Unexpected white light interference pattern

[Nickyvl]

Dear fellow Physicists,

For my project I wanted to measure the thickness of a glass plate with white light interference. Before being able to measure, I had to setup an interferometer. Unlike with a Laser, with white light it is hard to find the interference pattern since the coherence length is only a micrometer. Finally, after setting up the interferometer I was able to look for the interference pattern but unfortunately it was not what was expected. Attached are two pictures of the interference pattern that was found.
For this interferometer a white light source with fiber, A 10mm N-BK7 Beam splitter and two round silver mirrors were used.
The pictures show that the interference pattern is only visible around the white light spot, instead of the spot being part of the interference pattern. Now, my quetion is, why is the interference pattern so weak compared to the spot? And why are these four extinctions visible (marked in the edited picture). May it be because of bad alignment? Or is there any other cause?

Hopefully someone is able to help me.Nicky

 

[BvU]

Hello Nick, welcome to PF :)

Won't it be difficult to measure a path length difference if the coherence length is only a micrometer ? What is the setup ?

 

[Nickyvl]

Hi! Thanks for replying,
The goal is to measure the exact thickness of a glass plate. An estimation of the thickness can be made, after which you can search for the interference pattern from the reflection near that point. Yes it is indeed very hard to find the 0 OPD point with my setup. The setup is an Michelson-interferometer with a white light source with attached fiber, which is coupled in the setup with a collimator lens.

Info with the picture (it is Dutch):
Wit licht bron = white light source
Glasvezel=fiber
(translatie)spiegel = (translation)mirror

The detector can be, as seen in the thread picture, a white paper or can be a spectrometer, coupled out the same way as the light is coupled in(fiber+collimating lens).

Still, my greatest question is, why can i not see the interference pattern in the spot. I hope you can help me.

Sincerely.
Nicky

 

[mfb]

Where is the glass plate? And how thick (approximately) is it?

 

[Nickyvl]

Where is the glass plate? And how thick (approximately) is it?

The glass plate would replace the mirror on the translation stage. It's thickness is 1 or 2 milimeters, something like that.

 

[mfb]

It replaces the mirror? So you work with reflections at the front and back of the glass plate, together with reflections at the other mirror?

 

[Nickyvl]

Yes, i would replace the mirror and use the reflection(4%) from the glass. I will compensate this intensity difference with a filter in the other arm.