of 10 to 100km in the context of evolving requirements and future technical advances.
Optical transmission technology is a requirement for links with a large distance x data-rate. Applications range from intra data-center up to 2km, intra data-center up to 100km and Metro, Long-Haul, Submarine with distances greater than 100km. (Fig 1).
Fig 1: Optical transceiver reaches, and typical applications ranging from intra data-center up to 2km, data-center interconnect, up to 80km, and metro/long-haul/submarine
For data-rates ≥100Gbit/s and distances much greater than 100km only optical coherent transmission technology is possible for the required performance with the use of advanced and optimized digital signal processing (DSP).
The subject of the paper was for distances in the order of 10 – 100km where typical applications include connecting multiple smaller data-centers over an area such as a city. From an optical transceiver point of view, it is the area between short distances served with transceivers, based on inexpensive optical devices, simple On-Off Keying (OOK) modulation without digital signal processing (Fig) 2 and the long distances that are served with coherent transceivers utilizing sophisticated digital signal processing.
Fig. 2: 100Gbit/s optical transceiver based on 4 wavelengths each carrying 25Gbit/s.
Two technologies are competing for these 10 – 100km short reach links.
One technology uses coherent optical transceivers that whilst they can implement dedicated digital functions to meet performance requirements, cost, size and power consumption are a challenge.
The other technology is direct detection based transceivers. These are advantageous with regards to cost, size and power requirements but need significant technological advancements to improve performance.
Short reach 100Gbit/s optical transceivers
Many components are used in 100Gbit/s transceivers based on 4 wavelengths, each modulated at 25Gbit/s. However, a more attractive option is a single wavelength transceiver that reduces the component count from 4 lasers, photodiodes, drivers and transimpedance amplifiers (TIA) to just one of each component. To obtain the baud-rate of the On-Off Keying (OOK) signal in a single component would mean the baud rate would need to be raised from 25Gbaud to 100Gbaud which would be unrealistic as bandwidths requirements cannot be met. There are various other options including Pulse Amplitude Modulation (PAM) or Discrete Multi-Tone (DMT).
The paper discussed different approaches and concluded that DSP heavy optical coherent transceivers are the technology of choice for long reach applications >100km, whilst short-reach applications are best served with simple OOK transceivers. Distances in the order of 10-100km demand for 100Gbit/s the performance of coherent transceivers at the cost and power consumption of non-DSP based transceivers. For the interim, solutions are available with PAM/DMT based transceivers. However, with the trend for even higher data rates ≥400Gbit/s and the advances in DSP technologies, optical coherent transceivers are likely to be introduced for shorter distances in the very near future.
Whether the technical solution is coherent or non-coherent, with increasing data rates optical transceivers rely more and more on high speed Analog-to-Digital (ADC) and Digital-to-Analog Converters (DAC) – A Core Competence of Socionext.