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I'm taking an optics lab in which I'm required to construct an interferometer, and measure the wavelength of a laser, and the coherence length of the light emitted from a candle fire.

Now, I've been hinted by my instructor that when working with mirrors (which are present in an interferometer), it is best to make sure that the laser is polarized to either "P" or "S" polarization - otherwise, the mirrors might give an elliptic component to the polarization, which in turn will spoil the measurment, due to interference effects.

So now comes the question: should the light from the candle be polarized as well??

I have a reason to suspect that polarizing it will improve the coherence length, and on the other hand, I'm pretty sure that elliptical polarization will take effect if the light remains untouched.

What is your opinion on this dilemma?? Thanks!!

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There are two reasons why it may not be necessary...firstly the intensity of light from the candle is low unlike that of the laser. Secondly, the laser encounters two mirrors while light from the candle only encounters one before reaching the detector.

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Light from a thermal light source is unpolarized. That means that the electric field components of orthogonal polarizations are uncorrelated. This leads to a reduction of the contrast of your intererence pattern by 50% (because you average over 2 uncorrelated modes).
But the coherence length depends on the spectrum only. Hence, you measure the same coherence length with/without polarizer. The difference is that with a polarizer you have twice the contrast, but half the intensity compared to a measurement without polarizer.

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