I am designing an interferometer for an experiment. The setup consists of (1) the laser source, (2) the interferometer itself (consisting of optical components and photodetector(s)), and (3) the target object. Once the laser source emits into the interferometer setup, it first encounters a spatial filter. This spatial filter converts the low-quality, rapidly diverging beam of the laser diode into a high-quality, collimated beam. Once the beam exits the spatial filter, it enters a beam splitter. This beam splitter allows part of the beam to be emitted at the target, and part of the beam to go deeper into the interferometer setup. The interferometer then depends on light reflected back into the interferometer from the target object.
- Light being emitted at the target object:
- Light being reflected from the target object back into the interferometer:
One of my main concerns is managing beam quality in this setup. As I mentioned, I'm thinking of using a spatial filter and aspheric lenses at the point of emission (directly in front of the laser diode) to increase the beam quality (see slides 13/14 here). But I'm wondering about the light reflected back into the interferometer from the object: how does the quality of this reflected light compare to the spatially-filtered (again, see the slides) light being emitted from the setup (the light that is incident upon the object in the first place)? And how do I manage this (reflected) light quality well?
I have seen interferometer setups (see edit 2 below) that have an aspheric lens at the aperture of the interferometer system. The aspheric lens seems to be orientated so that the light leaving (being emitted from) the interferometer is focused, and the light entering (reflected back into) the interferometer is collimated. So, based on this design choice, I'm inferring that there does indeed seem to be some concern about collimating the returned/reflected light from the object. However, from what I can tell, doing this with my setup would defeat its purpose (the purpose of my setup), since it would then take the spatially-filtered, high quality light, and then emit a focused beam at the object, rather than just emitting my collimated, high quality beam. (See edit 3 below.)
I'm trying to think about how to get around this problem. One idea I had was to use two aspheric lenses (rather than one), orientated as shown here:
Using this setup, my spatially filtered, high quality light would enter the first aspheric lens, become uncollimated, and then enter a second aspheric lens, which would then re-collimate it before it leaves to be emitted at the object. Inversely, the reflected light from the object would enter the first (from the outside) aspheric lens, and then enter the second, which would collimate the reflected light before it enters the interferometry setup for the purpose of interferometry. However, I'm not sure if this idea makes sense: is the progress I made on improving the quality of the beam at the earlier point in the system with the spatial filter lost when the first aspheric lens focuses the light (in particular, is the improvement in the quality of the wavefront maintained, which, as I understand it, is particularly important for interferometry)? Or, perhaps, am I going about this the wrong way, and I might actually need to place a second spatial filter (and the accompanying two aspheric lenses) at the aperture of the system (since, as I understand it, spatial filters do the same thing from both ends)?
As you can see, fundamentally, what I'm trying to do here is maintain good beam/light quality in all parts of the system, including for the light that is reflected from the object and back into the interferometer, in order to maximise (within reason) interferometer performance.
EDIT 2
As an example, see how there's an objective lens focusing the emitted beam from the interferometer at the sample:
(From https://en.wikipedia.org/wiki/Interferometry#Biology_and_medicine )
And a more clear example:
(From https://pubs.rsc.org/en/content/articlelanding/2019/ay/c9ay00369j/unauth )
This is what I was referring to with regards to the single aspheric lens: the light being emitted from the interferometer is focused at the object, and the light reflected from the object back into the interferometer is collimated.
- Light being emitted at the target object (now including an aspheric lens):
- Light being reflected from the target object back into the interferometer (now including an aspheric lens):
EDIT 3
From this article:
Imagine that you have a collimated laser beam which has a clean Gaussian intensity profile. Now you use a lens to focus that beam down to a small spot. Will you obtain a nice Gaussian beam shape also in the focus?
I suppose that many people would indeed expect that, as they believe that the focus is basically just a demagnified copy of the original large beam. Therefore, they believe that it is sufficient to inspect the profile where it is large – which is often much easier to do than inspecting the beam around focus. (Particularly for Q-switched lasers, it is inconvenient to inspect the focus, as the intensities are very high, and it is not always trivial to attenuate a beam while preserving its profile.)
Unfortunately, the described expectation is utterly wrong. Without inspecting the focus itself (or knowing the details of your beam source), you don't really know your beam. The beam profile may contain phase distortions which you cannot see by looking only at intensity profiles. While the beam is collimated, such phase distortions may have only negligible effects. Once you get to the focus, however, they become important. In particular, you may get a non-Gaussian beam shape there, and the beam radius may be significantly larger than you expect based on a calculation for Gaussian beams.