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# FAQ - How to calculate optical power using OSA for 40G and 100G coheren lambdas

Issue Description
MCA boards and OSA analyzer, don't give the real values of power and OSNR of 40G and 100G coherent lambdas, so it is necesary calculate this values, using that values that OSA gives to calculate and setting the correct parameters to it. For this specific case, the OSA used is the JDSU 8000 series.

Alarm Information
None

Handling Process
1. Set the OSA with a resolution of 0.1 or 0.2nm.

2. Connect OSA analyzer at monitoring poing where it is necsary to take the measures and get the spectrum.

3. Scan and save the values gotten from the OSA. Centran frequency (Fc) must be taken.

4. Into the spectrum showed by the OSA, place the vertical cursor A in  (A = Fc - 0.2 nm) and the vertical cursor B (B = Fc - 0.2 nm). (Fc is the central frecuency of the lambda). For example, if the centra frecuency of the lambda is 1525.6 nm, cursor A must be placen in "1525.6 nm - 0.2 nm = 1525.4 nm, and cursor B must be placed in "1525.6 nm + 0.2 nm = 1525.8 nm as show in the picture below:

5. Get the integral power of the lambda (real power value) from A to B. In JDSU analyzer press the option  "Total Power A<->B" and the analyzer sheds the optical power value of the lambda (S+N). (For example, in the pictuare above, the optical power showed by the analyzer is -21.45dBm and this value is the signal value level + the noise value level S + N). Save that value.

6. Disconnect the lambda and scan again without change the cursors position. Then take the noise(N) power value by pressing the "Total Power A<->B" (For example in the next picture the power of noise N = 28.68 dBm).

7. The values gotten must be convert to mW units with the next formula:

8. Once there are the values in mW (S+N  and N) we must use the next formula to found the real OSNR of the signal:

Root Cause
Calibration of 10G lambdas is performed using values that directly the OSA sheds, but when there is a system with 40G/100G coherent lambdas, the values giving for the OSA for that last ones, are not the real values, so it is necesary to do the calculation included in the present document to get real values.

Suggestions
1. Before take values with the OSA, be sure that it is well configured.

2. Don't confuse the power units. (dBm, mW, dB).

3. The power values giving by the OSA are in dBm units.

END