He-Ne laser beam is spatially filtered and normally incident to the surface of reflective SLM. Then, when I observe the shape of the reflected beam at the far distance, its shape is weirdly distorted (kind of ellptic). When I replace the SLM with a flat mirror, I do not observe this phenomenon.
I didn't place any lens after SLM. Figure 1 shows the beam shape at the near distance from SLM (z ~ 15 cm), and Figure 2 shows the shape at the far distance (z ~ 1 m). In Figure 1, the beam is almost circular just as the incident beam, but in Figure 2, the beam becomes elliptic or distorted.
What would be the reason of this?
It might be related to polarization, among many other reasons.
Your SLM may not be absolutely flat, such as your mirror. Any even slight curvature will lead to beam diameter distortions.
This is not surprising. The mirrors tilt on one axis. The "off" position of the mirrors is not perfect and there is some variation in their pointing angles. The net effect is that the mirror is not flat in one dimension resulting in astigmatism. Per your images the astigmatism appears to be small. Do you need better?
One reason can be that the SLM backplane is not perfectly flat which is introducing wavefront aberration (primarily astigmatism) in the reflected output from the SLM.
- Badges
- Science topic
- Similar topics
- Philosophy
- Philosophy Of Science
- Reasoning