Questions about using Raman boards

Question:
Issues are likely to occur on Raman boards during deployment or maintenance, most of which are caused by customer lines and can be resolved by cleaning the optical path, replacing the ODF connectors, or fiber splicing. The following shows some examples of the questions of the customers and Huawei engineers:
1. Why is the maximum gain different before and after the RAU replacement for the same link?
2. Under certain conditions, the customer lines cannot be rectified in a short time. The actual maximum gain of the Raman board is only 5 dB or 6 dB (stable) whereas the system stability is excellent. Will the system run stably in this condition for a long time? Are there any pigtail burning risks?
3. The Raman lasers were closed before line operations, but the pigtails of some sites are still burned. Why cannot the mechanisms such as IPA or return loss detection avoid fiber burning?
Answer:
1. Why is the maximum gain different before and after the RAU replacement for the same link?
Answer: The actual gain of the RAU is closely related to fiber quality. Operators might contaminate the end faces of the pigtails or the Line port of the RAU, resulting in different end face environments before and after the replacement and different maximum gain of the RAU board. Fiber quality indicators include: a. Whether the insertion loss of long fibers is normal. b. Whether there are points of insertion loss change (detectable by OTDR meters). c. Whether the end faces of the fibers at connection points within 20 km of the near end have been burned (fiber splicing needed) or dirty (cleaning needed). Fiber end face microscopes can be used to observe whether the end faces are clean (For detailed instructions, see RAU Deployment Guide). d. Whether the pigtails at the near end have a bend radius of less than 3 cm.

2. Under certain conditions, the customer lines cannot be rectified in a short time. The actual maximum gain of the Raman board is only 5 dB or 6 dB (stable) whereas the system stability is excellent. Will the system run stably in this condition for a long time? Are there any pigtail burning risks?
Answer: No long-term testing has been carried out in Huawei R&D labs. If the actual maximum gain of the Raman board is only 5 dB or 6 dB (stable), Huawei cannot fully guarantee long-term stable running. Due to extremely high reverse output power of Raman boards, even slight abnormalities at the near end can cause the Raman amplifiers' failure to reaching the standard maximum gain. As a result, the system may not able to run stably in a long-term, leaving potential risks for future maintenance. Therefore, the customers are advised to rectify optical fibers.

3. The Raman lasers were closed before line operations, but the pigtails of some sites are still burned. Why cannot the mechanisms such as IPA or return loss detection avoid fiber burning?
Answer: Return loss detection cannot avoid 100% fiber burning. Instead, it lowers the fiber burning probability. The optics mechanism of return loss detection is that using detection light with lower power to ensure no fiber burning when the laser is turned on, and a part of the detection light are reflected to the Raman module when it reached the end face. The Raman module uses the ratio between the power of the detection light and reflection light (return loss) to determine whether the end face is normal. However, in actual situations, even the end face is already dirty, there is a low probability that the measured return loss can still be normal due to the random reflection angles, and the Raman laser will be turned on, and the fibers will be burned. To avoid these issues, the customers should be notified of the risks and requirements about Raman boards in advance. If abnormal line attenuation occurs, the customers should be encouraged to use fiber splicing for handling.

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