7.1.4. Interference of a doughnut laser beam: collinear beams.

When a doughnut beam interferes with a fundamental Gauss beam the resulting interference pattern shows a lobe which rotates depending on the difference in path between the two beams. Of course both beams must originate from the same laser source to have the same initial phase. The doughnut beam can be made by inserting a spiral phase plate in one of the two beams. The resulting beam looks like this:

When the two beams interfere and when they are collinear, the interference pattern shows a rotating lobe when the path difference between the two beams is varied:

(Source code, png, hires.png, pdf)