Coherent Optical Module Technology and Standards Trends

 With the increase of single channel transmission rate, more and more application scenarios in the field of modern optical communication have started to use coherent optical transmission technology, which has changed from the backbone network(>1000km) to metro area (100~1000km) and even edge access network (<100km). On the other hand, in the field of data communication, coherent technology has also become the mainstream solution for data center interconnection (DCI) (80~120km). These new applications also put forward new requirements for coherent optical transceiver systems, driving the evolution of coherent transceiver units from the original integration with line cards and MSA transceivers to independent, standardized pluggable optical transceivers.

Development of Pluggable Coherent Optical Transceivers

Compared to client optical transceivers used inside metro networks or data centers, coherent optical transceivers used in optical transport networks are usually built in or integrated into line-side veneers, which suffer from low port density, high volume and power consumption, and non-standard designs. For a long time, network operators have been hoping that the transmission optical transceiver has the same or similar package as the client optical transceiver, just as 10G network can be achieved using the standard SFP+ optical transceiver package. Recent years have seen advances in CMOS process DSP chips and integrated photonic technology, making smaller and lower-power-consumption pluggable coherent packaged optical transceivers possible.

After years of development, standardized, pluggable optical transceivers have been the inevitable choice for optical communication line side service transmission. The development trend of coherent optical transceivers applied in metro and backbone networks has the following characteristics.

High speed: evolution from 100G/200G to 400G and then to 800Gbps rates.

Miniaturization: from 100G MSA package form to CFP/CFP2 DCO/ACO package form, and the current package standards such as 400G OSFP DCO and QSFP-DD DCO are proposed.

Low power consumption: considering the overall system power consumption requirements, for example, the power consumption of coherent optical transceiver products in QSFP-DD package should not be higher than 15W.

Standardization of interoperability: traditionally, each equipment manufacturer uses self-developed special interface boards, using private high-order modulation methods and FEC algorithms, which are not interoperable between different manufacturers' interfaces; interconnection of coherent optical transceivers is the direction the industry is working on.

Comparative Analysis of 400G Coherent Standards

Current commercial coherent technology has evolved to single-wavelength 800G, but there are no standards for 800G in the industry, while 400G coherent technology is currently available in 400ZR, OpenROADM and OpenZR+ standards.

The 400ZR is a project started by the Optical Interconnect Forum (OIF) in 2016 to standardize interoperable coherent optical transceiver interfaces with power budgets that can support packages like QSFP-DD and OSFP for 400G coherent optical transceivers for Data Center Interconnect (DCI). This packaging proposed by OIF focuses on certain specific applications where transmission performance can be sacrificed because of its need to meet a 15W transceiver power target. OIF-400ZR targets at edge DCI applications with only 400GbE rates defined on the customer side and transmission distances of 80km to 120km with CFEC forward error correction. OIF has demonstrated that coherent interoperability standards are possible and its proposed 400ZR solution is supported in the industry. At the same time, system operators have demonstrated that there is room to further improve the thermal performance of these high-density packages, allowing optical transceivers using these packages to support additional features and thus provide higher performance.

Building on the success of the OIF, carriers led by AT&T have defined the standard OpenROADM MSA to support longer-distance transmission. OpenROADM is designed for OTN networks that need to support other protocols and increase the corresponding overhead bit ratios. OpenROADM MSA is primarily intended for telecom operator ROADM network applications, and defines interfaces of 100G, 200G, 400GbE rates & OTN with a transmission distance of 500km, using openFEC (oFEC) forward error correction algorithm.

400ZR and OpenROADM define pluggable coherent optical transceiver types and performance characteristics for data center interconnection and telecom optical transport networks, respectively. But each has certain limitations and drawbacks, for example, 400ZR only supports 400GbE customer-side interfaces, while OpenROADM only considers network scenarios for telecom operators.

Therefore, some mainstream vendors in the industry have integrated the respective advantages of OIF-400ZR and OpenROADM standards and introduced another MSA standard, OpenZR+.

The OpenZR+ MSA has a broader range of applications for metro, backbone, DCI and telecom operators and is designed to enable enhanced functionality and improved performance in pluggable forms such as QSFP-DD and OSFP to support multi-vendor interoperability. openZR+ not only maintains the simple Ethernet-only host interface of the 400ZR, but adds support for 100G, 200G 300G or 400G line interfaces for multi-rate Ethernet and multiplexing capabilities, and uses the oFEC already standardized by OpenROADM MSA and CableLabs, resulting in higher dispersion tolerance and higher coding gain. In September 2020, OpenZR+ released its first public version of the metrics book. OIF-400ZR, Open ROADM and OpenZR+ are three standards defined by the coherent optical transceiver main performance metrics comparison is shown in the table below.

The use of line side optical transceivers in the same package as the customer side is beneficial to network operators, reducing costs through simpler network architectures. Combined with the recent industry trend of Open Line System (OLS), these transport optical transceivers can be plugged directly into routers without the need for an external transport system. This simplifies the control platform while reducing cost, power consumption and land area. In the network scenario shown, users can choose to plug an OpenZR+ compliant coherent optical transceiver directly into a port on an OLS-enabled router, or they can plug it into the line-side port of the transport device used to implement the signal protocol conversion and then connect it to the router through the customer-side port of that device.

Analysis of the Technology Evolution After 400G Coherence

In terms of standardization evolution, the next generation of super 400G coherent pluggable products are likely to take single-wave 800G rates. Recently, OIF is discussing the development of 400ZR next-generation coherent technology standard 800ZR. The current initial consideration is to support 80~120km (amplified) DWDM links for DCI scenarios and 2~10km links without amplification for campus scenarios. The customer side interface supports 2×400GE or 1×800GE, and the line-side supports a single-wavelength 800G coherent line interface. Define the frame structure metrics mapped from the customer side to the line side and the signal metrics on the line side for interoperability. At the component level, the OIF is also discussing the next generation of coherent modulator specification OIF-HB-CDM2.0, which supports higher modulation rates.

In terms of optical and electrical chip technology development, 800ZR optical transceiver products may use 5NM or more advanced made of DSP chips, silicon-based hybrid integrated optical chip and Flip Chip technology and other advanced packaging technology, the coherent optical transceiver must be able to support high-order modulated signals of 96/128GBd and DP-64QAM/DP-16QAM. When the baud rate reaches 128GBd, the bandwidth of the optical chip should be at least 70~80GHz. The modulator based on silicon optical material may not be able to support such high rate, while the traditional III-V material optical modulator can reach theoretically, but the implementation difficulty will be quite high. Therefore the industry is also trying some new materials and device technologies, such as Thin Film Lithium Niobate (TFLN). Lithium niobate has been considered the preferred material for optical modulators, the traditional bulk material lithium niobate modulator due to the large size and bandwidth limited by the size of the device, can not support the baud rate of 64GBd or more applications. In recent years, the lithium niobate modulator can also achieve small size and high bandwidth due to the breakthrough in thin film lithium niobate chip processing technology, so it is considered as a potential technology direction to realize optical modulators with 100GBd and above. In addition to achieve high bandwidth at the device level, the electric drive chip and packaging technology is also one of the difficult points to be solved.