NEL

The ColorZ II and ClearLight modules support the 400ZR OIF standard used to link data centres up to 120km apart. They also support extended modes, known as ZR+, that is not standardised.

ZR+’s modes include 400 Gigabit-per-second (Gbps) over distances greater than 400ZR’s 120km and lower data rates over metro-regional and long-haul distances.

The announcements of the Canopus DSP and 400-gigabit pluggable coherent modules highlight the approaches being taken for ZR+. Optical module vendors are aligning around particular merchant DSPs such that interoperability exists but only within each camp.

The first camp involves Inphi and three other module vendors, one being NeoPhotonics. The second camp is based on the OpenZR+ specification that offers interoperability between the DSPs of the merchant players, Acacia Communications and NTT Electronics (NEL). Cisco is in the process of acquiring Acacia.

Market analysts, however, warn that such partial interoperability for ZR+ harms the overall market opportunity for coherent pluggables.

“ZR+ should be interoperable like ZR, and come along with the hard decisions the ZR standard required,” says Andrew Schmitt, founder and directing analyst at research form, Cignal AI.

The optical module vendors counter that only with specialist designs – designs that are multi-sourced – can the potential of a coherent DSP be exploited.

Applications 

The advent of 400-gigabit coherent optics within compact client-side form factors is a notable development, says Inphi. “The industry has been waiting for this inflextion point of having, for the first time, 400-gigabit coherent pluggables that go on router and switch interfaces,” says Pranay Aiya, vice president of product marketing and applications engineering at Inphi.

“IP over DWDM has never happened; we have all heard about it till the cows come home,” says Aiya.

IP-over-DWDM failed to take off because of the power and space demands of coherent optics, especially when router and switch card slots come at a premium. Using coherent optics on such platforms meant trading off client-side faceplate capacity to fit bulkier coherent optics. This is no longer the case with the advent of QSFP-DD and OSFP coherent modules.

“If you look at the reasons why IP-over-DWDM  – coloured optics directly on routers – failed, all of those reasons have changed,” says Schmitt. The industry is moving to open line systems, open network management, and more modular network design.

“All of the traffic is IP and layer-1 switching and grooming isn’t just unnecessary, it is more expensive than low-feature layer-2 switching,” says Schmitt, adding that operators will use pluggables wherever the lower performance is acceptable. Moreover, this performance gap will narrow with time.

The Canopus DSP also supports ZR+ optical performance and, when used within a CFP2-DCO module with its greater power enveloped than OSFP and QSFP-DD, enables metro and long-haul distances, as required by the telecom operators. This is what Neophotonics has announced with its ClearLight CFP2-DCO module.

Canopus

Inphi announced the Canopus DSP last November and revealed a month later that it was sampling its first optical module, the ColorZ II, that uses the Canopus DSP. The ColorZ II is a QSFP-DD pluggable module that supports 400ZR as well as the ZR+ extended modes.

Inphi says that, given the investment required to develop the 7nm CMOS Canopus, it had to address the bulk of the coherent market.

“We were not going after the ultra-long-haul and submarine markets but we wanted pluggables to address 80-90 per cent of the market,” says Aiya.

This meant developing a chip that would support the OIF’s 400ZR, 200-gigabit using quadrature phased-shift keying (QPSK) modulation for long haul, and deliver 400-gigabit over 500-600km.

The Canopus DSP also supports probabilistic constellation shaping (PCS), a technology that until now has been confined to the high-end coherent DSPs developed by the leading optical systems vendors.

With probabilistic shaping, not all the constellation points are used. Instead, those  with lower energy are favoured; points closer to the origin on a constellation graph. The only time all the constellation points are used is when sending the maximum data rate for a given modulation scheme.

Choosing the inner, lower-energy constellation points more frequently than the outer points to send data reduces the average energy and improves the signal-to-noise ratio. To understand why, the symbol error rate at the receiver is dominated by the distance between neighbouring points on the constellation. Reducing the average energy keeps the distance between the points the same, but since a constant signal power level is used for DWDM transmission, applying gain increases the distance between the constellation points. The result is improved optical performance.

Probabilistic shaping also allows an exact number of bits-per-symbol to be sent, even non-integer values.

For example, using standard modulation schemes such as 64-QAM with no constellation shaping, 6 bits-per-symbol are sent. Using shaping and being selective as to which constellation points are used, 5.7 bits-per-symbol could be sent, for example. This enables finer control of the sent data, enabling operators to squeeze the maximum data rate to suit the margins on a given fibre link.

“This is the first time a DSP with probabilistic shaping has been available in a size and power that enables pluggables,” says Aiya.

The resulting optical performance using the Canopus is up to 1,500km at 300Gbps signals and up to 2,000km for 200Gbps transmissions (see Table above). As for baud rates, the DSP ranges from 30+ to the mid-60s Gigabaud.

Inphi also claims a 75 per cent reduction in power consumption of the Canopus compared to 16nm CMOS DSPs found in larger, 4×5-inch modules.

Several factors account for the sharp power reduction: the design of the chip’s architecture and physical layout, and the use of 7nm CMOS. The Canopus uses functional blocks that extend the reach, and these can be turned off to reduce the power consumption when lower optical performance is acceptable.

The architectural improvements and the physical layout account for half of the overall power savings, says Aiya, with the rest coming from using a 7nm CMOS.

The result is a DSP a third the size of 16nm DSPs. “It [pluggables] requires the DSP to be very small; it’s not a paperweight anymore,” says Aiya.

400ZR and ZR+

The main challenge for the merchant coherent DSP camps is the several, much larger 400ZR eco-systems from Ciena, Cisco and Huawei.

“Each one of these eco-systems will be larger than the total merchant market of 400ZR,” says Vladimir Kozlov, CEO and founder of LightCounting. The system vendors will make sure that their products offer something extra if plugged into their equipment while maintaining interoperability. “This could be some simple AI-like features monitoring the link performance and warning customers of poor operation of devices on the other side of the link if these are made by another supplier,” says Kozlov.

LightCounting says that ZR+ units will be half to a third of the the number of 400ZR units shipped. However, each ZR+ module will command a higher selling price.

Regarding the ZR+ camps, one standardisation effort is OpenZR+ that adopts the forward-error correction (oFEC) scheme of the OpenROADM MSA, supports multiplexing of 100 Gigabit Ethernet (GbE) and 200GbE client signals, and different line rates – 100-400Gbps – to achieve greater reaches.

The backers of OpenZR+ include the two merchant DSP vendors, Acacia and NEL, as well as Fujitsu Optical Components, Lumentum, and Juniper Networks.

The second ZR+ camp includes four module-makers that are adopting the Canopus: Inphi, Molex Optoelectronics, NeoPhotonics and an unnamed fourth company. According to Schmitt, the unnamed module maker is II-VI. II-VI declined to comment when asked to confirm.

Schmitt argues that ZR+ should be interoperable, just like 400ZR. “I think NEL, Acacia, and Inphi should have an offsite and figure this out,” he says. “These three companies are in a position to nail down the specs and create a large, disruptive coherent pluggable market.”

Simon Stanley, founder and principal consultant at Earlswood Marketing Limited, expects several ZR+ solutions to emerge but that the industry will settle on a common approach. “You will initially see both ZR+ and OpenZR+,” says Stanley. “ZR+ will be specific to each operator but over time I expect OpenZR+ or something similar to become the standard solution.”