I am working on reflected microscopy. I want to focus both laser beams and white light on the sample - laser beam for producing some structural changes and white light (halogen lamp) for back reflection microscopy.
What I see is that the laser focuses earlier than white light. Is there any solution for this, which focuses white light and laser light (lambda 800nm) at the same point, so that I see the in-situ optical damage?
Hi Zia,
It may be tricky! I faced very similar problem when I wanted to see a location of a fs laser spot on a sample in reflection mode with one significant difference! I wanted to align the spot on a sample in a low power regime (for not having an interaction between the sample and the laser) and then confirm the details of the damage etc. after increasing the power. For this purpose, I developed an additional arm with a drop-in beam splitter that would redirect white light to a video camera. A drop-in beam splitter was a cubic beam splitter which one normally wouldn't keep in the optical path of fs laser because of beam distortions (aberrations, broadening etc.) A second beam splitter was used for white light to enter the optical path that was shared with a fs laser.
So, I literally put it in and out before and after the laser-sample interaction. Worked for me. Yes, I couldn't see the damage etc. in real time but immediately after the interaction. Of course, it requires for you not to move the sample and change the alignment. In short, I was happy with it. Hope, it helps.
How about your resolution ?i mean how much was your beam spot diameter?my beam diameter is 1 micron.Secondly did your white light was collimated properly or it was quasi-collimated?
My laser spot was about 1 um in diameter. White light source spot size - have no idea! White light wasn't collimated properly since I really didn't care about the quality of the image but some representative picture. See the attached picture for the set-up. It also shows a typical image taken from the camera that displays the fs laser spot on the sample (TEM mask on Au film).
However, I noticed that all thin-plate beam spiltters that were kept in the set-up all the time lead to beam aberrations (which is obvious). So, fs spot wasn't diffraction-limited.
You could also try an achromatic lens as the focusing optic, which maintains focal point in a range of wavelengths.
I faced a similar problem. In my case it was sufficient that the focused laser spot is exactly at the surface when I observe a sharp image of the surface, so I clearly observe the effects of the laser on the surface. (Note, this does not imply that you will observe a sharp laser spot on your camera).
To achieve this, you should try to focus the laser on the surface, looking for example at damage, two-photon fluorescence, ... and afterwards (without moving the objective lens) refocus the white light image of the surface by shifting the tube lens in fron of the camera.
Alternatively, shift the axial laser focus position with the help of a telescope (I guess you have already one in your setup to increase the beam diameter of your laser).
The focus of a laser beam always deviates a little of the given focus of the lens (even specified for this wavelength). It is something like df=(zR)^2/f (zR is the Rayleigh length and df the difference in the focal length, see Siegmann Lasers p. 678). The Rayleigh-length for the white light (if you define something like this for the "white" light) is different to the one of the laser light. But this shift is normally rather small...
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