There are errors in the setting of the Meter ONT-30, which causes optical signal-to-noise ratio test failure.
Symptom
There are four wavelengths added, dropped or transmitted transparently on station A. The four wavelengths are equalized before entering the FIU. When meter ONT-30 is used to test the RC and OUT interfaces on the FIU board, it is found that the difference of optical signal-to-noise ratio (OSNR) between the four wavelengths reaches about 6 or 7 dB after they pass through the FIU, while the optical power does not change. Remove the fiber from the RM interface on the FIU, the OSNR becomes flat.
Cause Analysis
Before the four wavelengths are received by the FIU, the optical power and OSNR are even. The difference between the OSNR before a supervisory channel is used and the OSNR after a supervisory is used is 6-7 dB. Therefore, the possible causes are as follows:
- The FIU board has poor performance. As a result, the supervisory channel affects the OSNR of the service channel.
- The output signal of the SC2 board is of poor quality. As a result, the OSNR of the service channel is affected.
- There are errors in the setting of the Meter ONT-30.
Procedure
- The FIU is a pure optical device. The possibility that the FIU is faulty is slight. When the RC and RM interfaces of the FIU are connected in another direction, the problem persists. That is, the supervisory channel still affects the OSNR of the service channel. Hence, the FIU is not faulty.
- Interchange the TM1 and TM2 interfaces of the SC2, and the problem changes. The signals from the TM1 interface can affect the OSNR while those from the TM2 interface cannot. Hence, the SC2 might be faulty. However, after the TM1 interface is tested, it is found that each index of TM1 interface is compliant with the standard. The evaluation on the optical power shows that the TM1 interface cannot cause a difference of 6 or 7 dB between the service channels. The problem cannot be caused by the SC2. Hence, the problem may lie in the setting of the meter.
- Meter ONT-30 performs always automatic scanning and is never manually set. It seems impossible for the meter to have any problem. However, after compare the parameter of the meter when the OSNR is flat with that when the OSNR is not flat, it is found that the parameter setting of OSNR DISTANCE changes from 50 GHz to 175 GHz. This might be the problem cause. Set the parameter to 50 GHz manually and perform the test again. The problem is cleared. That is, the OSNR of each service channel from the FIU is flat. Why does the parameter change? Compare the output of TM1 and TM2 interfaces. It is found that there is a spurious wavelength with the optical power of -39 dBm in the output of TM1 interface. There is no such spurious wavelength in the output of TM2 interface. In this case, the output side mode suppression ratio of the TM1 interface is compliant with the standard, but the spurious wavelength has lower SNR. This causes the change of OSNR DISTANCE.
Reference Information
Meters used in a WDM equipment test becomes more intelligent. However, you should be cautious of the parameter setting, because automatic scanning is not always reliable.
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