40G wavelengths are unavailable due to inappropriate polarization mode dispersion (PMD).
Symptom
An OptiX BWS 1600G network transmits both 10G and 40G wavelengths. The network diagram is shown in the following figure. 40G wavelengths are transmitted from station A to station C. They pass through station B. 10G wavelengths are added and dropped at station B. 40G wavelengths in two directions between station A and station C are unavailable. Performance of the 10G wavelengths between station A and station C and those between station A and station B fluctuates and the BER before correction exceeds two orders of magnitude; however, performance of the wavelengths between station B and station C is normal.
Cause Analysis
The possible causes of the performance fluctuation of the wavelengths are as follows:
- The PMD is excessively high.
- The optical amplifier boards work unstably.
The receive optical power on the OTU board at station C is stable. Therefore, the possibility that the optical amplifier boards work unstably is excluded. The measured PMD on the optical fibers during network design is low. In addition, the optical fibers are not rerouted. Therefore, it is preliminarily determined that the PMD is inappropriate.
Procedure
- Perform self-loops on the 40G OTU boards at stations A and C. The result is normal. This indicates that the 40G OTU boards work normally. The BERs of the 10G OTU boards fluctuate but there is only a slight change in the receive optical power. It is concluded that both the 10G and 40G OTU boards work normally.
- Check the total optical power and the optical power of each wavelength on each optical amplifier board along the links. The result is normal and the OSNR at the receive end reaches the specified value. This indicates that the links in the network are normal.
- Add a 40G board at station B. That is, add a 40G wavelength from station A to station B and from station B to station C each. The 40G wavelength from station A to station B is still unavailable but that from station B to station C is normal. It is concluded that the link from station A to station B is abnormal.
- Check the DCM modules at each station on the link from station A to station B. All DCM modules are properly connected. Therefore, it is determined that the PMD may be inappropriate.
- Measure the PMD span by span from station A to station B. It is found that the PMD of the optical fibers in the transmit and receive directions in a span is much higher than the design value. After the optical fibers in the span are replaced, the bidirectional 40G wavelengths between station A and station C are available.
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