Hyperscalers' needs drive a new class of coherent DSP

Coherent digital signal processors (DSPs) companies have supported two families of coherent chips for some time. That's because no single coherent DSP can meet all the market's requirements.

The coherent DSP used for highest-performance optical transmissions must include advanced coding techniques, forward error correction, and a high symbol rate to send as much data as possible on a single wavelength and maximise reach.

Helen Xenos at NGON & DCI World

In contrast, a DSP for coherent pluggable modules needs to be power-efficient and compact to meet the optical module's power envelope and size constraints; a 400ZR QSFP-DD and a CFP2-DCO 400ZR+ being examples.

According to Ciena, now there is a need for a third category of coherent DSP for 1.6 terabit-per-second (Tbps) and 3.2Tbps transmissions over short distances for next-generation switch routers.

Carrying data centre payloads

The need comes from the hyperscalers, as with most emerging coherent optical applications.

The new coherent DSP design is needed since it is the only way to support multi-terabit data rates for this application, says Ciena.

"Data centre switch routers with new 51.2- and 102.4-terabit switch chipsets will need greater than 400 gigabit-per-wavelength connectivity," said Helen Xenos, senior director, portfolio marketing at Ciena, during a talk at NGON & DCI World, held in Barcelona in June.

The coherent DSP will connect equipment within a data centre and between data centre buildings on campus. A 1-10km reach for the 1.6Tbps or 3.2Tbps wavelength transmissions is needed using an industry-standard pluggable such as a QSFP-DD or a OSFP pluggable form factor.

"It would have to be a specific, very low-cost design," says Xenos.

Coherent evolution

Applications using coherent optical technology continue to grow. 

Subsea, long-haul, metro, and 80-120km data centre interconnect are all well-known markets for coherent optics, said Xenos. Now, coherent is moving to the access network and for unamplified single-channel links.

"There is no one coherent optical design that will be cost-optimal across all of these applications," said Xenos. "This is why multiple coherent optical modem designs are required."

Xenos last presented at NGON & DCI World in pre-pandemic 2019. Then, the questions were whether 800-gigabit wavelengths would be needed and what optical performance 400-gigabit coherent pluggables would deliver.

Much has since changed. There has been a broad deployment of optical transport equipment using 800-gigabit wavelengths while the coherent pluggable market has gone from strength to strength.

For the high-end, up to 800 gigabits per wavelength, 7nm CMOS DSPs are used, operating at a symbol rate of 90-110 gigabaud (GBd).

For 400-gigabit coherent pluggables operating, the symbol rate is 60-70GBd, while the optics used is mainly silicon photonics.

800-gigabit market 

Ciena started shipping 800-gigabit capable optical modules in April 2020.

Since then, the company has seen strong uptake, with hyperscalers leading the way.

Ciena has shipped over 35,000 WaveLogic 5 Extreme 800-gigabit coherent modems in two years. Source: Ciena.

Also, a broad deployment of colourless, flexible grid optical line systems has helped 800-gigabit technology adoption.

Xenos cited, among others, Altibox, which brings high capacity connectivity from Norway to key digital hubs in Europe.

"They turned up the longest 800-gigabit wavelength between Copenhagen and Amsterdam, and that was over 1,100 kilometres," she said.

400-gigabit pluggables

Xenos points out that there has been a halving of the power-per-bit at 400Gbps.

Source: Ciena.

In 2017, Ciena offered a 400-gigabit 60GBd modem design in a 5x7-inch package.

"Now we have a pluggable 400-gigabit QSFP-DD at 60GBd pluggable, so the same type of design, the same simple feature set required with a 400ZR," said Xenos.

Optical performance is also being pushed to 70GBd in the QSFP-DD, with the module having a higher output power.

Near-term designs

Ciena says the next two to three generations of coherent DSPs will use 5nm and 3nm CMOS.

New promising materials for optical modulation are emerging, such as thin-film lithium niobite, and barium titanate, which is compatible with silicon photonics.

"[A] Higher baud [rate] will reduce cost-per-bit and get more capacity using a single wavelength," says Xenos. "Also, there will be more intelligence and programmability as we move forward to enable more automated networks."

She says a 160GBd symbol rate is needed to send 800 gigabits over long-distance spans.

The key for all the different modem designs is to develop something better while choosing the right technologies so that new products are available promptly.

"It's essential to make the right technology choice that will give the right reliability and be commercially available," says Xenos.

Three nanometre CMOS promises more significant performance benefits for a DSP design, but developing the process technology is challenging for the leading chip fabrication plants. In addition, a 3nm CMOS process will be costly.

Award

Ciena won the optical vendor of the year award, one of the five prizes presented at the NGON & DCI World show.