What Causes Lattice-Like Aberrations in a Gaussian Laser Beam Path?

[silin]

Does anybody know what can be the reason for such an aberration of initially Gaussian shaped laser beam (see attached picture)? Optical elements in the light path: linearly polarized diode 470nm laser, single mode polarization conserving fiber, collimator, mirror, lens, round 50 micrometer pinhole, lens, filter for 470nm line, dichroic beam splitter reflecting 470nm light, mirror, Olympus IX70 microscope. The picture is the beam profile after the tube lens in the microscope. I have already checked all elemets for causing this aberration by removing them one by one, the problem with this lattice like beam shape becomes really pronounced when I leave both lenses in the optical path, but it loos really strange, as if some kind of diffraction pattern. If somebody has any idea please tell, it´s pretty urgent.

 

[Claude Bile]

Wow, looks like you're getting some structure there.

It's not clear from your initial post whether you have a laser cavity set up. If you do, then it is possible there are higher order transverse modes being excited.

If not, it is probably a dirty optical element (from my past experience this is the cause of 90% of such problems). Failing this, I would think the laser itself would be the next thing to check out, make sure there is no feedback into the laser and make sure you are operating within the laser's coherence length.

Beyond this, it's tough to diagnose something like this without a detailed knowledge of the system, as there are lots of little subtlelties that might come into play.

Claude.

 

[Integral]

Claude,

Could this be, or is it already, used to check the cleanliness of a multi surface optical system? Do you have more information about it?

 

[Claude Bile]

Integral,

Not to my knowledge. For a precise measurement on the cleanliness of a surface, there are several tools we use, all of which are fairly expensive, They include scanning electron microscopes, optical profilers and Differential Interferance Contrast (DIC) microscopes.

Silin,

Another though came to mind; Double check that your polarisation preserving fibre is single mode at 470 nm.

Claude.

 

[silin]

Thank you for the replies. I have done what you said and found that the problem is in the microscope. Probably there is some dirt in it, but it's amazing that the pattern is so regular.