A little background: I've chopped up a small LCoS "Pico Projector" and turned it into a micro-projector. E.g. I removed all the projection lenses and replaced them with basically a microscope objective stack:
But it fails miserably with 405nm laser light. Here is the same test image, annotated to help see the "100" from the test:
Now, it's my understanding that LCoS projectors operate on the principle of S and P polarized light. E.g. incoherent light enters the optical assembly and is split into S/P by a polarized beam splitter. The P light goes straight through and is dumped, while the S light is reflected towards the LCoS.
If the pixel it hits is "on", the polarization is converted to P, bounces off the reflective portion of the LCoS and then exits the projector. If the pixel is "off", the light simply reflects without a change in polarization state, reflects again off the beam splitter and is sent back towards the light source.
So given all of that, what's going on with the 405nm laser light test? The test pattern is visible (barely), but there is a huge amount of "noise". My current theories are:
The polarized beam splitter is not rated for near-UV light. While technically still in the visible range (400+), it's pretty deep-visible. Perhaps the polarizer just doesn't work with these wavelengths? If you look closely at the "mirrored" section of the splitter, you can see a wavy texture that is similar to the noise in that above image
Something is amiss with polarization states. I did try to rotate the laser to see if that would help (e.g. maybe it was emitting S when I needed P, so rotating 90° would fix that) but to no avail. So perhaps my laser is emitting circularly polarized light?
Interference patterns of some sort. Maybe the light is interfering with itself somewhere in the light path (similar to an interferometer) and causing those standing wave patterns?
Do any of these sound plausible? I admit we are reaching the edge of my optics knowledge :)