mysterious filter action

From: smazzu (simone.mazzucato_at_roma1.infn-dot-it.no-spam.invalid)
Date: 12/01/04 

Date: 1 Dec 2004 07:29:28 -0600

dear all, I have a strange behaviour from my optical setup, and I ask
your help to understand the reason of it.
I am sending a 100fs 800nm laser beam through a polariser mounted on a
rotation stage, and an optical neutral density filter, to measure the
power transmitted after a nonlinear material and incident on a Si
photodiode. The scheme of the setup is the following one

LASER---->POLARISER---->FILTER---->SAMPLE--->PHOTODIODE

I change the polariser to change the power impinging the sample. The
photodiode follows an expected curve. As soon as I put the filter (I
have various filters, and all cause the same problem) and I return to
the optimal position of the polariser the transmitted signal is higher
than the one obtained just with the polariser, as shown in the
picture.
http://free.prohosting.com/smazzu/Immagine.JPG
The area of the photodiode is big, and the laser light is focused on
it with a lens. The problem is not related to the sample, as it
appears also changing the sample. The scale of the oscilloscope
(acquiring the signal) is kept constant, so it should not be this the
reason for this "jump".

Do you have any idea about this mysterious behaviour? for more info,
please contact me
thanks
Simone

*-----------------------*
        Posted at:
  www.GroupSrv.com
*-----------------------*