Re: Fabry-Perot question

From: Níkola Heímpel (niki0904_at_nurfuerspam.de)
Date: 12/01/04 

Date: Wed, 01 Dec 2004 17:22:02 +0100

Sam Goldwasser wrote:
> Níkola Heímpel <niki0904@nurfuerspam.de> writes:

>>If I have a spherical mirror Fabry Perot interferometer and do get
>>some signals, but they are way too small, what reason could that have?
>
> Are you doing a confocal or true spherical cavity? What
> is the radius of curvature and relfectance of the mirrors?

Confocal cavity, RoC 25mm, reflectance (self made) around 90% or even
above. I forgot to mention that we are using an slightly off-axis beam,
so at confocal distance, the two transmitted points can be seen very
clearly. When the adjustment is good, but wrong, we see "circles" of
points, just as one would expect (more than 4 reflections before it hits
the same place again, but still stable).

>>What would be the most likely reason:
>
>>- insufficient mirror surface quality, destroying the wavefront after
>>some cycles?
>
> Yes. However, this isn't as critical as for laser resonator mirrors in the
> sense that a speck of dust will affect it greatly.

Hm. I thought the sputtering process we use for the mirror (Ag) coating
might give a bad quality in terms of surface roughness.

>>- misalignment (hard to believe, since we have sensitive alignment
>>mechanics and tried for a very long time - also always get the same
>>values, even after taking everything apart and trying again, and the
>>maximum intensity we get is also very sensitive to adjustment changes)
>
> No, assuming a reasonable design, alignment is not that har.dYou should
> be able to do it mostly by just observing the scatter of the beam on the
> mirror surfaces if they are acessible.
>
> However, you may be setting the cavity at the wrong distance for a mode
> degenerate interferometer and getting confused thinking you have it set
> correctly. The output could be much lower in that case by an order of
> magnitude or more.
>
> How are you setting the distance?

By a micrometer screw. First I do some coarse alignment by looking at
the points, when they start "blinking", I turn to the scope. There I can
easily find the confocal distance of 25.7mm by looking for the maximum
signal. Fits every time. I don't really think the distance is wrong, or
rather: with an off-axis setup, I wouldn't have those signals at wrong
mirror distances. FSR is according to theory. The mirror holders can be
adjusted (almost) independent of mirror distance.

>>We use self-coated mirrors, so mirror quality could well be an
>>issue. But why are the peaks narrow then? I expected to get either
>>small and broad peaks, or huge and narrow ones. To me it looks like
>>I'm "losing" intensity somewhere.
>
> Have you tested the mirrors?

How can I do that? (what kind of testing do you mean exactly?)

> Don't expect textbook performance. I can easily get about 10 percent
> through for the peaks using HeNe laser mirrors. But 1/1000th is quite
> low.

Well, I'd be more than happy with 10%..

> More info in the Laser FAQ at:
> http://repairfaq.ece.drexel.edu/sam/laserlia.htm#liasfpi

I read most of your FAQ, especially the parts with the FPIs. We also see
the two modes of our HeNe, very nice.

Thanks again,
Nikola

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