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TOPIC: Laser front instability?

Laser front instability? 1 year 5 months ago #65

  • Piter
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Hi forum.

It may not be the best place to ask but I will give a try.
I tried to use 488nm diode OBIS laser with SLM, and got one problem.
Fringes in the light reflected from SLM were unstable.
On the beginning I was blaming SLM flickering, but I ruled it out.
If I replace the OBIS laser with HeNe 633 nm, fringes are dead stable.
It looks like OBIS laser becomes 'unstable' after a spatial filter.
If I look on fringes before spatial filter they are stable for both lasers.
I cannot come up with an explanation of the fact. Any hints?

Thanks,
Petro

Laser front instability? 1 year 5 months ago #68

  • sebastien.popoff
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Hi,

What do you call unstable? Do you see them changing in real time on a camera or with higher frequencies?


It is possible that you have a drift of the central frequency.
After modulation/filtering, you may make paths interfere with bigger optical path length differences than before. If the frequency move, the path length difference will change too.

The easy way to test is to build a Michelson interferometer and increase the path difference between the two arms and check if the stability changes.

Another thing to check is the AR coating of the SLM, if you work at the wrong wavelength, you may have parasite reflections that come into play and interfere with the signal.

Best,

Sebastien
Last Edit: 1 year 5 months ago by sebastien.popoff.

Laser front instability? 1 year 5 months ago #69

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Hi, thanks for reply
After SLM it looks like this


I am having parasitic reflections for sure as 488nm is not optimal for my SLM, it is built for 750.


A bit confused how can path length difference change stability.
It will change interference pattern but stability will be the same I guess. Where do I go wrong?
The same I think shall go for reflections from SLM, it will add some pattern but pattern itself shall be stable I believe.

Am I wrong?
Thank you.
Petro

Laser front instability? 1 year 5 months ago #70

  • sebastien.popoff
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I am just saying that if you have a drift in frequency in an interferometer, how much the interference figure changes depends on the difference of path length.

The phase shift between two arms in an interferometer depends on the difference of path length, the wavelength and the optical index. If the difference of path length is big, you will be more sensitive to a small frequency drift.

Does that make sens?

Best,

Sebastien
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