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

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Discussion Overview

The discussion revolves around the issue of achieving a dark central spot in a Michelson Interferometer setup. Participants explore the conditions necessary for achieving destructive interference, focusing on beam intensity and splitter characteristics.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant, Maurader, reports that despite having contrasty fringes, the central bright spot in their interferometer does not turn dark, even with approximately equal beam intensities.
  • Another participant suggests that for the central spot to be completely dark, the intensities of the two beams must be exactly equal and questions whether a compensator plate is being used to balance losses from the beam splitter.
  • A third participant notes the oddity of having good contrast in the rings while experiencing the bright/brighter issue only at the central spot, mentioning specific power intensity measurements of the beams.
  • A later reply expresses uncertainty and does not provide further insights or solutions.

Areas of Agreement / Disagreement

Participants express differing views on the reasons behind the inability to achieve a dark central spot, with no consensus reached on the underlying cause or solution.

Contextual Notes

There are potential limitations regarding the assumptions about beam intensity equality, the characteristics of the beam splitter, and the use of compensator plates, which remain unresolved.

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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