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FWHM Bandwidth with and without Optical Noise

Question asked by jtsou on Feb 1, 2010
Latest reply on Feb 2, 2010 by jtsou
I'm trying to understand some measurement I have made. I have a laser source and a relatively flat broadband source coupled with a 3dB coupler. I also have a variable optical attenuator to control the relative level of the broadband optical "noise". I have the OSA resolution bandwidth set to 0.1 nm.

First, I measure the FWHM maximum of the laser source by itself. Next, I turn on the broadband source and adjust the VOA to get close an OSNR of about 10dB. Now, when I measure the FWHM of the spectrum, it is wider than when I measured the laser by itself.

I can't think of a reason why adding a constant noise level would affect the measured bandwidth of the signal. Comparing my two measurements without and with noise, shouldn't each measurement point in the former case simply have a constant level added to it in the latter case? Why does the bandwidth broaden? Can anybody explain why this happens?