Generation of a doughnut-shaped beam using a spiral phase plate

To generate a doughnut-shaped beam, i.e., the first order of a Bessel beam, a spiral phase plate with 8 divided etching areas is fabricated with an etching accuracy of better than 6 nm. The etching depth of each area is designed so that the phase distribution of the laser beam passing through has a phase difference of π at a symmetric position with respect to the optical axis. Using a laser beam with a wavefront aberration of 1/10λ, the phase distribution of the beam passing through the plate is measured by a Shack Hartman wavefront sensor. It has been found that the beam has a spiral phase change of 2π along the optical axis. The focused beam has a circular doughnut pattern, as predicted by a theoretical calculation, and we succeeded to generate the ideal first-order of a Bessel beam.