400G Optical Modules in Data Center Interconnection (DCI)

400G Optical Modules in Data Center Interconnection (DCI)

         According to IDC data, by 2025, nearly 80% of global data traffic is expected to be stored in core and edge servers. Meanwhile, the growth rate of east-west traffic within data centers will be much higher than that of north-south traffic and traffic between data centers. Traditional data centers are gradually being replaced by cloud data centers with the popularization of cloud computing, which has significantly stimulated the market demand for 400G optical modules.

         Typically, customer needs adjust according to the application scenario. For long-distance WDM transmission, module performance is a key factor for customers pursuing higher capacity and longer transmission distances. In contrast, for short-reach transmission within data centers, transmission cost is more critical.

To achieve higher capacity, 400G optical modules have three main approaches to reduce cost per bit:

 * PAM4 Technology: PAM4 technology effectively improves bandwidth utilization efficiency. At the same baud rate, the bit rate of a PAM4 signal is twice that of an NRZ signal, thus improving transmission efficiency while reducing costs.

 * Multi-lane: Compared to 4-lane transmission, 8-lane transmission solutions offer advantages in balancing cost and power consumption.

 * Higher Baud Rate Optical Chips: These chips increase transmission rates without affecting transmission distance. Various 25G baud rate optical chips (DML, EML, VCSEL) are gradually being upgraded to 56G baud rate chips.

          400G optical modules are very common in data center applications. For instance, the 400G QSFP-DD XDR4 module can be used in 4x100G breakout applications from a QSFP28-FR-100G. For DCI applications, the 400G QSFP-DD FR4 module supports 2km transmission over single-mode fiber. The 400G QSFP-DD LR8 and 400G QSFP-DD LR4 modules support link lengths up to 10km by transmitting four CWDM wavelengths. Additionally, the 400G QSFP-DD ER8 module, designed for longer distances, can cover 40km on G.652 single-mode fiber links. The figure below shows the optical module solutions for 400G data center networks.

         With millions of optical modules expected to be used in 5G base stations, operators urgently need to reduce the cost of optical modules in network infrastructure investments. Furthermore, optical modules in telecom transport networks must have a service life of over 10 years and support transmission distances up to 80km, imposing higher requirements on reliability and performance in metro transport network scenarios.

        To achieve higher transmission rates and lower production costs, 400G modules for integrated metro transport networks adopt technologies similar to those used in data center networks:

 * Higher Reliability Modules: Hermetic packaging is used to meet the requirements of a 10-year service life and an operating temperature range of 0 to 70°C.

 * High-Performance APD Receiver: Improved receiver sensitivity.

 * Coherent Technology: To achieve transmission distances beyond 80km, 400G solutions employ coherent technology. Meanwhile, with the development of SiP and InP integration technologies and the continuous evolution of CMOS technology, coherent modules are trending towards smaller size and lower power consumption. The low power consumption and small size of 400G ZR modules position them for wide application in metro edge access scenarios.

Coherent optical modules are evolving in three directions:

 * Spectral Efficiency: Leveraging continuous advancements in oDSP algorithms to improve spectral efficiency and single-fiber capacity.

 * Baud Rate: Increasing the per-wavelength baud rate to achieve higher per-port bandwidth, thereby reducing cost and power consumption per bit.

 * Smaller Size and Lower Power Consumption: Adopting integrated optoelectronic components, advanced manufacturing processes, and specialized oDSP algorithms to reduce the size and power consumption of 400G modules.

Conclusion

        Current mainstream 400G optical modules are already widely used in various network scenarios, including data center networks, metro transport networks, and long-distance, high-capacity transmission networks. The demands for higher capacity, lower cost per bit, and lower power consumption are driving optical modules towards even higher data rates.