You can check the light reception (RX power) of an optical module using Digital Optical Monitoring (DOM) or an optical power meter to measure the received signal in dBm.

Using Digital Optical Monitoring (DOM/DDM)

Many modern optical modules, including SFP, SFP+, and QSFP, support DOM/DDM, which allows real-time monitoring of transmit (TX) and receive (RX) power levels directly from the device. To check RX power on a Cisco switch, for example:

1. Ensure the module supports DOM and is recognized by the switch (show inventory).
2. Use the command show interfaces transceiver detail (Catalyst) or show fiber-ports optical-transceiver (SMB switches) to view RX and TX power in dBm.
3. Compare the RX power against the module's receiver sensitivity and warning thresholds. Values that are too low indicate weak signal reception, potential fiber contamination, or misalignment . RX power is measured in dBm, where 0 dBm = 1 mW. More negative values indicate weaker signals (e.g., -20 dBm is weaker than -5 dBm), .

Using an Optical Power Meter

For modules without DOM or for independent verification:

1. Disconnect the fiber from the receiving module.
2. Connect the fiber to a calibrated optical power meter.
3. Measure the received optical power in dBm.
4. Compare the reading to the module's specified minimum receiver power to ensure proper operation .

Additional Troubleshooting Tips

• Inspect fiber connectors: Dust or damage can reduce RX power. Clean connectors and patch panels before testing .
• Verify fiber type and wavelength: Ensure both ends of the link use compatible transceivers and fiber types.
• Loopback testing: Connect the module's TX and RX ports with a loopback cable to confirm the module itself is functioning correctly .
• Check for alarms: Many switches report RX power warnings or alarms if the signal is outside acceptable ranges . By combining DOM readings, optical power meters, and proper physical inspection, you can accurately assess the light reception of an optical module and troubleshoot weak or unstable fiber links effectively.