400G Optical Interconnect Solutions for Metropolitan Area Networks

NADDOD Jason Data Center Architect Aug 9, 2023

NADDOD offers a range of 400G optical transceivers specifically designed for metropolitan area networks (MANs). These transceivers include the 400G LR8, 400G ER8, and 400G ER4 Lite, which comply with the 400GBASE-LR8/ER8 protocols defined in the IEEE 802.3cn standard. They provide comprehensive coverage for long-distance, high-speed optical interconnect applications ranging from 10km to 40km.


Now, let's delve into the design architecture and key technologies of these optical transceivers.


400GE-LR8/ER8/ER4 Lite Optical Transceivers

The 400GE-LR8/ER8/ER4 Lite transceivers consist of transmitter optical subassemblies (TOSAs), receiver optical subassemblies (ROSAs), EML drivers, PAM4 business chips (oDSP), controllers, and other components. The system diagram below illustrates the transceiver design. NADDOD strictly controls critical underlying components, covering end-to-end vertical integration solutions from optical chips (EML, PD, APD) to electrical chips (drivers, TIAs, oDSP).


System Diagram of 400GE-LR8/ER8/ER4 Lite

System Diagram of 400GE-LR8 ER8 ER4 Lite



Transceiver Connector receives 8-channel parallel 26.5625Gbps PAM4 electrical signals from the board. These signals are adaptively processed by the internal digital signal processing chip (oDSP) within the optical transceiver. After amplification by the driver, they drive 8 semiconductor lasers (TOSAs). The 8 semiconductor lasers output stable laser signals, which are then combined through a MUX and output, achieving 8-channel electrical-to-optical conversion.


Upon receiving optical signals, the transceiver decomposes them into 8-channel modulated optical signals through a DeMUX device, which enter the optical subassemblies (ROSAs). ROSA converts them into electrical signals, directly inputting them into the internal digital signal processing chip (oDSP) of the optical transceiver. The oDSP chip processes the received 8-channel parallel 26.5625Gbps PAM4 electrical signals digitally, ultimately achieving 8-channel optical-to-electrical conversion.


Key Technology

To ensure technological leadership in the optical transceiver, NADDOD's 400GE-LR8/ER8/ER4 Lite optical transceivers primarily adopt the following key technologies:


High-integration MLIC Chip

Compared to 100G and 200G optical transceivers with 4-channel lasers, 400G modules for telecommunications adopt an 8-channel laser architecture. In the QSFP-DD package, it requires small package volume and low power consumption for optical devices.


Additionally, to meet the link budget requirements of LR8 and ER8, high light-emitting efficiency and low power consumption are necessary for the transmitter lasers.


NADDOD's 400G optics utilize the Multi Laser Integrated Chip (MLIC) integrated chip. It integrates multiple lasers, Monitor Photodiodes (MPDs), MUX, and other building block transceivers. The chip employs novel laser designs and processes suitable for multi-channel integration, complex multi-channel RF designs and processes, and low insertion loss MUX designs and processes.


Compared to traditional discrete optical devices, it saves 80% discrete components, improves manufacturing efficiency by 70%, and achieves high light-emitting efficiency. The diagram below illustrates the concept. Additionally, the MLIC chip, combined with NADDOD's high-bandwidth EML Driver, enables 400G high-bandwidth transmission.


Traditional Discrete Device Solution vs. Integrated Solution

Traditional Discrete Device Solution vs. Integrated Solution



High-performance Receiver

The IEEE 802.3cn standard defines the link budgets for 400GBASE-LR8/ER8, requiring a link budget of 6.8dB for LR8 and 18.5dB for ER8. LR8 requires a receiver sensitivity of <-6.6dB, while ER8 requires a receiver sensitivity of <-16.1dBm. Achieving high receiver sensitivity poses significant challenges to the core components of the receiver, namely PIN and APD.


The performance of a 25G PIN receiver is typically around -7dB, which is insufficient for transmitting over 40km. Long-distance transmission requires the use of high-performance Avalanche Photodiode (APD) receivers.


NADDOD's 400G optical transceivers utilize newly developed APDs. Compared to traditional APDs, these APDs employ a new type of ultra-low noise material as the multiplication layer material, reducing the system's noise figure (NF) and improving the receiver sensitivity by 2dB. They also incorporate innovative structural designs and processes to enhance the APD's bandwidth.


Combined with NADDOD's independently developed high-bandwidth linear TIA, the overall system bandwidth and sensitivity are improved, meeting the high-performance requirements of 400G-ER8 optical transceivers.


High-performance APD based on New Materials

High-performance APD based on New Materials



High-performance oDSP

In the industry, commonly used PAM4 oDSPs are typically based on a 16nm process technology. However, the 400GE-LR4/ER8/ER4 Lite transceivers utilize a more advanced 7nm PAM4 oDSP. This advanced process technology not only reduces power consumption by over 30% but also reduces the size of the modules.


The oDSP architecture follows the ADC+DSP route, enabling precise link compensation and maintaining maximum compatibility. Additionally, the oDSP provides a rich set of powerful maintenance features, including SNR monitoring, link transfer function estimation, and BER monitoring, significantly improving the operational efficiency of fiber optic links.


Moreover, the oDSP offers precise delay measurement with an accuracy of up to 1ns, supporting high-precision synchronization requirements such as 1588, which is essential for applications like 5G. This greatly enhances the versatility of the optical transceivers.



What is clear through this article is that NADDOD's 400G optical interconnect solution, with its high-performance receivers and oDSP technology, offers significant advantages in meeting the demands of urban area comprehensive network infrastructure. The adoption of 400GE-LR8/ER8/ER4 Lite optical transceivers, equipped with advanced APDs and 7nm PAM4 oDSPs, demonstrates NADDOD's commitment to pushing the boundaries of optical interconnect technology.


With improved receiver sensitivity and enhanced link compensation capabilities, NADDOD's solution enables reliable and high-speed data transmission over long distances, catering to the requirements of various high-bandwidth applications. The integration of cutting-edge materials, innovative designs, and advanced processes ensures optimal performance, power efficiency, and compact form factors.


Furthermore, the oDSP's extensive maintenance features, including SNR monitoring, link transfer function estimation, and BER monitoring, enhance the operational efficiency of fiber optic links, making network management and troubleshooting more streamlined. The precision delay measurement capability also supports high-precision synchronization needs, addressing the demands of emerging technologies like 5G.


NADDOD's 400G optical interconnect solution stands at the forefront of technological advancements, empowering organizations to build robust and scalable networks capable of meeting the ever-increasing demands of modern data-intensive applications. With its focus on performance, reliability, and operational efficiency, NADDOD continues to drive innovation and shape the future of optical interconnect solutions.