As 400G/800G speeds become standard in data centers and PAM4 modulation technology is widely used, the traditional rules for using fiber optic connectors have been completely broken. The old rule of "using APC for single-mode and UPC for multimode" has now been gradually replaced by the combination of MPO multimode + APC.
As 400G/800G speeds become standard in data centers and PAM4 modulation technology is widely used, the traditional rules for using fiber optic connectors have been completely broken. The old rule of "using APC for single-mode and UPC for multimode" has now been gradually replaced by the combination of MPO multimode + APC.
Why are the traditional rules of "multi-mode + UPC" no longer applicable?
Before 2020, data center fiber connector selection followed a simple rule: single-mode fiber used green APC connectors, and multimode fiber adopted off-white/black UPC connectors, which suited low-speed operation well.
Yet as 400G/800G high-speed links and PAM4 modulation gained popularity, the scenario shifted. Unlike traditional NRZ with two signal levels, PAM4 uses four levels and is far more sensitive to signal interference. Tiny light reflections easily trigger noise and bit errors, reducing link tolerance.
This upgrade pushed multimode connectors to shift from UPC to 8° angled APC. Now, multimode + APC has become the new standard for high-speed data center connections.
The two key advantages of MPO multimode + APC
Extremely low reflection ensures signal transmission
Return loss is a core indicator for evaluating the performance of fiber optic connectors. The higher the return loss value, the less light reflection and the less signal interference. The return loss of traditional multimode UPC connectors is about 30dB, while the return loss of APC connectors can reach over 60dB, almost achieving a "zero back reflection" effect.
Adapting to extreme environments
Data center cabling environments are complex, and issues such as connector contamination and mating deviations are difficult to completely avoid. APC's 8° angled design effectively addresses this problem. Unlike UPC's planar contact, APC's angled design guides stray light to the fiber cladding, away from the laser emitter. Even with slight connector contamination or imperfect mating, transmission performance is not significantly affected. This feature greatly reduces the frequency and difficulty of connector cleaning and maintenance, thus reducing operational costs.
The widespread adoption of MPO multimode + APC is an inevitable result of data centers evolving towards higher speeds, higher densities, and higher reliability. For industry professionals, timely understanding of this technological shift and accurate selection and maintenance are crucial to gaining a competitive edge in the 400G/800G era.
