Orbital Angular Momentum

Home made spiral phase plate that kind of works

Top:  laser printer transparency material with cut

Bottom:  beam interfered in MZ interferometer.  More fringes on right of null than on left show change in phase.

Each beam is reflected twice in an MZ, so no phase difference if each mirror and beam splitter is at exactly the same angle.  Adjusting one mirror away from perfect results in linear fringes as the path length varies from one edge of the mirror to the other.  So, a sample in one path causing an abrupt change in phase shows up as a change in the number of fringes across the defect in the sample.

This is what they use in most of the papers, although I bought a dove prism to put in a polarizing Sagnac interferometer.  This way beams will have different polarization going in opposite directions in the ring.  Not sure yet how that's going to work, but it's why I wanted a polarizing interferometer.

I don't know how else to visualize the phase shift taking place from this SPP.  The null in the interferogram in the pic is pinhead size or less.  I have to use an old webcam with the lens and IR filter removed to see what's going on.  This also allowed me to try other colors.  Violet fringes are too small to see, and infrared too indistinct.  633 is the best.

  ​