In my previous post on hydrogen, sodium and neon light sources, I mentioned we would use these to test existing etalons, but the plan was also to experiment with our own home-made ones. Here is a progress report on our initial experiments. I reference first of all a nice article by Thomas Moses [Am. J. Phys. 83 (7), July 2015] whose students developed several Fabry-Perot interferometers for spectroscopy experiments. The approach is to use commercial beam splitters and mirror mounts. We chose the Thorlabs BSX10 beam splitter (90:10 reflectivity/transmission) and the Thorlabs KS1 precision kinematic mirror mount. The beam splitters are slightly wedged (0.5 degree) and have an AR coating on the non-reflecting surface. The mirror mounts are attached to a pair of ADM dovetail adapters (VPA), which can slide on a short Williams Optics dovetail (DSD 110). This allows the spacing of the mirrors to be varied from around 2mm to 20mm. By trial and error, I found that a gap of 3.0mm worked well.
Initial tests were with the low pressure sodium lamp. The sodium doublet was easily resolved and a fine line can be seen within each sodium line. As described in our previous post on spectrum tubes, I believe this is because the sodium is at different temperatures in the tube and there is self absorption in the cooler parts of the gas.
For a reflectivity of 90%, a finesse of around 30 is expected (ignoring for the moment mirror flatness issues). I found that how the light enters the etalon (collimation, etc) is quite critical and there is room for improvement on this issue to get the best finesse.
With not too much effort, we were able to get the sodium doublet pattern to be almost perfect but our initial attempts at hydrogen alpha have not been quite as good. The hydrogen light source is not as bright as the sodium lamp and we had to hurry the experiment in order not to let it overheat. The optics between the hydrogen source and the etalon was therefore probably not fine tuned as much as it was with sodium. It's also possible the mirrors, which are typically optimised for green light, may have lower reflectivity and therefore lower finesse at 656nm (the Thorlabs specifications state that the dielectric beam splitter coating is optimized for the 400 to 700 nm range).
This has been a good learning experience and we hope to refine the setup and use it to do some spectroscopy experiments in the near future.
A small red diode laser was used to do an initial alignment. As is well known, the laser creates a series of reflected spots which can be brought to the centre by vertical and horizontal adjustments. A small lens can then be put in front of the laser to broaden the beam so that the Fabry-Perot ring structure appears in order to fine tune the symmetry of the interreference pattern.Home-made Fabry-Perot etalon project
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- MalVeauX
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Re: Home-made Fabry-Perot etalon project
This is really awesome! I look forward to future tests!
Very best,
Very best,
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Re: Home-made Fabry-Perot etalon project
Cool experiments. Thanks for sharing.
Stu.
Stu.
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Re: Home-made Fabry-Perot etalon project
We also put some video data on YouTube so you can see the "scanning" of the etalon, done by hand by lightly squeezing the mirror mounts.
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Re: Home-made Fabry-Perot etalon project
Very good indeed!
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