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Eikonal
journeyman


Reged: 05/11/11

Anyone ever made a Fabry Pérot-Etalon? new
      #5908314 - 06/07/13 04:31 PM

Hello everyone,

I'd like to know whether anyone has experience building a tunable FP etalon.
The optical side is not really the problem, here: I will start out with NF2-disks of suitable surface quality I happen to have lying around, and then, if this works fine, I'll make a set of new plates made of quartz.

I want to build in three piezo elements as spacers.
So, what I really would like to know is whether someone in the forum has any experience with controlling piezo actuators electronically.
Brian Manning used piezo crystals from gas lighters to compensate his ruling engine.
Anyone ever done this?

Looking forward to hearing about any experience you may have had in this direction.
All the best,

Kai.


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John Carruthers
Skiprat
*****

Reged: 02/02/07

Loc: Kent, UK
Re: Anyone ever made a Fabry Pérot-Etalon? new [Re: Eikonal]
      #5909182 - 06/08/13 03:46 AM

I have recently recontacted an FB etalon for a PST. The 20mm clear aperture has plates at least 1/100 wave flat and getting them perfectly parallel takes many attempts. Cleaving the mica spacers to sufficient exactitude (+/- 0.002mm) takes practice too, (do it under water).

I tried other materials for spacers but couldn't get them as molecularly clean as freshly cleaved mica.

Do not try any form of adhesive, the variation in film thickness will badly affect the finesse of the etalon, if indeed it gives an image at all.


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skybadger
journeyman


Reged: 05/18/08

Loc: Berkshire UK
Re: Anyone ever made a Fabry Pérot-Etalon? new [Re: John Carruthers]
      #5909846 - 06/08/13 02:20 PM

Yes - I made several for my PhD.
They were not contacted etalons but air spaced for use in cryostats fro mid-infrared wavelengths.
The first plate of the pair was fixed and the second was carried in a plate suspended on concentric leaf springs. The drivers were not the Queensgate-type piezo elements since they couldn't get sufficient throw to tune through the wavelength range required. Instead I used solenoid coils driving against magnets arranged in three locations around the plate carrier. These were driven through a PID amplifier using capacitive sensors in the solenoids to maintain alignment.
FPs like these were used as the mid-IR spectrograph on the ISO satellite.
A 100micron piezo element was Ł100 last time I looked.
Shout if you need anything more.
Mike


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John Carruthers
Skiprat
*****

Reged: 02/02/07

Loc: Kent, UK
Re: Anyone ever made a Fabry Pérot-Etalon? new [Re: skybadger]
      #5912439 - 06/10/13 04:36 AM

would density tuning give you sufficient range?

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Eikonal
journeyman


Reged: 05/11/11

Re: Anyone ever made a Fabry Pérot-Etalon? new [Re: John Carruthers]
      #5912624 - 06/10/13 09:08 AM

Thanks for the answers, so far!

I am working on a first, coarse mechanics test setup. As far as the substrates for the mirrors go, I'll have to refigure them slightly:
one of the substrates is 1/5 fringe concave (which is fine for a first test), but the other one has a whopping two fringe hollow to it. It needs retouching, first.

One substrate will be placed on a fixed kinetic mount, the other on a short cross-roller stage.
Hopefully, I'll be able to post some pictures by the end of next week.

In the final setup, this second mirror will be placed on three piezo elements to fine-correct for tilt errors introduced by the linear stage and to fine-tune the main wavelength.

The scope of this project is to build an ultra-high-resolution spectrograph for solar work (mostly for dopplergrams and magnetographs).
I have set up one spectrograph with a resolving power of 180,000, which will work as a pre-filter for this etalon.
I want to see how far I can get with the light power we have in our setup (Zeiss Coelostat with several light paths of 150 mm diameter each, one of which will be dedicated for this purpose).

The other thing I'd like to try is using two cascaded etalons, slightly detuned against each other, for high-resolution, high contrast photographic work. The filter should be tuneable to any desired wavelength between 300 nm and 800 nm. (Hence the final version with Quartz substrates).

The whole setup will be placed in the amateur-owned Göttingen solar tower.


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skybadger
journeyman


Reged: 05/18/08

Loc: Berkshire UK
Re: Anyone ever made a Fabry Pérot-Etalon? new [Re: Eikonal]
      #5919222 - 06/13/13 05:47 PM

Err, I think the idea of a pair of FPs may be flawed. Modelling of this shows that you need a pre-filter since even a pair of FPs has a very narrow free spectral range. You need to carefully manage the relative gaps so that one sits in gap which is a prime ratio to the other. The pre-filter tends to be a medium-res spectrograph or narrow-band (ie 20nm or so) dielelectric imaging filter otherwise you lose the benefit of the 2-d fp's.
The best stability I achieved (not to say you can't improve on this ) is a step size of about 2 nm with limiting noise from the surroundings of 5nm. At 10microns this is entirely acceptable. At 550nm this probably isn't.
Mike

Edited by skybadger (06/13/13 05:53 PM)


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Eikonal
journeyman


Reged: 05/11/11

Re: Anyone ever made a Fabry Pérot-Etalon? new [Re: skybadger]
      #5939177 - 06/25/13 05:50 AM Attachment (14 downloads)

I do not quite agree (sorry this took me some time).
Please find attached the transmission through two cascaded FP-etalons, one has an arbitrarily chosen thickness of 791 microns, the thickness d of the other varies. Plotted is the logarithm of the product of the transmissivities of both etalons at a given wavelength.

This just an arbitrary example to demonstrate that the idea is not all nonsense: The target wavelength here would be the K-Line at ~393 nm But I claim that this arrangement can be optimized to any wavelength with independant adjustments made to both etalons.

The peaks are well separated by gaps in which the filter stack is about four orders of magnitude less transparent.

Bottom line: The main transmission peak can be finely tuned. And it can be so tuned that either a single reflection on a diffraction grating will sort the orders efficiently, or that a rather coarse interference filter, or even a dichroic mirror can be used to choose the main wavelength for the detector. But the question remains whether the transmission region can be made narrow enough this way.

Edited by Eikonal (06/25/13 08:49 PM)


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Eikonal
journeyman


Reged: 05/11/11

Re: Anyone ever made a Fabry Pérot-Etalon? new [Re: Eikonal]
      #5941917 - 06/26/13 06:36 PM Attachment (11 downloads)

For a slightly less asymmetric setup (which I would prefer), the transmission would look like this:

Edited by Eikonal (06/27/13 10:22 AM)


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skybadger
journeyman


Reged: 05/18/08

Loc: Berkshire UK
Re: Anyone ever made a Fabry Pérot-Etalon? [Re: Eikonal]
      #5944134 - 06/28/13 01:30 AM

Which kinda backs up what I was saying.
I don;t think I believe your first picture. Your second looks more like what I was expecting. The free spectral range between combined peaks is of the order of 2nm. Thats a narrow band filter at the best of times.
You want the asymmetry to increase the FSR and dampen the periodic beating effect, as seen in the first picture.
The loss is that you need to tune for a gap that resides near the peak to get the maximum side-band suppression. This is easily solved if you know the characteristics of each FP.
With a FSR of 2nm, your noise floor for positioning needs to be 1/100th of this, a positional accuracy of 20 picometres or 1/50 Angstrom to maintain stability while you build your exposure.
The theory is perfectly sound and do-able, I'm just trying to alert you to the pitfalls that make this hard in the optical. Quality of the glass is probably the least of your worries.
Cheers
Mike


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Eikonal
journeyman


Reged: 05/11/11

Re: Anyone ever made a Fabry Pérot-Etalon? new [Re: skybadger]
      #5948804 - 06/30/13 10:23 PM

Hi Mike,

thanks for the response.
Your are, of course, right in your pointing out the problems.
As to the first diagram: It's generated from the same expression for the transmission functions as the second: just the product of two transmission funtions for different thicknesses, really. And this is, mathematically, a rather trivial matter.

The original, somewhat naive idea I had was to try and rid myself of the necessity for a narrow band filter alltogether.
This is, as you quite rightly point out, not possible with just two FPs.
At best I realize I can get a more tuneable sharp peak within the passband of a broader pre-filter. As for the spectral resolution when emplying the FP etalon within a spectrograph, I still have some hopes, here.

So in any case, I will have to build an etalon, first, and only then I can measure the finesse and its spectral characteristics, and finally model this for real (but right now, I am waiting for the US postal service to finally deliver the quartz glass I ordered from surplus shed.

As for the accuracy requirements you have calculated: I have no reason to doubt your figures, but I would like to understand the physics underlying these estimates. Could you please point me to some literature?

I'll keep the community posted of any progress I make, assuming, of course, there may be some interest in this.

All the best,
Kai.


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