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Testing the ROADM


Commissioning the ROADM implies testing the OLA, the express route, as well as the dropped and added signals (all the different path light can take in the WSS).

Test Method Description Proposed Equipment
Optical spectrum analysis Optical spectrum analysis serves to quantify the gain and noise figure of the amplifier, as well as the insertion loss per optical path in the ROADM. It also calculates flatness, noise figure and gain, plus our units do this automatically.
With tunable laser The tunable laser source provides the optical signal required by the OSA to test ROADMs, with a capacity to sweep through the C- and L-band wavelength ranges. Resolution and a signal-to-spontaneous-emission ratio (SSER) are key specifications.
FLS-2600B Tunable Light Source
Variable optical attenuation Variable attenuation adjusts and controls the power level of the optical signal from the tunable laser source being injected into the ROADM or system under test.

FVA-60B Variable Attenuator
Testing the optical line amplifier (OLA) lightpath

The OLA lightpath is tested using an optical spectrum analyzer (such as EXFO`s FTB-5240) and a tunable laser source (EXFO`s FLS-2600) The purpose of this process is to test the OLA while the system is looped back upon itself (east-out into west-in, for example). The OLA lightpath must be tested at least at the first and last DWDM channel (e.g., tested at DWDM Channel 1 and DWDM Channel 44 in a 44-channel system), but testing at all channels is often preferred. A variable optical attenuator (EXFO FVA-60B) may be required to make sure that the power entering the OLA is representative of what will be present during live traffic operation.

Testing the wavelength-selectable switch (WSS)

These tests are used to verify the optical signal level for any band, half band or express port pass-through path. A tunable laser source (TLS) is connected to the main input, while the OSA is connected to the corresponding main output. The loss per channel as the TLS sweeps it should be within the published specifications of the ROADM, and all separate channel losses should be within 1 dB. All ports should be tested: used, unused as well as the express port. This is an extremely important test as the whole idea of the power meters and variable attenuators in the ROADM is to ensure that all exiting wavelengths have similar power (within typically 1 dB). This means that isolation loss that is too high on any of the express, add or drop ports, will automatically make all the other attenuators adjust, and power levels of all other wavelengths will be reduced, thus losing usable power and potentially increasing optical signal-to-noise ratio (OSNR).