400G Coherent Optics: Breaking Through Bandwidth and Distance Barriers in DCI and MAN

With the rapid advancements in technologies like machine learning (ML), artificial intelligence (AI), virtual reality (VR), and autonomous driving (ADS), the scale of Internet Data Centers (IDCs) has expanded dramatically, driving an exponential demand for data transmission capabilities. As the core component of next-generation high-speed transmission, 400G coherent optical modules are emerging as critical solutions for overcoming Data Center Interconnect (DCI) and Metropolitan Area Network (MAN) bandwidth bottlenecks, reducing latency, and enhancing network security and reliability. The previous article covered NADDOD's three types of 400G QSFP-DD digital coherent optics. This article will delve deeper into 400G coherent modules' specifics, composition, advantages, and application scenarios.

Challenges of Traditional DCI and MAN Systems

Traditional DCI and MAN systems often utilize Dense Wavelength Division Multiplexing (DWDM) technology, converting grey optical signals into colored ones for combined transmission over a single fiber. DWDM allows independent transmission of multiple wavelengths, conserving fiber resources and supporting individual channel rates of 10/100/200 Gbps. It can multiplex 4, 8, 16, 40, 48, or 96 channels, achieving total transmission capacities from 40 Gbps to 19.2 Tbps. DWDM can cover up to 100 km per span, extendable over hundreds of kilometers with optical amplification, ensuring high reliability with 50 ms failover protection using dual fiber pairs. While DWDM provides robust long-distance transmission, it presents several challenges:

• Limited Transmission Distance: Non-coherent modules struggle with dispersion and non-linear effects, degrading signal quality over long distances and limiting relay-free reach.
• Bandwidth Inefficiencies: Traditional direct detection techniques are less efficient in bandwidth utilization than coherent optics, potentially failing to meet growing data traffic demands.
• Subpar Interference Resistance: Non-coherent modules perform poorly in complex network environments, with higher susceptibility to noise and interference, affecting signal stability.
• Limited Flexibility and Scalability: The lack of adaptable modulation formats and wavelength options in non-coherent modules restricts dynamic network adjustments and scalability.
• Higher Power and Cost: Inefficiencies at high transmission rates lead to increased power consumption and operational costs, especially when multiple relays are needed.

Evolution of Coherent Optics Technology

As coherent optics technology advances, it has become the mainstream solution for interconnecting data centers. Coherent optical modules leverage advanced modulation and digital signal processing to overcome transmission challenges like dispersion and non-linear effects, simplifying network design and reducing operational costs. This has accelerated the development of coherent modules, with multiple standards like 400G-ZR, 400G OpenROADM, and Open ZR+ emerging to meet diverse network needs.

400G ZR Coherent Optical Modules

The Optical Internetworking Forum (OIF) introduced the pluggable 400G-ZR coherent optical modules for DCI and MAN applications. These modules comply with OIF 400ZR MSA and QSFP-DD MSA standards, using DWDM technology over the C-band spectrum. They leverage DP-16QAM modulation, supporting high-performance 400G transmission over distances of 80-120 km (up to 40 km on standard fiber, extendable to 120 km with optical amplification). Quadrature Amplitude Modulation (QAM) is a key technique here, utilizing both phase and amplitude of the carrier wave to transmit data. In 16QAM, each carrier supports 8 bits of data with 16 points in the constellation.

DP-QPSK:DP-16QAM Constellation Diagram

Composition and Packaging of 400G ZR Coherent Optical Modules

The 400G ZR standard includes three MSA packaging options: QSFP-DD, OSFP, and CFP2. QSFP-DD is preferred for its high density and flexibility. The QSFP-DD type 2 module measures 18.4 mm x 93.4 mm x 8.5 mm, support hot-pluggable connections via a 76-pin connector, and includes key components like a Digital Signal Processor (DSP) for analog-to-digital conversion and a Transmitter Optical Subassembly (TROSA) that supports high-order modulation.

400G-ZR Functional Block Diagram

400G ZR+ Coherent Optical Modules

To address diverse application scenarios, Open ROADM introduced additional standards based on OIF 400G-ZR, enhancing DCI transmission capabilities with flexible rates (100G, 200G, 300G, 400G), advanced forward error correction (oFEC), and adaptable line-side encoding and rate selection, supporting distances beyond 120 km.

Origins of OpenZR+

Advantages of OpenZR+ Over 400G ZR

Multiplexing Flexibility

OpenZR+ supports 4x100G multiplexing, dividing a single 400G signal into four 100G channels, improving fiber utilization and network flexibility. This gradual upgrade approach allows the coexistence of 400G and 100G equipment, enhancing practicality during network transitions.

Versatile Line-Side and Client-Side Compatibility

OpenZR+ supports various line-side encodings (16-QAM, 8-QAM, QPSK) and client-side rates (100GE, 200GE, 400GE), catering to different distance and capacity requirements. For long-distance applications, reconfigurable optical add-drop multiplexers (ROADMs) using Nx6.25 GHz increments, along with hybrid configurations of EDFAs and backward Raman amplifiers, maximize the optical signal-to-noise ratio (OSNR), enabling extended transmission ranges.

400G ZR+ High-Power Coherent Optical Modules

For metro data center interconnects around 100 km, the standard 400G-ZR may be more suitable than OpenZR+. However, 400G-ZR's reach is limited to about 40 km in bare fibre environments due to module output power and link loss constraints. For instance, NADDOD's 400G-QSFP-DD-ZR modules have a transmit power range of -6 to -10 dBm and a receiver sensitivity of -20 dBm, translating to a maximum link loss of 14 dB. With an estimated fiber attenuation of 0.3 dB/km, the feasible transmission distance is around 43 km. High OSNR requirements complicate stable long-distance connections, necessitating the use of amplification solutions to extend 400G bandwidth to 120 km for DCI/MAN applications.

The 400G-ZR+ High-Power coherent modules are engineered for longer distances and higher performance. They enhance transmission power to +1 dBm and integrate a Mini-EDFA (Erbium-Doped Fiber Amplifier), supporting adjustable DWDM transmission from -10 to +1 dBm and a maximum of +4 dBm in gray light mode, achieving a 26 dB link budget for point-to-point connections, with tested distances reaching 120 km.

NADDOD 400G ZR Series Coherent Optical Module Use Cases

Scenario 1: Direct Fiber Connection

• No amplifiers or multiplexers; direct fiber link.
• Fiber length: Up to 80 km.
• Attenuation: 0-31 dB.

Scenario 2: Metro Point-to-Point

• Employs amplifiers and multiplexers.
• Fiber length: Up to 120 km.
• Attenuation: Less than 30 dB.

Scenario 3: Standard Long-Haul Deployment

• Uses amplifiers and multiplexers with existing wavelengths.
• Ensures end-to-end OSNR greater than 23 dB per span.
• Requires 75 GHz or higher multiplexers and ZR/ZR+ power compensation.

NADDOD’s 400G ZR coherent optical modules provide high performance and reliability, delivering compact, low-power 400G DCI solutions that improve network flexibility and reduce costs and complexities associated with high-bandwidth DCI. These modules enable secure, point-to-point communication and high-speed data transmission, supporting the growth of data center infrastructure. Additionally, NADDOD offers a full range of high-speed optical connectivity solutions, including transceivers and cables for HPC, AI data centers, and edge computing. Contact our experts to find the best fit for your AI network needs.