The Main Reasons And Countermeasures Of ONU's Weak Light

Introduction

The reasons for the weak light of the ONU mainly include two aspects: equipment reasons and ODN link attenuation exceeding the standard. Due to equipment reasons, it is easy to find from the network management, and generally can be found and eliminated in time. At present, the weak light of ONU is basically caused by excessive ODN link attenuation.

For the low-light ODN link, we tested the downstream optical power of the PON at the trunk optical cross and the wiring optical cross respectively, and no obvious abnormality was found. When testing at the FAT, only about 30% of the attenuation anomalies were found. As shown in the table below, it shows that about 70% of the weak-light link problems are in the home segment.

The entrance section usually uses Drop Cable, which is less than 100 meters in length, which is only about 1% of the length of the entire ODN link. Why does it produce large attenuation?

Failure Phenomenon

In "What is the difference in bending resistance between commonly used G.657 and G.652 fibers? "In the article, we have analyzed that the use of G.652 pigtails at the end of the FTTH home segment optical cable at the FAT and Optical modem is the main reason for the excessive attenuation of the home optical cable. However, about 25% of the link loss is excessive due to the bending of the Drop Cable itself.

Drop Cable uses G.657A2 type optical fiber with good bending resistance. Generally, when the bending radius of Drop Cable does not exceed 7.5mm, the macrobending loss can be basically ignored. However, the phenomenon found at the fault site is indeed related to the bending of the leather cable.

When these bends are untied, the fault disappears; if it is not the macro-bending loss, what caused it?

Failure Cause Analysis

Taking into account that in some scenarios, the Drop Cable is under stress, we tested the additional loss of the Drop Cable under bending and stress, and found that bending and stress did not significantly increase the additional loss.

Could it be related to the knotting of the fiber optic cable? Because from the perspective of the failure phenomenon, the Drop Cable at the failure point is mostly knotted. After testing, it is found that knotting will not significantly increase the additional loss. Even if the Drop Cable is knotted and subjected to external forces, the additional loss still does not increase significantly.

After carefully studying the photos of the entrance section of the field failure, it was found that in most cases, the Drop Cable was twisted. Therefore, the Drop Cable was twisted and tested for additional loss.

The additional loss of Drop Cable after twisting is still relatively obvious. When the Drop Cable is twisted, the additional loss will increase more obviously.

When the Drop Cable is twisted and knotted, an external force is applied to test:

The additional loss is as high as 3.24dB. It seems that Drop Cable twisting is the main reason for the additional loss of the residential optical cable. This is basically consistent with the situation at the fault site.

Conclusion And Suggestion

Because Drop Cable uses G.657A2 optical fiber, as long as the bending radius is not less than 7.5mm, even if there is bending or knotting, even in the case of bending or knotting, the additional loss will not increase significantly. However, if twisting occurs, significant additional loss will occur. If the knot is twisted again or even subjected to external forces, the additional loss will increase significantly. Therefore, during installation and maintenance, this situation should be avoided.