100-gigabaud optics usher in the era of terabit transmissions

•  NeoPhotonics has unveiled a coherent modulator and receiver that operate at over 100 gigabaud.

Telecom operators are in a continual battle to improve the economics of their optical transport networks to keep pace with the relentless growth of IP traffic.

One approach is to increase the symbol rate used for optical transmission. By operating at a higher baud rate, more data can be carried on an optical wavelength.

Ferris Lipscomb

Alternatively, a higher baud rate allows a simpler modulation scheme to be used, sending the same amount of data over greater distances. That is because the fewer constellation points of the simpler modulation scheme help data recovery at the receiver.

NeoPhotonics has detailed two optical components - a coherent driver-modulator and an intradyne coherent receiver (micro-ICR) - that operate at over 100 gigabaud (GBd). The symbol rate suits 800-gigabit systems and can enable one-terabit transmissions.

NeoPhotonics’ coherent devices were announced to coincide with the ECOC 2020 show.

Class 60 components

The OIF has a classification scheme for coherent optical components based on their analogue bandwidth performance.

A Class 20 receiver, for example, has a 3-decibel (dB) bandwidth of 20GHz. NeoPhotonics announced at the OFC 2019 show Class 50 devices with a 50GHz 3dB bandwidth. The Class 50 modulator and receiver devices are now deployed in 800-gigabit coherent systems.

NeoPhotonics stresses the classes are not the only possible operating points. “It is possible to use baud rates in between these standard numbers,” says Ferris Lipscomb, vice president of marketing at NeoPhotonics. “These classes are shorthand for a range of possible baud rates.”

“To get to 96 gigabaud, you have to be a little bit above 50GHz, typically a 55GHz 3dB bandwidth,” says Lipscomb. “With Class 60, you can go to 100 gigabaud and approach a terabit.”

It is unclear whether one-terabit coherent transponders will be widely used. Instead, Class 60 devices will likely be the mainstay for transmissions up to 800 gigabits, he says.

Source: NeoPhotonics, Gazettabyte 

Design improvements

Several aspects of the components are enhanced to achieve Class-60 performance.

At the receiver, the photodetector’s bandwidth needs to be enhanced, as does that of the trans-impedance amplifier (TIA) used to boost the received signals before digitisation. In turn, the modulator driver must also be able to operate at a higher symbol rate.

“This is mainly analogue circuit design,” says Lipscomb. “You have to have a detector that will respond at those speeds so that means it can’t be a very big area; you can’t have much capacitance in the device.”

Similarly, the silicon germanium drivers and TIAs, to work at those speeds, must also keep the capacitance down given that the 3dB bandwidth is inversely proportional to the capacitance.

Systems vendors Ciena, Infinera, and Huawei all have platforms supporting 800-gigabit wavelengths while Nokia‘s latest PSE-Vs coherent digital signal processor (DSP) supports up to 600 gigabit-per-wavelength. 

Next-generation symbol rate

The next jump in symbol rate will be in the 120+ gigabaud range, enabling 1.2-terabit transmissions.

“As you push the baud rate higher, you have to increase the channel spacing,” says Lipscomb. “Channels can’t be arbitrary if you want to have any backward compatibility.”

A 50GHz channel is used for 100- and 200-gigabit transmissions at 32GBd. Doubling the symbol rate to 64GBd requires a 75GHz channel while a 100GBd Class 60 design occupies a 100GHZ channel. For 128GBd, a 150GHz channel will be needed. “For 1.2 terabit, this spacing matches well with 75GHz channels,” says Lipscomb.

It remains unclear when 128GBd systems will be trialled but Lipscomb expects it will be 2022, with deployments in 2023.

Upping the baud rate enhances the reach and reduces channel count but it does not improve spectral efficiency. “You don’t start getting more data down a fibre,” says Lipscomb.

To boost transport capacity, a fibre’s C-band can be extended to span 6THz, dubbed the C++ band, adding up to 50 per cent more capacity. The L-band can also be used and that too can be extended. But two sets of optics and optical amplification are required when the C and L bands are used.

400ZR and OpenZR+

Lipscomb says the first 400ZR coherent pluggable deployments that link data centres up to 120km apart will start next year. The OIF 400ZR coherent standard is implemented using QSFP-DD or OSFP client-side pluggable modules.

“There is also an effort to standardise around OpenZR+ that has a little bit more robust definition and that may be 2022 before it is deployed,” says Lipscomb.

NeoPhotonics is a contributor member to the OpenZR+ industry initiative that extends optical performance beyond 400ZR’s 120km.

800-gigabit coherent pluggable

The OIF has just announced it is developing the next-generation of ZR optics, an 800-gigabit coherent line interface supporting links up to 120km. The 800-gigabit specification will also support unamplified fixed-wavelength links 2-10km apart.

“This [800ZR standard] will use between Class 50 and Class 60 optics and a 5nm CMOS digital signal processor,” says Lipscomb.

NeoPhotonics’ Class 60 coherent modulator and receiver components are indium phosphide-based. For the future 800-gigabit coherent pluggable, a silicon photonics coherent optical subassembly (COSA) integrating the modulator with the receiver is required.

NeoPhotonics has published work showing its silicon photonics operating at around 90GBd required for 800-gigabit coherent pluggables.

“This is a couple of years out, requiring another generation of DSP and another generation of optics,” says Lipscomb.