Hello,
I have built a Mach-Zhender interferomer in order to irradiate thin films of polymer (PMMA) with an interference pattern. These polymer thin films are doped with metallic precursors, and the goal is to fabricate array of nanoparticles in situ by photoreduction.
I'm not familiar with this type of system and I would like to now is there is a specific way to align, tune the interferometer? (I joined a scheme of my setup in the attached file, the quarter wave plate can be removed). I specify that I'm only trying to generate a n intensity interference pattern (no weird polarization configurations).
Firstly, at the beginning I tried to use a spatial filter, as it is usually done (I did my bibliography by looking a Surface Relief Grating inscription on azopolymer films). However I always get circular fringes at the exit of the spatial filter, I don't really understand why as I really tried to choose the right pin hole to select only the center of the airy pattern. I'm now using a beam expander instead but I'm limited concerning the size of the output beam. Would someone know what I'm doing wrong with the spatial filter?
Then concerning the two mirrors used to orientate the beams, I just align them by eyes to there are superposed on my sample? Is it ok? How can I maximise the fringe's contrast?
Thank you,
Elie
Elie,
1. It is supposed that you use TEM 00 laser and coherence length is sufficient for your sample size. It is supposed that you set interfering beams part length difference as small as possible to reduce requirement for coherence length.
2. Why do you use spatial filter ? If you use singlemode laser, no any spatial filter is required. If your laser beam have diffraction structure, first try to increase distance between laser and your optical setup. But if laser is multimode, spatial filter will not help. But beam expander is needed .
3. Circular fringes may have different nature: due to interference within lens, due to bad beam collimation, else. In any case, I beleive, spatial filter is not needed.
4. Be careful with polarization. Remove waveplates, set linear laser polarization exactly horizontal or vertical to eliminate possible depolarization at mirrors.
5. I can recommend to start with classic Mach-Zender scheme and align to get zero order picture, then increase angle between beams to get strips with larger spatial frequency. If you see high contrast strips at low frequences, you can expect to get high contrast at larger frequences.
6. I don`t know, what spatial frequency you need, but if required frequency is high - then take in mind diffraction limit - it can restrict contrast and resolution.
7. Eliminate vibrations !
Good luck !
Aleksander,
Thank you very much for your advices.
Indeed my laser is TEM 00 and is supposed to produce a nice gaussian beam so I'wont bother using a spatial filter anymore. Concerning beam expander, is it easy to "built one" so I can choose directly the magnification I want or os ot likely that I will get fringes due to misalignement?
I set the polarization as you said.
To adjust the alignement, first, I don't really know how to visualise the interference pattern.How could I do? My spot size is currently around 3 mm in diameter.
Second, if i change the angle between beams, I need to move the sample to a different area so it is in fact a new alignement no?
Thanks again,
Elie
Be sure to use a DC-coupled detector when measuring fringe contrast. Reduce the laser beam size using your beam expander ,adjust the interferometer alignment to maximize the fringe contrast in the optical signal. Then, use electronic filtering and amplification to maximize the signal for analog-to-digital conversion.
Elie,
1. You need not think about visualisation. Set good overlapping of 2 interfering beams at your target. You will obtain interference pattern : strips with spatial frequency, determined by angle between beams. You need not "visualize" this pattern. As you expect, exposure of your material by 473 nm light can change optical properties of your material and you will obtain grating (Amplitude or phase). Your 633 nm probe beam will diffract at this grating (also be careful with polarisation. Diffraction efficiency may depend on polarisation). Basing on diffraction efficiency obtained and temporal behavour of diffraction response, you can conclude, what is going on in your material .
2. Select angle
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