Inspecting the Optical Fiber Link
This section describes the insertion loss and reflection requirements of optical links and the method of checking the quality of optical links for the application of 50G optical modules with the PAM4 coding technology.
Tools, Equipment, and Materials
Tools and instruments for checking optical fiber links are as follows:
OTDR meter
Fiber microscope
Precautions
Because the transmit optical power of the OTDR meter is much higher than the damaged optical power threshold at the receive end, the optical fiber must be removed from the optical module when the OTDR meter is used to test the optical path quality.
Currently, the Ethernet port rate is increasing. 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.
According to the national standard (GBT50312-2016), the loss of the optical fiber link connector must meet the requirements described in Table 1.
Type |
Maximum attenuation of an optical fiber connector (dB) |
|---|---|
Fiber splicing connector |
0.3 |
Optical mechanical connector |
0.3 |
Optical connector |
0.75 |
Fiber cores are connected through connectors, such as the ODF, optical attenuator, and flange, in splicing and mechanical modes.
Table 2 describes requirements for the reflection of the optical fiber connector when Ethernet ports (such as 50G) use PAM4 encoding to double the rate. More connectors bring lower requirements for the reflection.
Procedure
After the optical fiber at the peer end is disconnected, use the OTDR meter to test the local end. Check whether the loss and reflection of each link and node are normal. (The loss of a fiber splicing connector should be less than 0.3 dB, the loss of a connector should be less than 0.75 dB, and the reflection of a connector should be less than -30 dB.) If the test result is not within the required range, process the abnormal port.
Locate the equipment room where the port resides based on the distance between abnormal points in the OTDR test result. Preliminarily determine the port location, disconnect the port, and perform an OTDR test on the port that reports alarms. Check whether the distance is consistent with that in the previous test. If not, continue to test other ports.
After the abnormal port is found, test the port using a fiber microscope. If the port is dirty, clean it. For details, see "Inspecting and Cleaning Optical Fiber Connectors and Adapters".
After the port is cleaned, restore the port, and ensure that the connector is tightened. Perform an OTDR test on the port to check whether loss and reflection of each link and node are normal.
If the fault persists, replace the flange and perform an OTDR test on the port that reports alarms to check whether loss and reflection of each link and node are normal.
If the fault persists, replace the optical fiber and perform an OTDR test on the port that reports alarms to check whether loss and reflection of each link and node are normal.
If multiple abnormal points exist on the link, repeat steps 2 to 6.