Allied Telesis AT-MCF2300 - Manual (Page 127)

AT-MCF2000 Media Converter Series Installation Guide
Section II: Installation 127
Note
Do not remove the dust cap from an SFP transceiver until you are
ready to connect a fiber optic cable. Dust contamination can
adversely affect a fiber optic port.
6. Repeat this procedure to install the remaining SFP modules.