Interference pattern for a laser beam through an AOM

[Malamala]

Hello! I have a setup in which I send a laser beam (CW 1064 nm, 1 W) through an AOM (Acusto-optic modulator) and the 1st order beam coming out of it goes through a second AOM (they are both rated for this wavelength and more than 100 W of power). The original laser beam, as well as the first order (and zeroth order) coming out of the first AOM are TEM00 Gaussian beams (both in the specifications and as measured on a beam profiler). However, the beams coming out of the second AOM (both zero and first order), have a TEM00 Gaussian shape, but with an interference like pattern (wave-like) on top, along one direction (i.e. longitudinal pattern, not grid-like). Even when I don't apply any RF power at all to the second AOM, I still get this effect (in this case I only have the zeroth order beam). I even tried a different AOM as the second one and I am getting the same effect. I also measured the laser power reflected from the AOM crystal, and the reflection is <0.5%, as in the AR coating specifications. Has anyone seen this effect before?

 

[tech99]

It looks like the interference pattern created by the direct laser light plus the output of the first AOM. Presumably these have different path lengths and hence the fast ripple.

 

[Malamala]

It looks like the interference pattern created by the direct laser light plus the output of the first AOM. Presumably these have different path lengths and hence the fast ripple.

What exactly do you mean by the direct laser light? In my case the laser light goes through the first AOM which generates a zeroth order and first order mode which go on different path (in my case I just dump the zeroth order beam in a beam dump). Then the 1st order beam goes through the second AOM, so there is no other laser light coming into the first AOM.

 

[tech99]

What exactly do you mean by the direct laser light? In my case the laser light goes through the first AOM which generates a zeroth order and first order mode which go on different path (in my case I just dump the zeroth order beam in a beam dump). Then the 1st order beam goes through the second AOM, so there is no other laser light coming into the first AOM.

Could you send a diagram of the set up?

 

[Quarker]

Did you ever figure out what was causing the unexplained interference pattern?