Advancements in Coherent Optical Module Technology and Standardization Trends

As the single-channel transmission rate continues to rise, the application landscape in modern optical communication has witnessed a growing adoption of coherent optical transmission technology. This shift extends beyond the traditional backbone network (>1000km) to encompass metro areas (100~1000km) and even edge access networks (<100km). Simultaneously, coherent technology has emerged as the prevailing solution for Data Center Interconnection (DCI) applications, covering distances of 80~120km in the field of data communication. These evolving applications introduce new demands for coherent optical transceiver systems, steering the development of coherent transceiver units from their initial integration with line cards and Multi-Source Agreement (MSA) transceivers toward independent, standardized pluggable optical transceivers.

Revolutionizing Optical Networks: Pluggable Coherent Transceivers Lead the Way

In contrast to client optical transceivers deployed within metro networks or data centers, coherent optical transceivers employed in optical transport networks are typically embedded or integrated into line-side configurations. These configurations are associated with drawbacks such as low port density, large physical size, high power consumption, and non-standardized designs. Over an extended period, network operators have aspired to achieve a similar packaging approach for transmission optical transceivers as seen with client optical transceivers, analogous to the standardization achieved with 10G networks using the SFP+ optical transceiver package.

Recent advancements in Complementary Metal-Oxide-Semiconductor (CMOS) process Digital Signal Processor (DSP) chips and integrated photonic technology have paved the way for the development of smaller, lower-power pluggable coherent optical transceivers.

The progression toward standardized, pluggable optical transceivers has become an inevitable choice for line-side service transmission in optical communication. The developmental trajectory of coherent optical transceivers applied in metro and backbone networks exhibits the following key characteristics:

• High Speed: Evolution from 100G/200G to 400G, and further advancements to 800Gbps rates.

• Miniaturization: Transitioning from the 100G Multi-Source Agreement (MSA) package form to C Form-Factor Pluggable (CFP)/CFP2 Digital Coherent Detection (DCD)/Analog Coherent Optics (ACO) package form. Current package standards, such as the 400G OSFP DCO and QSFP-DD DCO, have been proposed.

• Low Power Consumption: Adhering to stringent overall system power consumption requirements. For instance, the power consumption of coherent optical transceiver products in the QSFP-DD package should not exceed 15W.

• Standardization of Interoperability: Departing from the traditional practice where each equipment manufacturer utilizes proprietary interface boards, employing private high-order modulation methods and Forward Error Correction (FEC) algorithms. The industry is now actively working towards achieving interoperability for coherent optical transceivers.

Navigating 400G Standards and the Emergence of OpenZR+

Current commercial coherent technology has progressed to the point of achieving single-wavelength 800G transmission. However, the industry currently lacks standardized specifications for 800G. In contrast, the 400G coherent technology is well-established and adheres to standards such as 400ZR, OpenROADM, and OpenZR+.

The 400ZR initiative, initiated by the Optical Internetworking Forum (OIF) in 2016, aims to standardize interoperable coherent optical transceiver interfaces suitable for power-efficient packages like QSFP-DD and OSFP. Specifically tailored for Data Center Interconnect (DCI) applications, the OIF-400ZR packaging targets scenarios where transmission performance can be somewhat compromised to meet a 15W transceiver power threshold. This solution is focused on edge DCI applications, supporting 400GbE rates on the customer side, with transmission distances spanning 80km to 120km, incorporating CFEC forward error correction.

OpenROADM, led by carriers like AT&T, defines the OpenROADM Multi-Source Agreement (MSA) standard to support longer-distance transmission in OTN networks. Geared towards telecom operator Reconfigurable Optical Add-Drop Multiplexer (ROADM) network applications, OpenROADM MSA outlines interfaces for rates of 100G, 200G, 400GbE, and OTN, with a transmission distance of 500km, utilizing an open Forward Error Correction (OFEC) algorithm.

While 400ZR and OpenROADM define pluggable coherent optical transceiver types for DCI and telecom optical transport networks, they exhibit limitations. For instance, 400ZR supports only 400GbE customer-side interfaces, and OpenROADM mainly addresses scenarios for telecom operators. Consequently, some industry-leading vendors have amalgamated the strengths of OIF-400ZR and OpenROADM standards, introducing a new MSA standard called OpenZR+.

OpenZR+ MSA extends its applications across metro, backbone, DCI, and telecom operator scenarios. It aims to deliver enhanced functionality and improved performance in pluggable forms, such as QSFP-DD and OSFP, ensuring multi-vendor interoperability. While maintaining the simple Ethernet-only host interface of 400ZR, OpenZR+ introduces support for 100G, 200G, 300G, or 400G line interfaces, accommodating multi-rate Ethernet and multiplexing capabilities. Leveraging the oFEC standard from OpenROADM MSA and CableLabs, OpenZR+ enhances dispersion tolerance and coding gain.

In September 2020, OpenZR+ released its initial public version of the metrics book. A comparative analysis of the main performance metrics of OIF-400ZR, OpenROADM, and OpenZR+ is presented in the table below.

The integration of line-side optical transceivers in the same package as the customer side brings cost benefits to network operators by simplifying network architectures. Aligned with the prevailing industry trend of Open Line Systems (OLS), these transport optical transceivers can be directly plugged into routers without requiring an external transport system. This streamlines the control platform, reducing costs, power consumption, and physical footprint. In the illustrated network scenario, users have the flexibility to plug an OpenZR+ compliant coherent optical transceiver directly into a port on an OLS-enabled router or connect it to the router through the customer-side port of a transport device that implements signal protocol conversion.

Anticipating 800ZR: Advancements and Challenges in Next-Gen 800G Coherent Pluggable Technology

In the realm of standardization evolution, the forthcoming generation of super 400G coherent pluggable products is anticipated to embrace single-wave 800G rates. Presently, the Optical Internetworking Forum (OIF) is in deliberations regarding the development of the 400ZR next-generation coherent technology standard, tentatively named 800ZR. Initial considerations include its support for 80~120km (amplified) Dense Wavelength Division Multiplexing (DWDM) links tailored for Data Center Interconnect (DCI) scenarios, as well as 210km links without amplification for campus scenarios. The customer side interface is envisioned to accommodate 2x400GE or 1x800GE, while the line side is set to support a single-wavelength 800G coherent line interface. The standard aims to define the frame structure metrics mapped from the customer side to the line side and establish signal metrics on the line side to ensure interoperability. At the component level, discussions within the OIF are also centered on formulating the next generation of coherent modulator specifications, denoted as OIF-HB-CDM2.0, which will support higher modulation rates.

Concerning the advancement of optical and electrical chip technology, forthcoming 800ZR optical transceiver products may leverage 5nm or more advanced Digital Signal Processing (DSP) chips, silicon-based hybrid integrated optical chips, Flip Chip technology, and other advanced packaging techniques. These technologies are essential to ensure that the coherent optical transceiver can effectively 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. Silicon optical modulators may face limitations in supporting such high rates, while traditional III-V material optical modulators, while theoretically possible, pose implementation challenges. Consequently, the industry is exploring novel materials and device technologies, including Thin Film Lithium Niobate (TFLN). Although lithium niobate has long been regarded as the preferred material for optical modulators, traditional bulk material lithium niobate modulators encounter size limitations and bandwidth constraints. Recent breakthroughs in thin film lithium niobate chip processing technology, however, have paved the way for smaller-sized modulators with high bandwidth, making it a potential avenue to realize optical modulators with 100GBd and above. In addition to achieving high bandwidth at the device level, challenges persist in developing the electric drive chip and packaging technology.