I don't understand it either, it's simply something I found while googling the subject in the hopes that it would help you.
What figures can be considered as good optical signal strength and bad optical signal strength?
It does give you normal ranges for tx and rx or transmit and receive power.
transmit optical power the normal range (0.5 dBm to +5 dBm)
receive optical power the normal range (-27 dBm to -8 dBm)
