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:
I have seen interferometer setups 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. This is illustrated in these diagrams:
- 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):
However, the problem is that my target object distance, from the aperture of the interferometer to the target object, varies between 10-100cm. This means that, if the optics at the aperture of the interferometer give the emitted beam a fixed focus, as an aspheric lens would, the beam would be out of focus when incident upon the target object. And, so, this would decrease the performance of my interferometer. Therefore, I don't want optics at the aperture of the interferometer that result in a fixed focus (such as an aspheric lens), since that wouldn't properly focus the beam for a varying-distance target.
But I had an idea to get around this. Since I'm using a spatial filter with aspheric lenses (see slides 13/14 here) immediately after the laser source in order to collimate the beam and get it to a high quality state, and since a collimated beam is, as I understand it, always in focus, can't I simply just emit the collimated beam over the varying distances as the target object moves (as shown in the first two diagrams), rather than having the fixed-focus optics (the aspheric lens) at the aperture of the interferometer (as shown in the last two diagrams)? It seems to me that, as long as the beam is relatively well-collimated (which it should be from the spatial filter), and as long as the distance isn't too far (which, for 10-100cm, it shouldn't be), this would mean that the beam will be in relatively good focus when incident upon the target object.
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