As data center networks move toward higher bandwidth, 400G optical connectivity has become an important option for high-speed switch-to-switch and data center interconnect applications. 400G QSFP112 modules are designed to provide 400Gb/s connectivity in a compact QSFP112 form factor. Among different 400G optical architectures, 400G FR4 stands out by combining four optical wavelengths with duplex single-mode fiber to support transmission distances of up to 2km.
At first glance, the use of a Duplex LC connector on a 400G module may seem surprising. Traditional parallel-optics solutions such as 400G DR4 use multiple fibers to carry separate optical lanes, while FR4 can transmit its four optical channels over just one fiber in each direction. The key difference is wavelength-division multiplexing, which combines multiple optical signals onto the same fiber.
So how does 400G FR4 achieve 400Gb/s transmission over a pair of LC fibers while reaching up to 2km? The answer lies in the combination of four 100G optical channels, four wavelengths around the 1310nm window, WDM technology, and single-mode fiber. Understanding these technologies makes it easier to see why FR4 is used for longer-reach 400G connections.
What Is 400G FR4?
400G FR4 is a 400Gb/s optical transceiver architecture designed for data center and high-speed networking applications. The “400G” represents the aggregate data rate, while “FR4” refers to a four-channel, four-wavelength architecture designed for a 2km-class reach.
Four 100G Optical Channels
The basic data structure of a 400G FR4 module is 4 Ă— 100Gb/s = 400Gb/s. Each optical channel carries 100Gb/s using PAM4 modulation. Instead of transmitting these four channels over separate groups of fibers, FR4 assigns each channel to a different optical wavelength.
The four wavelengths are located within the 1310nm optical window. The transmitter generates four independent 100G optical signals, and a wavelength-division multiplexer combines them into a single fiber for transmission. At the receiving end, a demultiplexer separates the wavelengths back into their individual optical channels.
This architecture allows four 100G signals to share one fiber in each direction while maintaining the overall 400Gb/s data rate.
How Does WDM Enable 400G Over Two Fibers?
Wavelength-division multiplexing is the central technology behind the FR4 architecture. Instead of dedicating one fiber to each optical lane, WDM allows multiple wavelengths to travel through the same fiber simultaneously.
Combining Four Wavelengths
On the transmit side, four 100G optical signals are generated at different wavelengths. These signals are combined by the optical multiplexer into a single optical stream. The combined signal then travels through one single-mode fiber toward the receiving transceiver.
At the other end, the receiving module uses a demultiplexer to separate the combined optical signal into four individual wavelengths. Each wavelength is then converted into an electrical signal corresponding to one 100G channel.
The process works in both directions. One fiber carries the four multiplexed wavelengths from the first module to the second, while the other fiber carries the four wavelengths in the opposite direction. This is why a 400G FR4 module can use a standard Duplex LC interface while still providing 400Gb/s bidirectional connectivity.
Why Does 400G FR4 Use 1310nm?
The 1310nm wavelength region is commonly used for single-mode optical transmission because it provides suitable transmission characteristics for data center links and other high-speed optical applications.
1310nm and Single-Mode Fiber
400G FR4 uses single-mode fiber because its intended reach is longer than many short-reach parallel-optics applications. A 2km transmission distance allows the module to connect network devices that may be located farther apart within or between data center facilities.
The use of four wavelengths around 1310nm also allows FR4 to maintain a relatively simple duplex-fiber architecture. Rather than requiring a large number of parallel fibers, the four optical channels are multiplexed onto a single fiber.
Why Can FR4 Reach 2km?
The 2km reach of FR4 is closely related to its use of single-mode fiber and wavelength multiplexing. Single-mode fiber has a small optical core that supports efficient long-distance transmission with low modal dispersion, making it suitable for high-speed links over distances such as 2km.
FR4 vs. Short-Reach Parallel Optics
A major difference between FR4 and architectures such as DR4 is how the optical channels are transported. A 400G DR4 module uses four parallel optical lanes, with each lane typically operating at 100Gb/s, and requires multiple fibers for the connection. FR4 also uses four 100G channels, but the channels are carried on different wavelengths and multiplexed into one fiber per direction.
This difference affects the physical cabling. FR4 can use a Duplex LC connection, while parallel-optics solutions generally require multifiber connectors. For applications where longer reach and conventional duplex single-mode fiber infrastructure are important, FR4 provides a different approach to delivering 400G connectivity.
What Role Does the Duplex LC/UPC Connector Play?
The physical interface is one of the most visible differences between FR4 and many parallel-optics modules. A Duplex LC connector contains two fiber interfaces, with one used for transmission and the other for reception.
Simple Two-Fiber Connectivity
Because WDM combines four wavelengths onto a single fiber, the FR4 architecture only needs one fiber for transmitting all four 100G optical channels and another fiber for receiving them. This results in a compact two-fiber connection despite the module carrying 400Gb/s of aggregate traffic.
The LC/UPC interface also provides a familiar connector format for single-mode fiber networks. This can simplify fiber management in environments where duplex LC infrastructure is already widely deployed.
Where Is 400G FR4 Used?
400G FR4 is suited to applications that require high bandwidth together with a reach of up to 2km. Typical deployments can include switch-to-switch connections, leaf-spine links, and other data center interconnects where network devices are separated by distances beyond the practical range of shorter-reach optical solutions.
For growing data centers, the combination of 400Gb/s bandwidth, 2km reach, single-mode fiber, and Duplex LC connectivity can provide flexibility when designing high-speed links across racks, rows, or different areas of a facility.
Conclusion
400G FR4 achieves 2km transmission over Duplex LC fiber by combining four 100Gb/s PAM4 optical channels with four different wavelengths around the 1310nm window. WDM technology combines these channels into a single fiber for transmission, while the second fiber carries the signals in the opposite direction.
This approach allows a 400G FR4 transceiver to deliver 400Gb/s of aggregate bandwidth without requiring a multifiber MPO connection. Its 2km reach, single-mode fiber interface, and Duplex LC/UPC connectivity make FR4 a useful architecture for high-speed data center links where both bandwidth and transmission distance are important.
