400G FR4 stands for 400 Gigabit Ethernet Four-wavelength 2 km Reach. A 400G single-mode optical interface multiplexing four 100G PAM4 wavelengths on duplex fiber for reaches up to 2 km. The term is most useful when a project connects that definition to a concrete use case such as Data center interconnects up to 2 km.
For 400G FR4, interoperability and acceptance start with the governing specification. The applicable IEEE 802.3, ITU-T, or multi-source agreement defines lane count, wavelength, fiber, reach, and interoperability assumptions. Project documentation should identify the edition, host assumptions, and any vendor-specific limits that apply.
A useful boundary is the difference between 400G FR4 and DWDM. The former describes a 400G single-mode optical interface multiplexing four 100G PAM4 wavelengths on duplex fiber for reaches up to 2 km. The latter describes a dense wavelength multiplexing technology that provides many closely spaced channels for high-capacity metro and long-haul networks. Keeping that distinction clear prevents an interface, performance value, or commercial condition from being assumed where it was never specified.
Before approving 400G FR4 for Data center interconnects up to 2 km, check lane count, wavelength plan, fiber type, connector, reach, FEC assumption, and optical power budget. The final decision should be based on the complete link or commercial requirement rather than a familiar label such as ER / ER4.
Frequently Asked Questions
How is 400G FR4 different from DWDM?
400G FR4 refers to a 400G single-mode optical interface multiplexing four 100G PAM4 wavelengths on duplex fiber for reaches up to 2 km. DWDM instead refers to a dense wavelength multiplexing technology that provides many closely spaced channels for high-capacity metro and long-haul networks. They can be related in one project, but one does not prove compliance with the other.
Where is 400G FR4 normally used?
Typical use includes Data center interconnects up to 2 km. The exact implementation still depends on the host equipment, link design, environment, and governing specification.
What should a 400G FR4 specification include?
It should state lane count, wavelength plan, fiber type, connector, reach, FEC assumption, and optical power budget. These details allow a supplier or engineer to verify the requirement instead of relying only on the term 400G FR4.
Need help selecting a compatible transceiver?
Tell us your target platform, data rate, fiber type, and transmission distance.