Fabry Perot etalon calibration

[Chen]

Hi,

I'm performing an experiment whose goal is to analyze the splitting of the green Mercury line under Zeeman's effect. To do this we use a Fabry-Perot etalon, which first has to be calibrated. This means both making sure that the two reflective mirrors are parallel, and then measuring the distance between them by watching the specturm of some known elements.

My question comes down to this:
When I see, in a specific angle, clear and sharp fringers coming out of the etalon, is this an indication that the plates are parallel (at least locally, i.e around the line of sight)? Or is there another set of circumstances under which I would get the same results with the plates not parallel at all?

If you feel like it, you can continue reading more about my situation and set up.

We are watching the fringes through a telescope, and in order to measure them the etalon is mounted on a rotating disk, and we have digital means to record its rotation angle. So the procedure would be rotating until the telescope crosshair hits a fringe, writing down the angle, rotating again until we hit the next fringe, and so on.

Now, I am busy with making the plates parallel. What bothers me most is that the plates themselves are not completely planar; I can clearly see that they are in fact concave (or perhaps convex - I don't remember the English word - either way it doesn't matter).

Those are the facts of the matter. Now here is are my observations and analysis:

Since the plates are, say, concave, you can never say they are actually parallel. What you can say, is that they are parallel for a specific incidence angle, in which the concavity of the plates is such that they are locally parallel.

I have three screws through which I control the alignment of the plates relatively to each other. I can clearly see in the lab, that it's possible for me to get very very thin and clear fringes for a specific angle, however when I rotate the etalon the fringes become blurry and start expanding/contracting (depending on the direction in which I rotate the etalon).

Which brings up back to the question I asked above.

Many thanks,
Chen

 

[Chen]

Can whoever moved the thread to this forum please move it back? I don't see how it is even remotely relevant to this forum.

Thanks,
Chen

 

[charng]

Dear Chen,

Thank you for sharing your experiment and question about the calibration of a Fabry-Perot etalon. Based on your description, it is likely that the clear and sharp fringes you are observing are a result of the plates being parallel at that specific angle. The Fabry-Perot etalon works by allowing only a specific wavelength of light to pass through the narrow gap between the two plates, creating interference patterns or fringes. In order for these fringes to be sharp and clear, the plates need to be parallel to each other and perpendicular to the incident light.

However, even if the plates appear concave or convex, it is still possible to achieve parallelism for a specific angle of incidence. This is because the curvature of the plates can be compensated for by adjusting the spacing between the plates. Therefore, it is important to continue adjusting the screws to find the angle at which the fringes are the sharpest and thinnest, as this will indicate the best parallelism for your experiment.

Additionally, it is important to note that the etalon must remain stable and undisturbed during your measurements, as any movement or vibration can affect the parallelism and accuracy of your results. It may also be helpful to use multiple known elements to calibrate the etalon, in order to ensure that your results are consistent and accurate.

I hope this helps answer your question and wish you success in your experiment. Please don't hesitate to reach out if you have any further questions or concerns.