intensity noise measurement of fs laser

Hi guys,
Anyone has experiences of measuring intensity noise (IN) of fs laser?
I am now try to measure IN of a mode lock fs laser and IN after
supercomtinuum generation (SGC). I used a si PD from Newfocus with 125M
bandwidth and set it directly face to lthe aser. The DC was monitored
by OSC and AC is monitored by a RF Analyzer.

The weird thing was:
when DC was low like below 100mv, the noise floor was increased with
DC and flat;
when DC was increased further, the noise floor began to go down and
then increased again. The nosie floor also began to become unflat. A
big dip was shown around the first harmonics. The maximum DC I can saw
was about 3 V. I do not think the PD is saturated at that time.

I know the IN is come from shot noise and laser IN. In order to
measuring IN, the power on PD should be high. But why the noise floor
began to go down?

If the laser wavelength and SCG wavelength is very different.In my case
laser is at 1.05um and SC is at 0.8 um. How can I compare their IN? I
mean I should control the average power on the PD or keep the same PD
current ?

Thanks a lot

Hui

.

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