Infinity corrected lens - expanding before tube lens?

donroy81

Hello folks, I am on a project that involves combining Raman microscopy + optical tweezers, and I made a setup over the past year. All of this is home built on minimal funds so its not the neatest of all setups. It involves an upright microscope (coupled to trapping laser) and collecting the back reflected Raman signal at 180^o and also the optical image along the same axis.

To make things easy i converted to a NIKON infinity corrected 100x objective and added a tube lens. While the back reflected laser beam from the laser seems to be collimated when the sample is at focus, the white light (illuminated from the bottom) diverges very fast so I catch only 25% of l the light on my tube lens (which needs to be 200 mm away anyway). Should not the white light collected be the same size as the back aperture of the objective (8mm) : I need to catch all the light on 25mm dichroic filters (which eliminate the laser).

Why is the white light expanding out of the objective - Is something wrong? We are a spectroscopy group and not a microscopy one, so all this is pretty new to me and my research group. Any help is appreciated.

Andy Resnick

How are you using the white light? Is there a condenser (a lens that focuses the light onto the sample)?

donroy81

Hi Andy, thanks for replying. I have a 10x (.25NA)objective as a condenser, I was using a 1.25 NA condenser before with the same effect.
I know its not Kohler illumination (since the objective is 1.25 oil) but this is also what Thorlabs uses in their "optical trapping kit".

Andy Resnick

I'm not surprised that the white light is diverging- it has to, in order for the objective to image objects that are off-center. Some of the diverging light may be 'spurious', in that it would normally be rejected by the tube lens- objectives have a limited field of view. Have you (without the trapping beam) simply looked at the image your setup produces?

I'm not sure how much control you have over the divergence angle (does it change between using the 0.25 NA and 1.25 NA condensers? Can you ring up a field stop?), so one solution may simply be to move the tube lens closer (or use a larger diameter tube lens). AFAIK, the tube lens on my Leica is about 100mm off the back aperture of the objective.

donroy81

Andy, sorry for the delay, I wanted to review everything once. I also went over your conversation with Kubikat. I would need to show you some pictures to explain. Slide1 is a closeup of the problem region.I have 1" optics, the filter and the mirrors before the tube lens. DSC09636 gives you another take on it. Everything was fine when I was using a 160mm objective and a complete Nachet, we decided that the infinity will help with the raman and hence this. 2" dichroics run too many $$. In anycase, I dont see why a parallel flux of light from an infinity objective with a back aperture would expand so fast to 2".

Additionally, if I may, I would like to know a few more things, for starters : how do you check that the trapping beam is on center of the back aperture of the objective and perfectly perpendicular to it?

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