Hi all,
I am focusing a Gaussian beam using a High NA objective (NA=1.3). The laser is linearly polarized. I focus the laser at an oil/water interface and image the back reflected light. The image is attached. The profile is not near what you expect, i.e. asymmetric Gaussian. If I lower the NA of the objective the profile is Gaussian again which is expected. I can trap fine. At this point am not sure what is causing this. I appreciate any help.
Regards
Hi,
I found the answer
https://www.researchgate.net/publication/5808269_Reflected_image_of_a_strongly_focused_spot
Thanks for you help
Article Reflected image of a strongly focused spot
Is the objective immersed in oil? Or are you focusing from air through a layer of oil?
Do the circular fringes disappear at lower NA?
Thanks for the replies
Boris, it is focused using a high NA oil immersion objective. So the interfaces are oil, glass, water. The fringes starts to disappear as the NA is lowered.
Young, I understand where you coming from but I can trap beads using the laser as deep as 12um form the interface. From my experience with optical tweezers, if something, this is a good sign of a very well behaved Gaussian profile. My concern is that the profile is good but hat the reflected image is somehow aberrated.
I carefully align the laser using pinhole. Giving that I am not using a commercial microscope body but a home build microscope, I have access almost to the whole laser path and so far all looks good.
it is possible that its an issue with the interface. I will misalign the laser and see what happens.
When the fringes go away, do they lose contrast or does their spacing become larger and larger? You must be dealing with a parasitic reflection somewhere. If it's the former, then the two reflection planes are likely coincident (field curvatures are the same). If it's the latter, reflections probably take place on different surfaces.
Could you briefly describe the optical setup? I'm having trouble visualizing just how this is set up.
Hi,
I found the answer
https://www.researchgate.net/publication/5808269_Reflected_image_of_a_strongly_focused_spot
Thanks for you help
Article Reflected image of a strongly focused spot
From that paper, the pattern you are measuring seems to be the pattern of the cross-polarized component, so it would seem that your detector is cross-polarized - is that right?
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