Hello,
I have noticed that when a collimated laser beam (532nm) is reflected from the DLP 7000 DMD (0.7 XGA) the incident polarization is rotated depending upon the orientation of the DMD. A rotation of 0 degrees (that is the DMD is square with the optical table as are the micromirrors) results in unchanged polarization. If the DMD is rotated ~22.5 degrees a 45 degree rotation of the polarization is seen and at a 45 degree rotation of the DMD (such that the micromirrors are now diamond in shape relative to the table) results in a 90 degree rotation of polarization. Further rotation is difficult due to the ribbon cable connecting the DMD to the controller, but I have noted that if the DMD is rotated through 90 degrees the original incident polarization is restored; indicating a 180 degree rotation of polarization.
I was hoping that you may be able to identify the cause of this rotation. To the best of my knowledge DMDs are thought to be entirely polarization insensitive, and so I would not have expected to see any effect of the DMD on the incident polarization. My current hypothesis is that there may be some amount of stress-induced bifringence in the Corning 7056 glass used to hermetically seal the DMD, but I am unsure as to how to confirm this. Interestingly, a DLP 3000 that I have also tested (which uses Corning Eagle XG) shows no change in polarization.
I would appreciate any help or insight into the cause of this phenomenon.
