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ROADM Turn-Up and Commissioning

After validating the amplifier and the insertion loss on all optical paths, the next step is to check the impact of the ROADM on transmission signals—especially the impact on OSNR, which should be within the acceptance level determined by the system manufacturer. In short, this means that the OSNR of the transmitted wavelengths must be tested; and this can be done using an optical spectrum analyzer.

To ensure an optimal OSNR measurement, the measurement should be performed in-band, since noise contribution can be different for each wavelength. Traditional, out-of-band OSNR measurements are based on the noise level in-between the channels, so in this application, the measurements fail. EXFOs FTB-5240 and FTB-5240B Optical Spectrum Analyzers offer this unique in-band testing feature.

Test Method Description Proposed Equipment
Optical spectrum analysis Since wavelengths originate from different networks, noise contribution can differ for each wavelength. In-band OSNR measurements with an OSA allows for error-free quantification of the OSNR levels through the ROADM for each wavelength.


OSNR: A Key Issue with ROADMs

OSNR measurements are calculated as the average between the left and right OSNRs (which are themselves measured as the difference in power between the peak power and the noise at half the distance between the peaks).

Unfortunately, the above description is not suitable for ROADMs. Let us consider, for example, the wavelengths going through the express route; since they can be remotely rerouted in any direction, they need to be demultiplexed. The wavelengths going through without being dropped are then remultiplexed. Lets examine the effects this has on the DWDM spectrum:

Figure 1. Before the demultiplexer (demux)

Figure 2. Individual wavelengths after the demux (the red line represents the filter shape)

Figure 3. After remultiplexing; The IEC-61280-2-9 measurement method is shown by the red arrow above, while the real OSNR is shown in blue. OSAs that base their measurements on the IEC method induce significant errors

The example above illustrates a 100 GHz ROADM and a 10 Gbit/s transmitter. Even though the automatic OSNR measurement is wrong, one can manually measure OSNR with visual markers. On a 50 GHz spacing device, the hump on either side of the peak visually disappears, but it is still present as noise in the channel (known as in-band noise). The same remains true for the broader 40 Gbit/s transmissions.

Therefore, the risk of error is even greater since the odd shape cannot be seen. Furthermore, with ROADMs, signals can come from different networks and can be combined (multiplexed)all these signals from different networks have different noise contributions. As a result, the interpolation of noise taken by measuring the left/right OSNRs becomes false and unreliable.