Talk:Fabry–Pérot interferometer

message: Isn't the graph of transmission against wavelength against slightly incorrect.  Shouldn't it be F = 1 for the blue line as the full-width-half-maximum equals the distance between peaks.  Hence delta-lamda equals 2*pi and hence F=1.  Or am I completely incorrect?  129.67.53.158 19:39, 23 April 2007 (UTC)
replies: text: I don't see your point. The full width at half maximum is clearly about half the distance between the peaks, as it should be for a finesse of 2. (Don't use "F" for finesse. F is the symbol for the coefficient of finesse, which is a different quantity. Finesse is denoted by a curly  \mathcal{F}.)--Srleffler 02:57, 1 October 2007 (UTC)
thread_title:  Incorrect graph? 

message: Shouldn't the coeff. of Finesse be
 F = \frac{4R} instead of  F = \frac{4R}?
Such as http://scienceworld.wolfram.com/physics/FinesseCoefficient.html
--Emplynx 18:43, 28 April 2007 (UTC)
replies: text: The Wolfram page doesn't give quite the formula you propose. Their formula is different from ours because their r (note the lowercase) is the amplitude coefficient of reflectivity, while our R is the intensity coefficient.--Srleffler 03:03, 1 October 2007 (UTC)
thread_title:  Coefficient of Finesse 

message: There seems to be some uncertainty about whether the coinventor's name is Perot or Pérot. I started a discussion on this at Talk:Alfred Pérot. We need a reference that is agreed to be authoritative, that establishes the correct spelling of his name. --Srleffler 02:44, 1 October 2007 (UTC)
replies: text: "Correctness" is the wrong criterion.  And we are concerned here with the correct spelling of "Fabry-Perot interferometer", not so much with his name.  In other words it is entirely possible that the correct spelling here could differ from the spelling in the article about him. Gene Nygaard 10:36, 4 October 2007 (UTC)
replies: text: I agree, but let's keep the discussion together at Talk:Alfred Pérot.--Srleffler 17:32, 4 October 2007 (UTC)
thread_title:  Perot or Pérot? 

message: According to Encyclopedia of Laser Physics and Technology, the formula for free-spectral range should use the group index instead of the normal (phase) index. But I couldn't find a discussion of dispersion in reference books. I tried to derive it:
Assume normal incidence for simplicity. Then the optical path difference is L=2n\ell. The peak-transmission criterion is kL=integer. [I'm using k=1/wavelength, not 2pi/wavelength]. Normally that means \Delta k = 1/L. But L is wavelength-dependent too. The zeroth-order approximation neglects this: (\Delta k)_0 = 1/L_0. The first-order approximation of the change in L from one peak to the next is:
:\Delta L = (\partial L/\partial n)(\partial n/\partial k)(\Delta k)_0 =(1/n)(\partial n/\partial k)
Then the corrected increment of k is:
:\Delta k = (\Delta k)_0 + \frac{\partial \Delta k}{\partial L}\Delta L = 1/L + (-1/L^2)(1/n)(\partial n/\partial k) = \frac{1}{L}-\frac{\partial n / \partial k}{nL^2}
Supposedly this equals 1/(2n_g \ell). But obviously not -- for one thing, in my formula, \Delta k is not inversely proportional to \ell. Anyone know what's going on here? Is Encyclopedia of Laser Physics and Technology totally wrong? Am I making a mistake or using the wrong approach? Is there a reference that discusses this?
[FYI: For glass, the difference between the Encyclopedia of Laser Physics and Technology formula and the wikipedia formula is about 1%. The difference between my formula and the wikipedia formula is about (1 nm)/L, e.g. 3×10-7 for 1mm thick.]
Thanks in advance!! :-) --Steve (talk) 21:34, 29 August 2011 (UTC)
thread_title:  Index vs group index 

message: Hi, just a quick comment: as it is, I see that the discussion is valid for normal incidence, but is it also true for a variable angle of incidence? The way the finesse is defined, it should be dependent on whether the field is p or s polarized.  — Preceding unsigned comment added by Anglyan (talk • contribs) 21:02, 3 October 2011 (UTC)
replies: text: The reflectivity R is a function of angle and polarization. See Fresnel equations.--Srleffler (talk) 01:17, 4 October 2011 (UTC)
replies: text: Just so. Shouldn't this be mentioned somewhere in the text? The normal incidence case avoids this problem since both p and s polarizations provide the same result. My concern is that figures represent the oblique incidence case, giving the impression that these equations work for oblique incidence as well, whereas they only apply to purely p or s polarized light. Simply stating: "In the case of oblique incidence, the finesse will depend on the polarization state of the light" would clarify things for the general reader.  — Preceding unsigned comment added by 24.11.236.211 (talk) 05:42, 5 October 2011 (UTC)
replies: text: Sounds good. Why don't you go ahead and add that to the article?--Srleffler (talk) 00:42, 6 October 2011 (UTC)
thread_title:  what about polarization 

message: I just noticed that the image at right, which appears in the article, is flawed. It fails to show the reflection from the point where the incident beam strikes the surface. Without including that reflection, one can never analyze the total reflection and get the right result. --Srleffler (talk) 01:24, 4 October 2011 (UTC)
replies: text: I never did like the first "derivation", it seemed to present results via a discussion that was opaque to me. This is why I added the section "detailed analysis". In this section, the incident beam has amplitude 1. When it hits the left face, the reflected beam has amplitude √R, the transmitted beam √T. When the transmitted beam hits the right face, the reflected beam has amplitude √T√R and the transmitted beam has amplitude √T√T=T. etc. etc. The reflected beam is not shown, and probably should be. As for the first "derivation" I don't want to go there. PAR (talk) 04:22, 4 October 2011 (UTC)
replies: text: What confused you is that it is not a derivation, just a presentation of the key results. Wikipedia is not a textbook; derivations are often not appropriate material for WP articles.--Srleffler (talk) 05:14, 4 October 2011 (UTC)

text: You're a little late with the correction, but thanks! Stigmatella aurantiaca (talk) 05:01, 24 May 2013 (UTC)
thread_title:  Bad image. 

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thread_title:  External links modified 

message: The factor 4 in the denominator of the last equation of part 1. of the Theory section is incorrect, as can be easily seen by inserting the expression from the line above:
 \gamma_{q,L} (\nu) = \frac{\pi}{2} \Delta \nu_c \tilde \gamma_q (\nu) = \frac{\pi}{2} \Delta \nu_c \frac{1}{\pi} \frac{\Delta \nu_c / 2}{(\Delta \nu_c / 2)^2 + (\nu - \nu_q)^2} = \frac{(\Delta \nu_c / 2)^2}{(\Delta \nu_c / 2)^2 + (\nu - \nu_q)^2}. 
The same error is present in equation (17) of the citation given for this section (Opt. Express 24, 16366-16389 (2016)). 130.183.91.36 (talk) 12:42, 15 October 2018 (UTC)
thread_title:  Incorrect expression in part 1. of Theory section 

message: Free spectral range is slowly varying. For a limited band, FSR can be considered fixed i.e. FSR is approximately invariant over many FSRs of a practical resonator.
I do not understand therefore the origin of the cosine in the denominator. I suggest that the approximation to the right would be correct if the cosine were deleted. I cannot give a correct expression for the exact equation to the left, but do not believe it should contain a cosine. Could somebody please either explain or revise?  — Preceding unsigned comment added by Oh, I'm just here for (talk • contribs) 11:35, 26 February 2021 (UTC)
replies: text: The free spectral range varies with the angle of incidence of light on the etalon, hence the \cos \theta.--Srleffler (talk) 02:53, 1 March 2021 (UTC)
replies: text: Thanks Srleffler, that is obvious now! Apologies Oh, I'm just here for (talk) 14:56, 6 April 2021 (UTC)
thread_title:  Description of the Fabry-Perot resonator in wavelength space: free spectral range (FSR) of the etalon, Δλ