Some switches like Cisco, Hpe, Brocade will offer CLI (Command-Line Interface) reference for users to view SFP modules DDM/DOM information, which includes transceiver temperature, the supply voltage in the transceiver, laser bias current, RX power TX power (mW or µW or dBm).
Log in to the switch console to run the privileged EXEC mode of the Cisco switch, use the fiber-ports-optical-transceiver command. The Output Power (mWatt) field in the command output indicates the received power of the optical module, and the Input Power (mWatt) field indicates the transmit power.
The transceiver information displayed depends on the show interfaces transceiver [port-list] detail command executed. The information in DOM information, Alarm and error information (GBIC/SFP transceivers only), and Alarm and error information (XENPAK transceivers) is only displayed when the transceiver supports DOM.
SFPSHOW command will print stats pertaining to a particular port on the switch.
The commands to show the DDM information of transceiver optics may vary among different switches. Refer to the specification before operation.
Generally speaking, there are three common methods to measure optical power strength: milliwatt (mW) microwatt (µW), and dBm. Different vendors may adopt one of them to describe signal power. For instance, Cisco switches are used to adopt mW to measure the power, while other switches get accustomed to using dBm. Since the optical power is small, sometimes microwatt (µW) is also used in some switch vendors.
There are conversions between these methods:
dBm=10*lgP (P indicates optical power, in mW.)
For example, 1 mW can be converted into 0 dBm.
1mW = 1000µW
Here are some figures recommended by EMC.
Each vendor will publish the normal operating specifications for SFPs. These are also very useful when troubleshooting faults, as operations outside of these parameters will narrow down the search.