Huawei 3 U Quick Installation Manual page 22

Appendix Inspecting and Cleaning
Optical Fiber Connectors and Adapters
Since the 50G optical module link uses the PAM4 encoding technology,
there are higher requirements on the optical fiber and cable quality
and the link is more sensitive to multipath reflection interference of
signals. If the fiber link connector, fiber section, or fiber splicing surface
is dirty, optical signals are reflected back and forth on the fiber link,
causing interference due to co-channel noise on the receive side. As a
result, the optical link is unstable or intermittently disconnected. To
prevent this issue, you need to check and clean the optical fiber
connectors before installation. For details, see Installation and
maintenance > Preparing for the installation> Inspecting and
Cleaning Optical Fiber Connectors and Adapters in the product
documentation.