Amplitude modulation sidebands


Hello everybody,

I am modulating a CW laser using an optical amplitude modulator with a sine wave (see attached file). Even if I increase the number of samples in the optical spectrum analyzer, I can not see the sidebands corresponding to the modulation if the frequency modulation is below 400 MHz. Is it due to numerical limitations?

Thank you very much in advance,

Best regards,

modulation.icp (134.0 KB)


Hi @Anna,

Indeed it is a resolution limitation. By default, we use short format to plot figures, so when the sidebands are too close to the carrier frequency, you won’t be able to see them appear in the plot. One possible solution is to lower the bit rate; the attached file modulation_modified.icp (159.4 KB) is what I set the bit rate to 2.5e9 GHz, and you can see the sidebands in the OSA, but the frequency axis resolution is not high enough to tell the actual values. However, even though the frequency points are collapsed together, you can still see a difference in the sidebands with different modulation frequencies. The following figure plots the power spectrum for a 0.4 GHz modulation signal and a 0.5 GHz modulation signal, respectively.

To get the accurate frequency points in long format, you could get the results from the analyzers and print them in the script prompt window using format long:

powerSpec = getresult("OSA_1", "sum/signal");
freq = powerSpec.Frequency;
format long;

I hope this could help :slight_smile:


Hi Guanhui,

It’s just what I needed. Thank you very much! :slight_smile:

Best regards,