Why won't the bright spot in my Michelson Interferometer turn dark?

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SUMMARY

The discussion centers on a Michelson Interferometer built with a 635 nm HeNe laser, where the user, Maurader, is unable to achieve a dark central spot despite having contrasty fringes. The issue arises from the need for exact intensity matching between the two beams, which is critical for achieving complete dark spots. Maurader uses a cubic beam splitter with power intensities measured at 0.30 mW and 0.33 mW, indicating a slight imbalance. The suggestion to use a compensator plate to equalize beam losses is highlighted as a potential solution.

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Maurader
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Hi all,

I've made a Michelson Interferometer with 635 HeNe, and I get nice, contrasty fringes. I have one problem - I cannot get the bright spot in the middle to turn dark! The intensity it has is like bright and brighter, but never dark. Any ideas why? The intensity of the two beams are approximately equal.

Thanks,

Maurader
 
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Maurader said:
The intensity of the two beams are approximately equal.
If you really want the dark places to be completely dark, then you must have exactly the same intensity for the two beams that combine. Are you using a compensator plate to even out the loss that one of the beams encounters through the splitter? Is your splitter 50-50? What is the tolerance of the splitter's ratio?
 
What I find strange is that my rings have extremely good bright/dark contrast, and this problem of bright/brighter only occurs at the central spot.

I am using a cubic beam splitter, and have measured the power intensities of the two beams to be almost identical (.30 mW and .33 mW)
 
I don't know what to tell you. Sorry.
 

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