Talk:Glan-Foucault prism

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Isn't the drawing misleading here? I think some of the e-ray is also reflected at the gap, just due to ordinary Fresnel reflection. The transmitted beam is 100% polarized. The reflected beam is not.--Srleffler 16:52, 10 January 2006 (UTC)

Wow, I just did the calculation, and I get that if the o-ray is above its critical angle, the e-ray's reflection is at least 20% per surface. Am I doing something wrong? If this is true, it explains why I have only ever actually seen Glan-laser (Glan-Taylor) polarizers, never a true Glan-Foucault.--Srleffler 17:27, 10 January 2006 (UTC)

Not sure. I can't find a mention of that as a drawback in the OSA Handbook of Optics (where I got most of the info for the polarizing prism articles.) I do know the air-gap distance is extremely critical, perhaps some kind of evanescent effect reduces the loss? --Bob Mellish 19:35, 10 January 2006 (UTC)

I found a paper on arxive, which seems to confirm the high loss for the transmitted beam [1]. The key graph is fig.3. Not only is there high reflection from the gap surfaces, but there are etalon effects due to multiple reflections in the gap. The curve in fig.3 looks about right for an air-gap etalon with around 20% reflectivity. I added this as a reference to the Glan-Taylor article. The etalon effects should be reduced if the beam diameter is small or the gap is large, but the transmission would still be low. I don't think evanescence can help, because it would destroy the total internal reflection on which the polarizer depends. United Crystals sells Glan-Foucault prisms[2]. They claim transmission >60%. Their Glan-Taylors[3] have >85% transmission.

This raises another issue, regarding polarizer. Right now, that article breaks polarizers up into absorptive and beamsplitting polarizers. Really, there should be three categories. Due to the imperfect polarization of the reflected beam, Glan-type polarizers are not really polarizing beamsplitters. The Wollaston-type prisms are, as are the dielectric plate and cube splitters. --Srleffler 23:05, 10 January 2006 (UTC)

• I've finally got around to uploading a slightly tweaked image, to show that the deflected beam is only partially polarized. --17:39, 15 March 2006 (UTC)